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• Local condensation heat transfer in smooth channels and pin fin array is presented. • The effect of operating parameters on the heat transfer characteristics was analyzed. • Sensitivity analysis is performed of heat transfer to identify the critical parameters. • Condensation heat transfer increases with heat flux in the range of vapor quality above 0.4. In this paper, an experimental study of flow condensation was carried out of R134a in a multiport mini channel and a mini channel with pin fin array. The former comprised 20 parallel rectangular channels with an equivalent diameter of 0.64 mm. The latter is a narrow rectangular mini channel containing 10 rows of diamond pin fins with a staggered configuration, and height and longitudinal/transverse pitch of 0.5 mm and 2 mm respectively. To acquire the local value of heat flux and heat transfer coefficient, air jet impingement cooling devices were adopted to condense the vapor of refrigerants. The effects of vapor quality, mass flux, heat flux, and saturation pressure on flow condensation heat transfer coefficient were investigated with the operating conditions: vapor quality from 1 to 0, mass flux from 160 to 450 kg/(m2s), heat flux from 10.0 to 39.8 kW/m2, and saturation pressure from 600 to 1500 kPa. The experimental results indicated that the pin fin array significantly improves the condensation heat transfer coefficient up to nearly 186 % compared to the multiport mini channel in the high vapor quality region. For both channels, the local heat transfer coefficient increases with an increase in vapor quality, mass flux, and heat flux whereas decreases with increase in saturation pressure. The influence of heat flux and mass flux on heat transfer coefficient was more pronounced in the high vapor quality region than in the low vapor quality region. A sensitivity analysis was performed at different vapor quality levels. The most influential parameters in the smooth channel and the pin fin array are saturation pressure and mass flux, respectively. The relative contribution of heat flux is more obvious in the high vapor quality region. Some of the existing correlations have good prediction performance for the smooth channel experimental data in this paper, and the corresponding MAD is within 20 %. However, their deviations are much greater for the applications of pin fin array. [ABSTRACT FROM AUTHOR]

• HFE-7100 boiling curves from the onset of boiling to dry-out, in a vertical and rectangular mini-channel. • Local heat transfer coefficient (HTC) calculated with an inverse heat conduction method. • Influence of subcooling and pressure on heat transfer, pressure drop and flow patterns. • Increasing pressure improves the critical heat flux (CHF) and HTC, delays the dry-out, and reduces pressure drops. • Increasing subcooling improves CHF and HTC, delays the dry-out, and modifies the flow pattern. The background of this study is the problem of thermal runaway in the battery cells of electric vehicles. Increasingly powerful and compact battery packs require highly efficient thermal management. One potential solution is the direct liquid cooling of lithium-ion battery cells using a dielectric liquid which boils over a safety limit temperature. This approach intends to prevent in a first step the cell thermal runaway and if it fails in a second step its propagation to its neighbors: the liquid boiling helps to mitigate the cell temperature rise and to transport and spread the heat. The arrangement of the cells in the pack and their close spacing induce a scientific problem of convective boiling in a mini-channel. To enhance designs using this solution, a comprehensive understanding of flow boiling and a precise local heat transfer quantification is essential. The HFE-7100 has been selected as a working fluid for its attractive thermophysical properties and a suitable saturation temperature. The present paper introduces new experimental results of flow boiling dielectric fluid, the HFE-7100, in a vertical rectangular mini-channel. Measurements are carried out from start of boiling to dry-out. Boiling curves are obtained for a mass flux of 391 kg/(m².s), at different pressures (0.7, 1, 1.5 bar) and at different subcoolings (Δ T s u b = 0 ; 15 ; 30 ; 45 ∘ C). The test section is an aluminum block instrumented with three lines of five K-type thermocouples located all along the flow. The visualization of the flow is achieved through a glass pane with a high-speed camera to identify different flow patterns. The local heat transfer coefficient is calculated by a resolution of a 2D inverse heat conduction method. The experiment mainly aims at determining the heat transfer coefficient, the critical heat flux (CHF) and the pressure drop and at characterizing both the flow regimes and the dry-out phenomenon. The influence of subcooling and pressure on flow boiling is analyzed. An increase in subcooling enhances the CHF and the heat transfer coefficient, and delays the dry-out. Subcooling has an impact on the flow pattern and modifies the type of CHF. In the operating conditions tested, a decrease in pressure enhances CHF and delays dry-out. On the other hand, the heat transfer coefficient increases with rising pressure. Similarly, pressure drops are lower with higher pressure. [ABSTRACT FROM AUTHOR]

• The mathematical model can accurately predict the melting heat transfer process of PCMs. • The Rayleigh number increased with the heat flux, and the higher the heat flux, the larger the amplitude. • Increasing the heat flux improved the integrated heat transfer coefficient and thermal storage performance of the CPCMs. • Conduction was the primary heat transfer mechanism; however, the proportion of natural convection increased with the heat flux. To investigate the effect of heat flux on the enhanced heat transfer in phase change materials (PCMs) composed of copper metal foam (CMF) and paraffin wax (PX), a semi-cylindrical visualized heat storage device with a varied heat flux of 7.3 kW/m2, 8.5 kW/m2, 9.7 kW/m2, and 10.9 kW/m2 was established in this paper. Moreover, a 3-D mathematical model of composite phase change materials (CPCMs) was established using hybrid grid to study the temperature distribution, imbalance effect, flow, heat transfer mechanisms, and heat storage performance during the melting process. The results showed that the melting time of the CPCMs decreased as the heat flux increased. Compared with a heat flux of 7.3 kW/m2, the melting time was shortened by 10.05 %, 20.71 %, and 27.21 %, respectively. In addition, the Rayleigh number (Ra) increased with the increase in heat flux, and the amplitudes of Ra were 1.45×108,1.54×108,1.61×108, and 1.74×108, respectively, which indicated that the higher the heat flux, the larger the amplitudes. Moreover, the heat transfer mechanism during the melting process was dominated by conduction; however, with the increase in heat flux, the maximum flow velocity of the liquid paraffin increased, which caused the proportion of natural convection to increase from 27.80 % to 31.30 %. Hence, the integrated heat transfer coefficient of CPCMs increased from 5.69 W/(m·K) to 5.81 W/(m·K). However, the temperature imbalance effect of the CPCMs was exacerbated, as evidenced by the increased maximum temperature difference in the vertical direction of the center of the CPCMs from 24.7 K to 35.6 K. Besides, the heat storage performance was enhanced. Compared with a heat flux of 7.3 kW/m2, the heat storage capacities of the CPCMs increased by 4.21%, 9.46%, and 12.66 % with increasing heat flux, and the heat storage rates of the CPCMs increased by 15.3 %, 34.9 %, and 52.3 %, respectively. Furthermore, the relative error of the overall melting time was 3.4 %, indicating that the model provided reasonable predictions. [ABSTRACT FROM AUTHOR]

• 1 Flow and boiling heat transfer in a 5 mm tube were studied experimentally. • 2 Heat transfer enhancement is obviously founded in the region where x e < 0.05. • 3 New empirical correlations are developed based on the experimental data. • 4 New correlations have higher prediction accuracy than the existing correlations. Experimental investigation on flow boiling heat transfer in a small tube under high system pressure has been conducted in this study. The test-section is constructed with a 316L stainless steel tube. The inner diameter and heated length of the test-section are 5 mm and 930 mm, respectively. The working fluid is deionized water. The operating conditions of the experiments are as follows, the mass flux from 548 to 978 kg/(m2·s), the wall heat flux from 136 to 238 kW/m2, and the system pressure from 1.0 to 2.9 MPa. The experimental results showed that the inlet subcooling mainly affects the onset of nucleate boiling, but its impact on the heat transfer coefficient at the same vapor quality is almost negligible. The heat transfer coefficient increases with the increase of mass flux and wall heat flux, especially in the region where x e < 0.02. The average heat transfer coefficient is higher under high system pressure owing to the variation of the thermophysical properties. Additionally, the existing flow and heat transfer correlations cannot predict the flow boiling heat transfer coefficient and frictional pressure drop in the experiments accurately enough. Therefore, the new flow and heat transfer correlations are developed based on the experimental data in this paper, and the mean absolute deviation of the flow boiling heat transfer correlation and frictional pressure drop correlation are 13.3% and 12.1%, respectively. [ABSTRACT FROM AUTHOR]

• Heat transfer coefficient of highly subcooled water during flow boiling. • Challenges in minichannels during subcooled flow boiling. • Wall temperature distribution of mini and conventional channels during flow boiling. • Temporal temperature variations in minichannel at near to zero vapor quality. • Wall temperature fluctuations in minichannel for bo > 1.76 × 10−4. Channels of different sizes are employed in diverse engineering applications to enhance the efficiency of heat transfer system. Minichannels are preferred for their compactness, enhanced heat transfer, larger surface area to volume ratio, and high heat transfer coefficient values. There is a dearth of literature addressing flow boiling in minichannels with highly subcooled water at atmospheric system pressure. This research paper analyses the flow boiling phenomenon in a 2 mm diameter minichannel, comparing it with conventional 4 mm and 11.7 mm diameter tubes while maintaining a constant length to diameter ratio of 150. The analysis investigates the wall temperature distribution, local heat transfer coefficient, and temporal heat transfer fluctuations at a mass flux of 150 – 2400 kg/m2s and 109 – 1080 kW/m2 heat flux. The study explores the impact of boiling number on wall temperature fluctuations. Results reveal that mass flux doesn't impact heat transfer in subcooled flow boiling but does in saturated flow boiling in conventional channel. There are no significant temporal fluctuations of wall temperature in a 4 mm tube. However, vapor bubble size affects the circumferential area of minichannels, leading to abrupt flow disturbance. Wall temperature fluctuations occur in the minichannel for the boiling number exceeding 1.76 × 10−4. Temperature fluctuations in minichannel increase with boiling number, but decrease as mass flux is increased. Delving into the depths of minichannel heat transfer, this intriguing study unveils the pivotal role of the boiling number in ensuring process stability. [ABSTRACT FROM AUTHOR]

• A compact jet impingement boiling heat transfer device achieves a high-powder cooling power of 1532 w. • The variation trends of critical heat flux, heat transfer coefficient and pressure drop with mass flux, saturated pressure and liquid subcooling are obtained. • Compared with smooth surface, a micro-pin-finned surface can increase CHF and HTC by up to 60 % and 32 %, while only increasing a pressure drop of 14 %. In this paper, jet impingement boiling heat transfer performance was studied using a novel new environmentally friendly refrigerant medium (HP-1) based on a high-power chip heat sink. A series of experimental investigations of jet impingement boiling were conducted on smooth and micro-pin-finned surfaces (PF0.5–0.5–1, PF0.5–0.5–2 and PF0.3–0.3–1). Heat transfer performances were evaluated over the saturated pressures from 450 to 590 kPa, liquid subcoolings from 2 to 15 K, nozzle sizes from 1 to 2 mm, mass fluxes from 71 to 858 kg/(m2s). The results show that critical heat flux (CHF) and heat transfer coefficient (HTC) increases with the increase of mass flux and saturated pressure. At ΔT sub =15 K, the CHF of the micro-pin-finned surfaces can be increased by 63 % compared with smooth surface, in which the CHF of PF0.5–0.5–2 surface is up to 1532 kW/m2. Boiling heat transfer coefficient first increases and then decreases with the increase of heat flux. Due to the increase of the inlet liquid subcooling, leading to a large temperature difference, so the heat transfer coefficient is smaller at a high liquid subcooling. Several factors, such as mass flux, saturated pressure and liquid subcooling, have a great impact on pressure drop. With the increase of vapor quality, the shear action between the vapor-liquid phase is enhanced, and the pressure drop will gradually increase. Bubble behaviors have also been captured by a high-speed camera to help analyze the mechanism of jet impingement boiling heat transfer in a confined space for high-power chips. [ABSTRACT FROM AUTHOR]

• Numerical study on falling film flow and heat transfer on porous tubes. • Compared falling film flow and heat transfer on plain and porous tubes. • Studied porosity, layer thickness, and material effects on flow and heat transfer. • Porous layer leads lower flow, thinner film, fewer rupture, better heat transfer. • Smaller porosity, thicker layer, aluminum and copper facilitate heat transfer. Porous surfaces often exhibit exceptional performance in liquid spreading and heat transfer, making them promising candidates for enhancing falling film heat transfer on solid surfaces. This paper delves into the flow and heat transfer characteristics occurring within falling films on horizontal tubes coated with a porous layer. To achieve this, a three-dimensional numerical model that couples the Volume of Fluid (VOF) method with porous media considerations was employed. The investigation comprehensively covers film spreading behavior, flow patterns, film velocity, film thickness, temperature distribution, and heat transfer coefficients, for both plain and porous tubes. Furthermore, we delve into the effects of key parameters, such as porosity, layer thickness, and material properties, on the overall heat transfer process. The results demonstrate the accuracy of our model in reproducing film thickness and heat transfer coefficients within falling films on horizontal porous tubes. When comparing plain tubes to porous ones, some key differences emerge. Specifically, the liquid film on porous tubes spreads more slowly, leading to increased film thickness and fewer occurrences of dry patches. Additionally, porous tubes result in lower film velocity and a more uniform temperature field compared to plain tubes. Interestingly, the findings reveal that smaller porosities yield thicker films and higher heat transfer coefficients, particularly when the porosity is below 0.4. Moreover, thicker porous layers result in thinner films, higher wetting ratios, and increased heat transfer coefficients. Among the materials we tested, aluminum and copper porous layers exhibit the highest heat transfer performance, while steel demonstrates the lowest. [ABSTRACT FROM AUTHOR]

• The 3D hybrid surface with a tall HB pillar array surrounded by a short HP pillar array is proposed to enhance boiling heat transfer. • The 3D hybrid surface could enhance CHF and HTC simultaneously. • Compared with the plain SiO 2 surface, the 3D hybrid surface gave a 107% higher CHF and a 360% higher HTC. • The height difference between the HB and HP pillar arrays could mitigate liquid–vapor interference and increase the CHF. • The hydrodynamic instability limit corresponded closely with the experimental CHF values of the surfaces. Boiling heat transfer is widely used in numerous industrial systems because of its excellent heat dissipation performance. The critical heat flux (CHF) represents the upper limit of efficient heat removal. The most widely applied CHF model is Zuber's hydrodynamic instability model, which posits that the interference between vapor and liquid counterflows results in the CHF. In this paper, a three-dimensional (3D) hybrid micropillar array surface for hindering the liquid–vapor interface is proposed to enhance the CHF and heat transfer coefficient (HTC). The 3D hybrid surface comprises a tall hydrophobic (HB) pillar array surrounded by a short hydrophilic (HP) pillar array. The tall HB pillar array acts as a potential nucleation site for increasing the HTC, and the short HP pillar array acts as a wicking structure for facilitating liquid rewetting. In addition, the height difference between the HB and HP pillar arrays can potentially reduce the interference between liquid and vapor counterflows and increase the CHF. Relative to a plain silica surface, the 3D hybrid surface increases the CHF and HTC by 107% and 360%, respectively. Theoretical modeling suggests that the CHF on the 3D hybrid surface was still limited by pool hydrodynamics. [Display omitted] [ABSTRACT FROM AUTHOR]

• A revised self-induced jet impingement device is introduced in pool boiling using R1336mzz(Z). • Liquid-vapor separation promoted by the revised device is confirmed from the visualization results. • The revised device does nearly no effort on the nucleate boiling heat transfer coefficient until a relatively high heat flux. • Parametric investigations on the characteristics of the revised device are conducted. • The revised device gains up to 69.5 % and 36.7 % in the critical heat flux and the maximum nucleate boiling heat transfer coefficient, respectively. Pool boiling performance enhancement has attracted considerable interest on high power electronic cooling. In this paper, we introduce a self-induced jet impingement device for practical boiling enhancement, achieved by integrating the guidance tube with an orifice plate. With R1336mzz(Z) as the working fluid, we conduct visualization and parametric investigations on the pool boiling performance, considering the varying characteristics of this device. Visualization snapshots confirm the liquid-vapor separation and indicate that the two-phase flow pattern in the guidance tube eventually stabilizes as a churn-annular flow. Our findings suggest that self-induced liquid jet impingement has a minimal effect on the nucleate boiling heat transfer coefficient (h NB) under a certain heat flux, around 75 % CHF of the standard pool boiling, as the single-phase heat transfer is weakened due to the low thermal conductivity of R1336mzz(Z). As the heat flux intensifies, we observe substantial enhancements in both the Critical Heat Flux (CHF) and the Maximum Nucleate Boiling Heat transfer coefficient (h MNB) resulted from the delayed onset of boiling crisis, owing to the additional liquid supply and the promotion on vapor exhausting. The parametric studies reveal that the flow rate of the liquid jet, amplified by either extending the length or the inner diameter of the guidance tube, or by increasing the number of jet holes, positively impacts the boiling performance. However, these improvements show diminishing returns. Our study shows enhancements in CHF and h MNB of up to 69.5 % and 36.7 %, respectively, compared to the standard pool boiling. [ABSTRACT FROM AUTHOR]

• Deep learning (DL) was used to predict supercritical flow field and heat transfer with drastically changing thermo-physical properties. • The deep neural network with a Multilayer Perceptron (MLP) model was established. • The relative errors between the DL prediction and the CFD calculation were within 4 %. • The prediction by the DL model was of several orders (106) faster than the CFD simulation. • DL method provided an efficient tool for design and rapid control of the regenerative cooling system. Flow dynamics and heat transfer of supercritical hydrocarbon fuels have significant influences on the regeneratively cooled advanced aero-engines. In this paper, based on the principle of deep learning (DL), a deep neural network with a Multilayer Perceptron (MLP) model was developed to predict the flow field and heat transfer characteristics of supercritical hydrocarbon fuels. The network was trained by the calculation of Computation Fluid Dynamics (CFD). The analysis shows that the predicted values of bulk fuel temperature, fuel velocity and heat transfer coefficient by the trained DL model match well with the CFD simulations. The linear correlation coefficients (R) between the DL model predicted values and the CFD calculated results are all bigger than 0. 99. Most of the relative errors are lower than 2 %. The consumed time of the DL model is of several orders (106) less than the CFD simulation. The effects of inlet Reynolds numbers, pressure and heating rate on the flow field and heat transfer of supercritical hydrocarbon fuels inside the regenerative cooling channel have been successfully learned by the DL model. Therefore, the proposed deep learning model provides an efficient tool for the sensitivity analysis and design optimization of the regenerative cooling system. [ABSTRACT FROM AUTHOR]

• Application of porous materials to heat dissipation of LFR-PV system. • A new metal foam porous-wall mini-channel is proposed. • The MFP-channel increases the heat transfer coefficient by 35%. This paper takes the heat dissipation problem of linear Fresnel concentrating photovoltaic devices as the research background, this paper conducts an experimental investigation on the boiling flow characteristics of the dielectric liquid R141b, and discusses the enhancement effect of the electric field on the foam metal porous wall microchannel heat sink. In this paper, the effects of electric field strength (applied voltage) on wall temperature, heat transfer coefficient and critical heat flux (CHF) of mini-channel heat sink are investigated. The bubble departure diameter and incipience location in the mini-channel heat sink under different electric field strengths are recorded using a high-speed camera. Combined with the theory of electrohydrodynamic (EHD), the behavior of bubbles appearing in mini-channel heat sink under the action of electric field is analyzed. It is observed by visualization that the number of bubbles in the channel increases and the volume of bubbles becomes smaller under the electric field force. Research shows that when the voltage is 200V, the heat transfer coefficient of the mini-channel heat sink increases by a maximum of 178%, and the critical heat flux can be increased by a maximum of 123%. As the applied voltage increases, the wall temperature decreases. [ABSTRACT FROM AUTHOR]

• An adaptive heuristic fluid-solid interface condition is proposed for the first time. This new condition, called "X-R", has been developed to switch automatically between the Dirichlet-Robin (D-R) and Neumann-Robin (N-R) conditions on the basis of the Biot number. • A temporal interpolation of the fluid conditions for dealing with "non-coupling instants" is defined on the basis of a physics-based approach by naturally considering the flux-temperature linearity. This approach replaces the generally adopted time-dependent linearity, which cannot be justified. • The first test case demonstrates that the adaptive interface condition always produces smooth solutions, which is not the case when fixed conditions like D-R or N-R are adopted. • The second test case shows that the physics-base approach based on the flux-temperature linearity is by far the most satisfactory. However, this interpolation method remains valid only if each ramp defined initially remains in a linear domain. • Indeed, a full transient cycle, entirely based on the existence of particular time points can lead to non-linear problems, difficult to deal with. • As a direct consequence of the previous point and shown in the third test case, the initial problem can be not well-posed. In this case, adding a small number of temporal points can keep the coupling algorithm in a quasi-linear domain. This paper is devoted to the study of numerical methods used in the analysis of a thermal transient flight cycle. Two priorities are stressed. The first one concerns a stability issue, namely the fluid-solid interface conditions. The main properties of the Dirichlet-Robin and Neumann-Robin conditions are first recalled, before proposing for the first time an adaptive heuristic condition combining them. The second line of research is dedicated to an accuracy issue, the temporal interpolation of the fluid conditions for dealing with non-coupling instants. We propose here to adopt a physics-based approach by naturally considering the flux-temperature linearity. Then, using a simple numerical test under severe thermal boundary conditions, three test cases are proposed. The first one demonstrates that the adaptive interface condition always produces smooth solutions. The other two cases show that the physical interpolation approach is much more accurate than the one based on temporal linearity, using a costly but accurate reference solution built for the validation. Moreover, important improvements were made by adequately defining the integral contribution of the boundary condition using the Gauss points in the finite element solid code. It was also revealed that a relevant approximation of the heat transfer requires a sufficiently fine solid mesh at the interface. Finally, potential improvements are proposed at the end of this paper and more particularly two other interface conditions. One of them has the remarkable property of not using the convective heat transfer coefficient. [ABSTRACT FROM AUTHOR]

• The flow and heat transfer characteristics of single and multiple-synthetic jet impingement cooling is presented. • The equivalent Reynolds and Nusselt numbers were determined for the purpose of describing multiple orifice configuration. • The merging jet process is presented and the merging point is determined. In this paper, the comparison between synthetic jet actuator with single axisymmetric orifice and multiple axisymmetric orifices is presented. The aim of this paper is to compare the flow and heat transfer characteristics of single and multiple synthetic jet impingements cooling. The multiple orifice configurations had a central orifice and a satellite orifices placed in the pitch circle. The synthetic jet actuator had central orifice with 0, 3, 4, 5 and 6 orifices placed in the diameters of the relative pitch circle p / d = 2.5, 3 and 4. A total of 13 various geometric configurations were tested experimentally. In the multiple orifice configurations, the total cross-section area was approximately equal to the cross-section area of the single orifice configuration. The input power delivered to the synthetic jet actuator was maintained constant and equal to 3.00 W. The synthetic jet actuator energetic efficiency, Reynolds number, equivalent Reynolds number, radial and axial distributions of pressure coefficient, and heat transfer characteristic were presented. The synthetic jet issuing from multiple orifice plate merged at some axial distance, creating a single axisymmetric jet. The merging jet axial distances were determined for all investigated configurations. In the present paper, the integral quantities averaged over all orifices cross section area, or over impingement plate much larger than total orifices area, were analyzed for the purpose of objective comparison. From all various investigated parameters obtained from five different and independent experimental methods, the following conclusion may be made: if the total orifice cross section area in the case of single and multiple orifice synthetic jet is similar and real power deliver to the SJA is maintained at the same level the resulting equivalent Reynolds number and equivalent dimensionless stroke length achieved similar values and the impinging jet equivalent Nusselt number also achieved comparable values and distributions. [ABSTRACT FROM AUTHOR]

• This paper presents R290 (propane) and R1270 (propylene) local condensation heat transfer coefficients inside a BPHE. • The transition between gravity-dominated and forced convection condensation occurs at a refrigerant mass flux around 10 kg m−2s−1. • Nusselt (1916) analysis for vertical surface shows a fair agreement with the data in gravity-dominated condensation. • Longo et al. (2015) model predicts the data in forced convection condensation with a MAPE around 20%. • R290 and R1270 exhibit condensation heat transfer coefficients similar to those of traditional HFC refrigerants, such as R410A and R404A. This paper presents the local heat transfer coefficients of R290 (propane) and R1270 (propylene) condensation inside a Brazed Plate Heat Exchanger (BPHE). A new test section was designed for measuring the local heat transfer coefficient in refrigerant two-phase heat transfer in a BPHE. The test section includes two corrugated plates instrumented with 36 T-type thermocouples for measuring the local plate surface temperature, the local heat flux and the local heat transfer coefficient in 9 positions along the refrigerant channel. The experimental tests were carried out at a condensation temperature of 30°C, in the refrigerant mass flux range 5 − 17.5 kg m−2s−1 with an inlet vapour quality around 1.00 and an outlet vapour quality ranging from 0.13 to 0.17. A transition point between gravity-dominated and forced convection condensation was found for a refrigerant mass velocity around 10 kg m−2s−1. The experimental data was compared against theoretical models: Nusselt (1916) analysis for vertical surface shows a fair agreement with the data in gravity-dominated condensation, while Longo et al. (2015) model predicts the data in forced convection condensation with a Mean Absolute Percentage Error (MAPE) around 20%. The experimental data were further compared with some original measurements obtained with R404A and with data available for R410A. R290 and R1270 exhibit condensation heat transfer coefficients similar to those of traditional HFC refrigerants, such as R410A and R404A. [ABSTRACT FROM AUTHOR]

• The heat transfer process of NH 4 Cl fouling was studied experimentally. • The local heat transfer characteristics around tube wall was analyzed. • The effects of flow rates and temperature on heat transfer process were investigated. • The fouling resistance and average fouling thickness on the HET wall were obtained. The crystallization fouling of ammonium chloride (NH 4 Cl) often occurs on heat exchange tube bundles in petroleum refining equipment, which seriously reduces the heat transfer and operational efficiency of the equipment. In this paper, the heat transfer characteristics of heat exchange tube (HET) with NH 4 Cl crystallization fouling was studied experimentally. The effects of velocity, temperature and mass flow rate of hot and cold fluids on the overall and local heat transfer characteristics were analyzed. The results indicate that the local heat transfer coefficient around the tube wall decreases first and then increases with the increase of the circumferential angle (θ) under clean condition. At the position of 0°-45°, the local heat transfer coefficient is the largest. At 90°-135°, the local heat transfer coefficient is the smallest. Under fouling condition, the local heat transfer coefficient increases with the increase of circumferential angle. In the region of 0°-45°, the local heat transfer coefficient is the smallest. The region with the largest local heat transfer coefficient is 135°-180° The heat transfer coefficient (h), fouling resistance (R f) and average fouling thickness increase with the increase of Reynolds number (Re), temperature of hot fluid (T h) and mass flow rate of cold fluid (q m,c), but decrease with the increase of temperature of cold fluid (T c). [Display omitted] [ABSTRACT FROM AUTHOR]

• Synergistic enhancement spray cooling use micropillars and capillary-driven wicking hybrid structures. • The coolant can self-supplied reach the target surface under the capillary force. • The circulation heat transfer enhancement process of wicking-rewetting-vapor evacuation. • HTC and maximum heat flux were improved 79.6% and 80.0%, respectively on the hybrid structure surface. • A maximum reduction of approximately 30.8 °C was obtained for ONB. Spray cooling, which has a fast thermal response and low flow rate consumption, is a common and effective thermal management approach for high power electronic systems. Typically, to improve the heat transfer coefficient (HTC) for spray cooling, the target surface wettability is decreased, promoting nucleation boiling by reducing the surface energy barrier for bubble nucleation. However, the merging of bubbles and the formation of a vapor film have limitations in further increasing the critical heat flux (CHF). Hence, maximizing both the HTC and CHF of spray cooling can be mutually exclusive on the same heated surface. This paper presented a novel design for a vapor-liquid separation structure surface. It consisted of lower micropillars that serve as the vapor separation channel. On top of that, there was an upper multi-layer copper micromesh that wicks and spreads the coolant. Additionally, nano-grass attached to the micromesh to enhance capillary force and increase the nucleation sites simultaneously. Subsequently, we calculated the forward velocity and pressure drop of the droplets passing through the copper mesh to study the effects of droplet velocity and capillary force on heat transfer performance. The results show that the forward velocity of the droplets decreases rapidly as the increase of copper mesh layers and it drops to zero when passing through the third mesh. Therefore, the capillary force generated by the copper mesh is another important factor for the atomized droplets to pass through the copper mesh and reach the target surface. We have proven that all the vapor-liquid separation structure surfaces satisfy the condition ΔP cap > ΔP pore + ΔP local. Hence, the synergistic effects of the micropillar, copper micromesh, and nano-grass enable a circulation process of wicking-rewetting-vapor evacuation, which improves the onset of nucleate boiling (ONB), enhances the HTC, and increases the maximum heat flux dissipated by the system. Enhanced HTC (7.94 W/cm2/K) and maximum heat flux (817 W/cm2) were achieved at 1.50 ml/s on the vapor-liquid separation structure surfaces (#200 2-layer), representing an enhancement of 79.6% and 80.0% compared with that for the flat surface and the temperature of the heated surface at the ONB maximum decreased by approximately 30.8 °C. [ABSTRACT FROM AUTHOR]

Fluid transport through microvascular networks—a hallmark for homeostasis in living systems—has transcended to engineered materials, primarily made possible because of modern manufacturing advancements. Vascular-enabled multifunctionality, including thermal regulation and self-healing, holds great potential for extending the lifetime of structural materials and expanding the operational envelope. Prior studies on vascular-based active cooling use a "combined" heat transfer coefficient (HTC): a single parameter lumps convection and radiation effects. Although the resulting mathematical models are linear—an attractive feature for computational modeling, the combined coefficient approach may not be accurate or even applicable if the operating temperature is unknown, which is the case with many thermal regulation applications (e.g., space probes). In this paper, we illustrate the remarked limitations of the lumped approach and advocate the need to use a decoupled HTC by splitting convective and radiative heat transfer modes. We show the broad applicability of the proposed method by applying it to three material systems: glass and carbon fiber-reinforced polymer composites and an additive manufactured metal. We show, using numerical simulations, the differences in the predictions from the decoupled approach with that of the combined HTC; these differences are prominent at higher heat fluxes. Also, the decoupling has enabled us to establish a scaling law that allows transferring of solutions fields across material systems, strengthening further the validity and utility of our approach. This work's significance is two-fold. First , the research is fundamental, providing accurate measurement protocols for critical model parameters. Second , this work facilitates the development of mathematical models for vascular-based thermal regulation that are predictive even for hostile environments (which are often difficult to realize in laboratories), such as outer space. • Theoretically consistent heat transfer coefficient (HTC) measurements and analyses. • A heat transfer framework for improved prediction of microvascular active-cooling. • Multi-material experimental validation of a reduced-order model. • Self-similar scaling strategy to transfer solution fields across material systems. [ABSTRACT FROM AUTHOR]

• Subcooled flow boiling sequences in microchannels at high aspect ratios are explored. • Effects of aspect ratio, hydraulic diameter and subcooled degree on thermal- fluid characteristics are studied. • Increasing aspect ratio and inlet subcooled temperature realize better cooling performance. • A decrease in hydraulic diameter can augment thermal/frictional outcomes. • Channel with an aspect ratio of 5.0 achieve coefficient of performance up to 116277.8. With the fast progress of new technologies applied to cloud computing, artificial intelligence, and fifth generation communication, the researches on the heat dissipation of high-power electronic devices have concentrated on the two-phase flows in the microchannel, with the phase change used to achieve excellent cooling efficiency. The objective of this paper is to conduct the computational and experimental studies for investigating the thermal-fluid behaviors of subcooled flow boiling in microchannels. In the computational fluid dynamics (CFD) model, the numerical solver is built in ANSYS/Fluent® implementing the multiphase formulation with the full consideration of the effects of turbulence, surface tension, and phase change to appropriately imitate the interfacial movements between liquid water and vapor for probing the subcooled boiling and bubble nucleation processes over the microchannel. In the experimental investigation, a syringe pump and a programmable DC power supply is employed to regulate the heat flux and subcooled temperature for determining the heat transfer and pressure drop outcomes across the microchannel at varied aspect ratios and hydraulic diameters. In addition, this study sets up a high-speed camera with a LED fiber optical light source to acquire the close-up observation images over the complex flow boiling phenomena. The accuracy of CFD predictions is assessed by comparison against both the measured heat transfer coefficients and pressure drops as well as the photo-captured interfacial behaviors. The numerical simulations are then conducted to resolve the temperature gradients on the heating wall surface due to intense disturbances incited by the confined bubbly flow and sweeping flow at high aspect ratios, enhancing the cooling performance over the microchannels. The experimental measurements show that a reduction in inlet subcooling from 65 to 50 °C tends to enlarge the average heat transfer coefficient and pressure drop by 21.5 and 17.1%, respectively. The design impact study also reveals the average heat transfer coefficient and pressure drop at an aspect ratio of 5 greater than those at an aspect ratio of 2 by up to 41.1 and 27.2%, respectively. In contrast, increasing hydraulic diameter from 0.92 to 1.38 mm can strengthen the thermal and frictional outcomes by 17.2 and 20.3%. The microchannel design having an aspect ratio of 5.0 can realize the estimated coefficient of performance (COP) up to 116,277.8, achieving the satisfactory overall thermal and frictional outcomes. [ABSTRACT FROM AUTHOR]

• This paper presents R32 and R410A local condensation heat transfer coefficients inside a BPHE • The transition between gravity controlled and forced convection condensation occurs at a refrigerant mass flux around 20 kg m−2s−1. • Nusselt (1916) analysis for vertical surface predicts very well the data under gravity control • Longo et al. (2015) model shows a fair agreement with the forced convection condensation data This paper presents the local heat transfer coefficients of R32 and R410A complete condensation inside a Brazed Plate Heat Exchanger (BPHE). A new test section, with one channel on the refrigerant-side and two channels on the water-side, was specifically designed and manufactured for measuring the local heat transfer coefficient in refrigerant two-phase heat transfer in BPHE. The test section includes two thick corrugated plates instrumented with 36 copper-constantan thermocouples for measuring the plate surface temperature, the heat flux and the heat transfer coefficient in 9 different positions along the refrigerant channel. The experimental tests were carried out at a saturation temperature around 30°C, in the refrigerant mass velocity range 10 − 39 kg m−2s−1 with an inlet vapour quality around 1.00 and an outlet vapour quality ranging from 0.04 to 0.27. A transition point between gravity controlled and forced convection condensation was found for a refrigerant mass velocity around 20 kg m−2s−1. The experimental data was compared against theoretical models: Nusselt (1916) analysis for vertical surface predicts very well the data in gravity controlled condensation, while Longo et al. (2015) model shows a fair agreement with the forced convection condensation data. [ABSTRACT FROM AUTHOR]

• Bed-to-tube heat transfer in fluidized bed with mixed-size particles are simulated. • Average HTC increases nonlinearly with the proportion of small-size component. • Local HTC is improved by enlarging the size difference between components. • A modified heat transfer correlation for complex particle systems is proposed. Mixed-size particles with low processing cost have great potential to act as the absorbing media in the next-generation concentrated solar power plants. However, the research on the heat transfer from the particles to heated surfaces in gas–solid fluidized beds is limited. In this paper, the bed-to-tube heat transfer coefficient (HTC) in a shallow fluidized bed with immersed tubes is determined numerically. The Eulerian–Eulerian approach that incorporated the kinetic theory of granular flow has been implemented in a two-dimensional model. The distribution of the local HTC around the tube is investigated for various particle systems. The results show that the HTC reaches a maximum at the upper right (45°) and upper left (135°) of the tube, respectively, indicating a strong relation between the local HTC and solid volume fraction (SVF). The average HTC increases as the mean particle size gets smaller. For monosized particles, the HTC increases from 925 W/(m2·K) to 3033 W/(m2·K) as the particle size decreases from 300 to 100 µm. For binary particles, the growth in the HTC is nonlinear, with an increase in growth rate being observed as the small-size component becomes dominant. For mixed-size particles with the same mean size, a higher HTC can be achieved by enlarging the size difference between the components. A modified correlation for predicting the bed-to-tube HTC is established. Besides, the impacts of gas velocity and temperature, and heat radiation on the bed-to-tube HTC are discussed. It is found that an optimal gas velocity exists to maximize the average HTC and SVF simultaneously, while the gas temperature has a minor contribution. Due to the small heat transfer temperature difference, the effect of thermal radiation on the HTC can be neglected even though the bed temperature reaches 800 °C. [ABSTRACT FROM AUTHOR]

• The end-state isobar depending on the force field is introduced. • The evolution of PCM temperature field is predicted using the end-state isobar. • The negative force field can significantly improve the thermal performance of PCM. To make a better application of phase change material (PCM) in the thermal management of aerospace systems, this paper investigates effects of the variation of foam-metal pore structure on the thermal performance of composite PCM (CPCM) subjected to different force fields. A two-dimensional numerical model based on the enthalpy-porosity method and the two-temperature energy equation is developed and verified with the experimental data. The thermal behavior of CPCM with different porosities (ω = 20 PPI, ε = 0.91, 0.93, 0.95 and 0.97) and different pore densities (ε = 0.95, ω = 10, 20, 40 and 80 PPI) are investigated under centrifugal acceleration conditions of 0, 5 and -5 g, respectively. A novel index of "end-state isobars" is introduced to describe the melting process of CPCM. The results show that the isotherms of CPCM liquid phase evolve gradually towards the interface parallel to end-state isobars in all conditions. At 0 g, decreasing the porosity effectively improves the thermal conduction of CPCM and slows down the approaching of the isotherms to the end-state isobars. Similarly, increasing the pore density significantly weakens the natural convection of liquid PCM and can also slows down the approaching isotherms to the end-state isobars. At 5 g, the isotherms can match exactly with the end-state isobars, after which the heat transfer process mainly depends on conductive. And decreasing the porosity can significantly improve the thermal performance of CPCM, while adjusting the pore density has almost no effect. At -5 g, the natural convection strength of CPCM is enhanced, changing porosity has limited effect on the heat transfer performance of CPCM, while decreasing pore density can significantly enhance the heat transfer performance. The results may provide some valuable reference for the evaluation and design of latent heat thermal energy storage (LHTES) system suitable to the aerospace field. [ABSTRACT FROM AUTHOR]

• Boiling heat transfer is significantly increased via surface functionalization. • Boiling heat transfer at the functionalized surface is understood via MD analysis. • High ITC and FGs-water interactions collaboratively boost boiling heat transfer. • High ITC stems from strong bonding and strong vibrational coupling effects. • Hydrogen bonds benefit energy exchange between FGs and water molecules. Liquid-vapor phase change heat transfer, i.e., boiling heat transfer, has been demonstrated to be an effective thermal management strategy for high-power electronics and power generators. Improving the boiling heat transfer performance can largely increase the energy conversion efficiency in these corresponding devices. In this paper, we demonstrate that the boiling heat transfer coefficient (HTC) at functionalized Au surfaces can be improved 3.3 times at most by introducing functionalization groups (FGs) to Au surfaces. Our molecular dynamics simulations show that the increase of HTC is resulting from the high thermal conductance across functionalized Au/water interfaces and the strong interaction between FGs and water molecules. The high interfacial thermal conductance of functionalized Au/water interfaces stems from the strong bonding between the functionalized Au surface and water, and the strong vibrational coupling at 0∼4 THz between Au and FGs. The strong interaction between FGs and water molecules comes from their mutual adhesions including van der Waals and electrostatic interactions, and bridging effects. For the FGs without electronegative atoms (e.g., -CH 3 FGs), FGs will facilitate thermal energy transfer via their van der Waals interactions with water molecules. When FGs with electronegative atoms (e.g., -CF 3 , -OH, and -COOH FGs) are introduced, hydrogen bonds will form due to their electrostatic interactions which benefit the thermal energy exchange between FGs and water molecules. Meanwhile, the vibrations for the FGs with -CF 2 and -CF 3 terminal groups are coupled with Au at low frequencies of 0∼4 THz and water molecules at middle frequencies of 6∼8 THz. The FGs can then bridge the thermal energy from Au to water through the dual vibrational couplings. The Au-CF 2 (COOH) FGs surface, which is designed to include all the mechanisms mentioned above, can therefore increase the boiling HTC 3.3 times compared to that of the planar Au surface. Our results here provide insights into the design of surfaces with high boiling heat transfer performances using chemical FGs. [ABSTRACT FROM AUTHOR]

• Heat transfer coefficients were identify using experimental data • Radiative effects have been included and analyzed in the identification methodology • Thermocouple position and initial temperature impact the coefficient values This paper is devoted to the identification of the heat transfer coefficient between a high temperature glass and a cast iron punch taking radiative effect into account. A 1D numerical thermal model taking the radiative effects into account has been developed to simulate the contact between a punch and a glass cylinder. It was coupled with an inverse identification methodology to determine the heat transfer coefficient using experimental data carried out in the LAMIH. First, the sensitivity of the identification of the heat transfer coefficient with respect to different parameters (initial glass temperature, thermocouple position, radiation) will be introduced. A second part will show the identification of heat transfer coefficient using experimental data taking into account radiative effects. [ABSTRACT FROM AUTHOR]

• This paper explores the evaporative heat transfer of falling spray films over the horizontal circular tube array. • Cooling outcomes of spray films on tubes is experimentally and numerically studied. • Effects of water flow rate, tube location and upward air velocity on thermal performance are examined. • Counter current airflows thicken liquid layers with deferral of spray film development. • An upward velocity of 2.0 m/s can enhance average heat transfer coefficients of 14.3%. The spray film evaporative cooling equipment is useful to circumvent the scaling with expanded liquid coverage for attaining good thermal performance and energy efficiency. This paper examines the evaporative heat transfer phenomena of falling spray films over the horizontal circular tube array. In the experimental study, a high-speed camera with a LED fiber optic light source is implemented to capture the optical images for illuminating the distributions of spray films over the tube array under the counter current airflows. The average wall temperature, outlet temperature of spray nozzle and heat flux regulated by a programmable DC power supply are measured to determine the heat transfer coefficient. Moreover, the computational simulation is conducted using the CFD software ANSYS/ Fluent® in conjunction with a user defined function (UDF) to investigate the evaporative heat transfer of liquid spray films over the tubes. The calculated liquid film thicknesses and average heat transfer coefficients around the upper and lower tubes agree reasonably with the measured data to validate the numerical model. To explore the development of falling spray film dispersion and thermal performance over the tubes, the measurements and simulations are then extended to examine the characteristics of film thickness and heat transfer of splashed water sprays on the tube array at varying water flow rates of 3–7 l/min, heat fluxes of 10–50 kW/m2, tube placement positions of 0–142.5 mm (i.e. w1-w6) and counter current airflow velocities of 0.8–2.0 m/s, respectively. In measurements, the average heat transfer coefficients of four positions for the tubes at an upward airflow velocity of 2 m/s can reach 5.0 kW/m2 K for the water flow rate of 5 l/min and a heat flux of 30 kW/m2K. [ABSTRACT FROM AUTHOR]

• Heat transfer coefficient and pressure gradient of R1233zd(E) and R1336mzz(Z) measured in a 0.643 mm microchannel tube. • Flow pattern captured at exit of each data point and related to the heat transfer and flowing behavior of the two fluids. • Result compared to existing correlations and the best predictive correlations are recommended. This paper presents the heat transfer coefficient, pressure gradient, and flow patterns of R1233zd(E) and R1336mzz(Z) in a microchannel tube. Both heat transfer coefficient and pressure gradient are measured simultaneously and presented in the same plot in this paper. The experiment was conducted on a 24-port microchannel tube with an average hydraulic diameter of 0.643 mm. Experiment covers mass flux from 100 to 200 kg- m −2s−1, heat flux from 0 to 6 kW- m −2, vapor quality from 0 to 1. The saturation temperature of R1233zd(E) is from 30 to 50 °C, and R1336mzz(Z) is from 40 to 60 °C. The pressure gradient is a strong function of mass flux and saturation temperature (properties). Heat flux has an insignificant effect on the pressure gradient because of low accelerating pressure drop. The heat transfer coefficient is strongly affected by mass flux, heat flux, and saturation temperature. When mass flux is higher, the flow has more waves and turbulence, and the HTC and dP/dz are higher. R1233zd(E) and R1336mzz(Z) have much smaller HTC and higher dP/dz than R1234yf and R1234ze(E) under the same condition. R1234yf has almost the same HTC as R1234ze(E) at high quality (x >0.5), but higher at low quality (x <0.5). Hwang and Kim (2006) is recommended for predicting pressure gradient of the two low pressure fluids (R1233zd(E): MAE=14.1%, ME=-3.7%, Dev=16.8%; R1336mzz(Z): MAE=16.7%, ME=-2.0%, Dev=22.0%). Liu and Winterton (1991) is recommended for predicting HTC of R1336mzz(Z) (MAE=15.2%, ME=-4.6%, Dev=17.9%). None model has good agreement with the measured HTC of R1233zd(E). [ABSTRACT FROM AUTHOR]

• The effect of continuous pressure waves on heat transfer of pipe bundle was studied. • Continuous pressure waves were stimulated by vibrating membrane in CFD models. • A larger affected space was observed in the presence of continuous pressure waves. • The heat transfer is enhanced by 7.01% to 156.22% at the frequency of 20–160 Hz. • Continuous pressure waves have the great potential in enhancing the heat transfer. This paper presents a numerical study of the heat transfer enhancement outside the pipe bundle under the effect of continuous pressure waves (PW) stimulated by a vibrating membrane as a PW generator. Model 1 and 2 made of one and two rows of pipe bundles were investigated, respectively. An oscillating wall boundary condition was applied to the vibrating membrane. Based on the RNG k-ε turbulent model and full cavitation model, the computational fluid dynamics (CFD) model was developed. The prediction results of the model were in good agreement with the experimental data of other researchers. The validity of PW was investigated from the viewpoint of the affected scale by comparing the flow and heat transfer with ultrasound and PW. The effect of the frequency and amplitude of the membrane on the heat transfer enhancement was analyzed. The result shows that the continuous PW had significantly larger affected scale than ultrasonic waves, which can cover the whole pipe bundle space and increase the velocity amplitude of the fluid around the bundle. While the affected scale of ultrasonic waves was limited to the center place of the pipe bundle because of the acoustic stream. In addition, the effect of continuous PW on the convective heat transfer increased with increasing the frequency and amplitude of the membrane. The heat transfer was enhanced by 7.01% to 156.22% at a frequency of 20–160 Hz and an amplitude of 2 mm. Considering the benefits and the energy consumed of PW, the optimal frequencies of models 1 and 2 were 110 and 120 Hz, and the heat transfer coefficient were increased by 119.19% and 150.36% compared to the steady-state flow. The results confirm continuous PW has the great potential in enhancing the heat transfer of large-space pipe bundles. [ABSTRACT FROM AUTHOR]

• We propose an optimized structure of spiral finned tube by using delta winglet on the surface of fins to enhance the heat transfer. • The influence of delta winglet on the performance of finned tubes was studied and analyses by the assessment parameter - Nusselt number, fanning friction factor and comprehensive performance parameter. • The effect of the position and number of winglets on performance of spiral finned tubes and the mechanism of the effect are further studied. • A scheme of combining orthogonal experimental design and multivariate linear fitting is proposed to simplify the number of tests and obtain the performance correlation of optimized spiral finned tubes. • Through the error analysis and comparison with the literatures, it is found that the new correlation proposed in this paper has satisfactory accuracy of heat transfer performance. Spiral finned tube (SFT) has been widely used in various applications, including heat exchangers in power plants and metallurgy, energy and chemical industries because of their compact design and ease of production. This study aims to enhance and optimise heat transfer and decrease the flow resistance of SFTs in heat exchangers, which could help reduce manufacturing costs and fuel consumption. In this study, a mathematical model of heat transfer coupling with the fluid and solid components of SFTs is established, and the flow field of SFTs is experimentally verified by particle image velocimetry. To improve the heat transfer over the air side, triangular winglets are built by stacking evenly spaced on the surface of SFTs. The heat and flow characteristics are numerically computed using large-eddy simulation in the same way as the experimental model, and these SFTs are compared with ordinary spiral fin tubes (OSF) at Reynolds numbers ranging from 500 to 9000. The influence of the location and deployment of the triangular winglets is also investigated for performance improvement. On this basis, the heat transfer and resistance of sixteen different SFTs are calculated using an orthogonal experimental design method, In addition, the correlations between the Nusselt (N u) number and fanning friction coefficient (f) of an optimised model are presented. The results show that SFTs with triangular winglets exhibit significant performance compared with OSFs. The heat transfer coefficient also improves as the number of triangular winglets increases. The optimised model exhibits good accuracy above 95% in terms of the correlation between heat transfer and fanning frictional coefficient. [ABSTRACT FROM AUTHOR]

• New database for flow boiling of R1233zd(E) for moderate and high reduced pressures. • New database for pressure drop of R1233zd(E) for moderate and high reduced pressures. • Reduced pressure considered in the experiment ranged from 0.2 to 0.7. • Enhanced in-house heat transfer model. The paper presents the results of experimental study of flow boiling of R1233zd(E) in minichannel at moderate and high saturation temperatures. The heat transfer coefficient and pressure drop were measured for flow boiling in a stainless steel tube with 2 mm inside diameter and a length of 300 mm. Experiment has been conducted for saturation temperatures ranging from 83 to 145 °C which corresponds to values of reduced pressures ranging from 0.2 to 0.7. Three different heat fluxes were analyzed, namely 20, 30 and 45 kW/m2 and mass velocities ranged from 200 to 1000 kg/m2s. The effects of reduced pressure, mass velocity, heat flux, vapor quality and flow regime were analyzed. At low reduced pressures (p r ≤ 0.3) heat transfer is driven by convective boiling mostly with nucleate boiling having effect only in the case of low mass velocities. Heat transfer in this case increases with mass velocity and vapor quality and is independent of heat flux. For reduced pressures of 0.4 and 0.5 both nucleate and convective boiling are present, however the nucleate boiling starts to be more dominant here, especially for low mass velocities and high heat fluxes. For low mass velocities and/or higher heat fluxes the heat transfer decrease or is independent of vapor quality, whereas for higher mass fluxes and/or lower heat fluxes it is initially independent of vapor quality and starts to increase close to the transition from intermittent to annular flow. At the range of high reduced pressures (p r ≥ 0.6) nucleate boiling is the dominant heat transfer mechanism. Experimental results were compared with some well-known two-phase flow correlations. [ABSTRACT FROM AUTHOR]

• The water heat transfer coefficients for 18 different OSFs are tested under a constant heat flux. • The high and close-spaced OSFs with large A ffe / t 2 are unsuitable for the water heat transfer. • A j correlation of water for different OSFs, which provides the water results that fall within the 2 σ confidence interval with a standard deviation of 9.63%, is developed. This paper experimentally investigates the heat transfer behavior of water in offset strip fin (OSF) channels. 18 different OSF geometries, which are divided into three groups by the ratio of the front-fin-end area of a single cell to fin thickness squared (A ffe / t 2), are tested under a constant heat flux. The results show that when the A ffe / t 2 is large, the high and close-spaced OSFs with small fin length are unsuitable for water heat transfer, while high fin density, small fin height and small fin length can improve the water heat transfer performance for the OSFs with small A ffe / t 2. Further analysis indicates that there is a negative relationship between the j factor and Prandtl number, as well as between the j factor and fin efficiency. The comparison between the data for water and air reveals that the large difference between the heat transfer coefficient and the heat transmittance coefficient due to the low fin efficiency of water heat transfer makes it difficult to extend air data to water. Moreover, a j correlation of water for the different OSFs is developed based on the regression analysis. The correlation provides the water j factors that fall within the 2 σ confidence interval with a standard deviation of 9.63%. The present study not only provides useful data for the design and development of OSF heat exchangers, but also offers strong support for a deeper understanding of liquid heat transfer behavior in OSF channels. [ABSTRACT FROM AUTHOR]

• The steam-water mixture layer and interfacial vortexes are captured. • Local heat transfer coefficient calculated model is developed with entrainment theory. • The maximum local heat transfer coefficient locates at tail of the steam cavity. • Averaged local heat transfer coefficient is about the half of that with inner interfacial area. Steam-water interface and mixture layer are the key window for understanding the heat and mass transfer mechanism of steam submerged jet condensation. In this paper, the steam-water two-phase mixture layer is captured and the interfacial vortex is observed with a quasi-two-dimensional submerged steam jet visualization experimental platform. Then, the volume heat transfer capacity of steam-water mixture layer and heat transfer coefficient with steam-water inner interface are obtained. Furthermore, a new turbulent eddy entrainment model is developed to calculate the local entrainment mass rate and local condensation heat transfer coefficient. Both the local entrainment mass rate and local condensation heat transfer coefficient increases along steam jet direction, and reaches the maximum value at the tail of the mixture layer. The calculated condensation heat transfer coefficient ranges from 0.88 to 2.94 MW· m −2· K −1, which is about the half of the value of hypothetical heat transfer coefficient with inner interfacial area. [ABSTRACT FROM AUTHOR]

• The in-line high shear mixer (HSM) was investigated as a liquid-liquid direct contact heat exchanger for the first time, using kerosene and thermal water as the heat transfer fluids. • The effects of operating parameters and structural parameters of the high shear mixer on the volume heat transfer coefficient and heat transfer efficiency were systematically studied by experiments. • The maximum volume heat transfer coefficient and heat transfer efficiency can be up to 2522.8 kW·m−3·°C−1 and 92.4% with the optimal structure. • The dimensionless correlations were established respectively for the volume heat transfer coefficient and heat transfer efficiency of HSM, which can be adopted to guide the operating mode and structural design of high-efficiency HSM heat exchanger with low-energy consumption. As an indispensable unit equipment in industrial production, the design of efficient heat exchangers is of great significance to improve energy utilization rate. In this paper, the in-line high shear mixer (HSM) was investigated as a heat exchanger for the first time by experiments and assisted with CFD simulations, and the heat transfer performance was measured using kerosene and thermal water as the heat transfer fluids. Effects of operational parameters, including rotor speed, fluids temperature and flow rate, as well as the structural parameters, involving the rotor teeth number, teeth height, and the shear gap, were studied on the volume heat transfer coefficient and the heat transfer efficiency of such inline HSM. The results indicated that the volume heat transfer coefficient can be significantly improved by increasing the rotor speed and the flow rate of the heat transfer fluids, but reducing the heat transfer temperature difference, whereas increasing the thermal fluid flow rate deteriorates heat transfer efficiency. Furthermore, the structural parameters including rotor teeth number and teeth height, the shear gap within the stator and rotor were optimized to enhance the heat transfer of HSM. The optimal HSM possesses a shear gap of 0.5 mm, a teeth height of 10 mm and a teeth number of 6, which can generate larger turbulent energy dissipation rate and higher shear rate, meanwhile make more fluids jet out of the shear gap and stator hole. The maximum volume heat transfer coefficient and heat transfer efficiency can be up to 2522.8 kW·m−3·°C−1 and 92.4% with the optimal structure. Finally, the dimensionless correlations of heat transfer performance were established to guide the operating mode and structural design of high-efficiency high shear mixer. [Display omitted] [ABSTRACT FROM AUTHOR]

• A new numerical model was established to simulate pyrolytic hydrocarbon fuel. • Heat fluxes and heat transfer coefficients on interior channel walls was analyzed. • The 3-D phenomenon of reacting flow affects fuel heat chemical heat absorption. • Regulation methods were proposed to improve heat transfer performance. Endothermic hydrocarbon fuel cooling in hypersonic vehicles has recently drawn significant attention. There exists significant 3-D phenomenon of the supercritical reaction flow which exerts great influences on heat transfer process. In this paper, an efficient multi-dimensional numerical model is established based on OpenFOAM to investigate the steady heat transfer of supercritical hydrocarbon fuel with pyrolysis in mini-channels. Numerical tests indicate that the present numerical model is able to reach better convergence and offer a higher computational efficiency than transient models for its simplicity. The numerical results reveal that there exists an unbalanced heat flux distribution and a conspicuous difference of heat transfer coefficients among interior walls in the square channel. Violent near-wall pyrolysis and resulting strong thermophysical property variations, reduced turbulent intensity in the lower part of the core flow, and bulk fluid reaction play important roles in the heat transfer performance. The 3-D phenomena of the reacting flow also make also significant impact on heat transfer. Based on the analyses above, two methods are proposed to mitigate heat concentration by enhancing fin effects of solid walls: one is to increase solid thermal conductivity, and the other is to change the square shape of cross-section to inverted trapezoid. This work helps raise awareness of the flow behavior inside asymmetrically heated channels and provides useful information for the engineering design of active regenerative cooling systems. [ABSTRACT FROM AUTHOR]

• Surface characteristics of copper and aluminium microchannels are different. • The flow boiling patterns observed in Cu and Al microchannels were similar, i.e. bubbly, slug, churn and annular flow. • Flow reversal was observed. • Flow boiling heat transfer coefficient in Al microchannels was higher by 12% average. • Pressure drop in Al channels was higher but overall pumping power is low in both Cu and Al microchannels. • Overall thermal performance of Cu and Al microchannels was similar. • Flow boiling in both Al and Cu microchannels is a viable option in cooling high heat flux devices. The substrate material and active side characteristics can affect the bubble ebullition cycle and consequently the heat transfer rate and pressure drop in microchannel evaporators. This paper presents an experimental study on flow boiling patterns, heat transfer rates and pressure drop in multi-microchannels evaporators made of copper and aluminium. HFE-7100 was used as the test fluid at atmospheric pressure, 5 K inlet sub-cooling, mass flux of 50-250 kg/m2s and wall heat flux up to 174 kW/m2. All heat sinks were made with channel width 0.46 mm, channel height 0.46 mm, giving a 0.46 mm channel hydraulic diameter. The heat sink base area was 25 mm in length and 20 mm in width. The experimental results showed that similar flow patterns were visualised for copper and aluminium namely bubbly, slug, churn and annular flow. The heat transfer coefficient in the aluminium heat sink was 12% (average value) higher than that found in the copper heat sink. The measured pressure drop in the aluminium heat sink was 28% (average value) higher compared to the copper heat sink. However, the additional pumping power required to move the fluid through the heat exchanger is small for this factor to be significant. The SEM images of the surface revealed that the number of cavities (possible nucleation sites) was higher in the aluminium surface with clear-cutting marks compared to the copper surface. This may explain the different pressure drop and heat transfer behaviour. The results of the present study indicate that aluminium heat sinks can offer comparable thermal performance to that of copper heat sinks and can also be recommended for cooling high heat flux systems. [ABSTRACT FROM AUTHOR]

• An experimental investigation of flow boiling on micro-pin-fined surfaces in a horizontal narrow microchannel was performed. • The CHF and HTC were enhanced up to 51% and 240%, respectively. • The bubble behaviors were captured to explain the heat transfer enhancement mechanism. • A new correlation was proposed to predict flow boiling HTC in the microchannel by considering the micro-pin-fins. In this paper, an experimental investigation of flow boiling in a horizontal narrow microchannel with hydraulic diameter of D h = 952 μm was conducted. Flow boiling heat transfer characteristics of micro-pin-finned surfaces were evaluated over the mass fluxes range from 200 - 500 kg/m2s (corresponding flow velocity: 0.2 - 0.5 m/s), using deionized water as the working fluid with different inlet temperatures T in = 30 - 50 °C. The outlet pressure of the microchannel was about 1 atm. The boiling curves, heat transfer coefficients, bubble behaviors and heat transfer enhancement mechanism were discussed with the variation of heat flux, mass flux and inlet temperature. The results indicated that the micro-pin-finned surfaces could improve the critical heat flux (CHF) and heat transfer coefficient (HTC) greatly due to numerous nucleate sites and large heat transfer area enhancement ratio. The CHF up to 360 W/cm2 has been demonstrated at a mass flux of 500 kg/m2s. Compared with a smooth surface, a 240% higher heat transfer coefficient (18.4 W/cm2K) on the S30-120 surface has been achieved at a mass flux of 500 kg/m2s with an inlet temperature of 30 °C. The boiling curves of the micro-pin-finned surfaces presented an obvious "hook back" phenomenon after reaching the onset of the nucleate boiling (ONB) and then the wall temperature had a slight rise with the increase of the heat flux. Moreover, the intensive micro-pin-finned arrangements showed a significant wicking effect and promoted liquid replenishment, which could destroy the liquid boundary and enhance turbulence. The bubble behaviors were also captured to explain the heat transfer enhancement mechanism in a horizontal narrow microchannel. [ABSTRACT FROM AUTHOR]

• Replication is used to fabricate open-cell aluminum foams with bimodal cell size. • Coarse (C – 7 mm) and fine (F – 1 mm) cells are combined in different proportions. • The foam containing 67%C-33%F (C67F33) has a remarkable total porosity of 76%. • C67F33 exhibits the highest heat transfer coefficient and the lowest pressure drop. • C67F33 shows excellent thermal efficiency and great potential in thermal management. Recent advances in cellular materials for active thermal management applications involve the integral design of their porous structure. In this work, open-cell aluminum foams with porosities in the range of 0.60–0.76 are developed, containing pores with spherical geometry and a bimodal pore size distribution. The fabrication strategy involves the use of the replication method by infiltration with liquid aluminum of porous preforms formed by packing NaCl spheres of two largely different sizes (7 mm and 1 mm average diameter), which are then removed by water dissolution. The paper describes cellular materials with different proportions of coarse and fine pores. Their detailed characterization by pore volume fraction, pressure drop, permeability, thermal conductivity and heat transfer coefficient is supported by analytical schemes. In addition, these materials were characterized under realistic operating conditions using computational fluid dynamics simulations. The material with 67 percent coarse pores and 33 percent fine pores, which has the highest porosity (and thus permeability), has the most appealing heat dissipation properties with the lowest pressure drops, making it an excellent candidate for emerging thermal management applications in electronic systems. [Display omitted] [ABSTRACT FROM AUTHOR]

• Two-dimensional heat transfer coefficient maps presented for obliquely impinging jet. • Effect of jet-to-target spacing (z / d), and jet Reynolds number (Re) studied. • z/d had a dominant effect on local heat transfer characteristics. • The upstream shift of the primary peak heat transfer moves closer to the geometric center with increasing z/d. This paper presents an experimental investigation on the primary and secondary peak heat transfer characteristics of an obliquely (45°) impinging jet on a flat and smooth target surface. Local convective heat transfer coefficients have been calculated through a transient liquid crystal thermography technique employing one-dimensional semi-infinite conduction model for the target surface. The effects of jet Reynolds number (Re) and jet-to-target spacings (z/d) have been investigated where Re was varied from 9,000 to 18,000 and z/d was varied from 2.5 to 6. Local heat transfer measurements reveal several important features of oblique jet impingement, e.g., the location of the primary and secondary peaks with respect to the geometric center (GC), its dependence on Re and z/d, and the shape of enhanced heat transfer zone with stagnation, separation, and reattachment features. The two-dimensional heat transfer map is presented in a normalized form Nu/Nu max , where Nu max is the primary peak heat transfer. It has been found that for a certain z/d, the primary peak occurs at a fixed distance from the geometric center for the range of Re investigated, while the secondary peak location varies with Re. For the smallest z/d, the primary peak heat transfer location was offset by one jet diameter from the GC in the uphill direction, whereas for the moderate z/ d = 4, the primary peak occurred near the GC. For the highest z/d, the primary peak was again in the vicinity of GC, however, slight offset on the downhill side. The spent flow reattachment (post the first impingement) which also determined the secondary peak location and heat transfer level was strongly dependent upon the Re. The oblique jet impingement radial variation is also compared against 90° (orthogonal) jet impingement at z/ d = 2.5 and 6 at Re = 18,000. It has been observed that the primary peak for above two z/d configurations were sharper for the 45° case when compared to 90°, and the secondary peak was further offset in the downhill direction with higher heat transfer levels for the 45° impingement case compared to 90°. [ABSTRACT FROM AUTHOR]

• Visualizations of flow patterns of two-phase flow in smooth and hierarchical tube. • Measurements of heat transfer coefficients of two-phase flow in smooth and hierarchical tube. • Flow boiling correlation in three-dimensional hierarchical tube. • Flow condensation correlation in three-dimensional hierarchical tube. Development of novel enhanced surface tubes is an important way to advance two-phase flow heat transfer. To make correlations of two-phase flow heat transfer in enhanced tubes with acceptable prediction has been a challenge for many years. The paper presents a method: two-phase heat transfer correlations in enhanced tubes are directly developed through existing two-phase heat transfer smooth-tube models by introducing several modifying factors and dimensionless numbers. An enhanced heat transfer tube containing staggered arrays of three-dimensional dimples and secondary petal-shaped protrusions (EHT) is one of the most complicated internal surface-tube configurations on the market. Visualizations of flow patterns and measurements of heat transfer coefficients of two-phase flow using refrigerant R410A in smooth and EHT tube are conducted. Detailed visualization experiments during two-phase flows show good agreements between flow regimes and flow-pattern maps in smooth and EHT tube. The equation of continuity is adopted to provide understanding of the mechanism of the agreements. Both theoretical analysis and experimental studies support the method. As a result of using it, new two-phase heat transfer correlations are developed in enhanced tubes as the EHT tube is an example. Flow boiling correlation can predict data points within +10% to -20% and flow condensation correlation can predict data points within ±10% in the EHT tube. [ABSTRACT FROM AUTHOR]

• Heat transfer was studied during gas flow in a heated quasi-triangular tube. • Comparison of the flow of helium-xenon mixture and air. • The distribution of the tube wall temperature is shown. • A strong non-uniformity of the heat transfer coefficient along the tube was obtained. • A method is proposed for calculating local heat transfer coefficients for unsteady flow. The paper considers experimentally the process of heat transfer during the flow of gas mixtures in a heated thin-walled quasi-triangular tube. The cross-section of the tube corresponds to the shape of the channel formed by the dense packing of the reactor rods for generating heat at a power plant. The working fluids were a helium-xenon gas mixture with a Prandtl number of 0.23 as a promising coolant for a nuclear installation generating electricity, and air with a Prandtl number of 0.71. Temperature distributions of the heated wall are obtained in a wide range of flow velocities, and the effect of laminar-turbulent transition at the initial section of the tube and high-velocity turbulent flow at the outlet on these distributions is analyzed. An approach for calculating local heat transfer coefficients for an unsteady flow, based on determining the local velocities of the coolant in the channel, is proposed. Significant non-uniformity of the heat transfer coefficient along the channel length was obtained with a decrease in its value at the inlet area and an increase towards the outlet due to flow acceleration. An increase in the heat transfer coefficient during the flow of helium-xenon mixture is more intense than in the air flow. It is shown that this circumstance should be taken into account when designing compact power plants. [ABSTRACT FROM AUTHOR]

• The effects of non-wetting fraction and pitch distance of adjacent dots on flow boiling heat transfer and pressure drop are characterized. • Bubbles coalesced more easily, and flow patterns transited faster in the microchannel with smaller pitch distance. • HTC, CHF and pressure drop are found to significantly rely on the pitch distance of dots and the mass flux. • Bubble detachment diameters are predicted in hydrophilic, hydrophobic, and wettability-patterned microchannels, respectively. Flow boiling in microchannels offers a promising and attractive solution for thermal management of electronic devices and power systems. In this paper, microchannels composed of a hydrophilic surface with hydrophobic dots were studied to characterize the effects of non-wetting fraction and pitch distance of adjacent dots on flow boiling heat transfer and pressure drop. The pitch distances ranging from 122 µm to 172 µm were studied. Using deionized water as the working fluid, highly subcooled flow boiling experiments were conducted at different mass fluxes ranging from 41.1 to 246.6 kg/m2s over a heat flux up to 146.2 W/cm 2. Bubble dynamics and flow patterns were visualized using a high-speed camera. It was found that bubbles coalesced more easily, and flow patterns transited faster in the microchannel with smaller pitch distance. Heat transfer coefficient (HTC), critical heat flux (CHF) and pressure drop were found to significantly rely on the pitch distance of dots and the mass flux. Furthermore, based on a force-balance model, bubble detached diameters were predicted in hydrophilic, hydrophobic and wettability-patterned microchannels, respectively. This provides a useful insight to optimize the wettability pattern design and then improve flow boiling heat transfer in a microchannel. [ABSTRACT FROM AUTHOR]

• A manifold immersion cooling scheme for LIB thermal management is developed. • Influence of structural parameters are numerically studied for cooling optimization. • Manifold system can adapt its cooling ability for lateral surfaces of batteries. • This scheme achieves high cooling performance with extremely low pressure drop. Thermal management is very important because lithium-ion batteries generate a lot of heat under high rate charging and discharging. Inspired by the manifold microchannel (MMC) structure for chip cooling, this paper proposes a new manifold immersion (MI) cooling structure used for battery thermal management. The optimization analysis is performed in numerical simulation models. The local convective heat transfer coefficients of the lateral and baffle surfaces are analyzed separately. The results show that unlike the MMC for chip cooling, MI cooling structure's demands high heat transfer in the lateral surface of batteries but not the bottom or baffle surface. Subsequently, the influence of the design parameters on the cooling capacity is analyzed. The results show that the manifold channel length L ch,m and the battery spacing channel width W ch greatly influence the MI cooling performance. Stable wall jet flows are beneficial for MI cooling. It can be formed when L ch,m or W ch is large enough. MI cooling structure requires smaller vortex regions leading to better thermal load and temperature uniformities. The optimized MI cooling structure achieves the maximum temperature of 35.06 °C for a lithium ion battery pack at 5 C discharging, with the bulk temperature non-uniformity θ bulk of 6.66 °C, and the surface temperature non-uniformity θ sur of 3.52 °C. [ABSTRACT FROM AUTHOR]

• The effect of pulsating flow on heat transfer efficiency of ice slurry is studied. • The cycle-averaged heat transfer coefficient can be increased by 34%. • Heat transfer is more efficient for large particles and high ice volume fraction. • The prediction correlation for the time-averaged nusselt number is proposed. In this paper, the heat transfer enhancement of ice slurry in a horizontal pipe with constant heat flux has been investigated numerically. The model is validated with the experimental results from the literature. Then the axial distribution of the heat transfer coefficient, the bulk and wall temperature and the ice volume fraction are obtained. The factors that affect the heat transfer coefficient are analyzed. Furthermore, the heat transfer efficiency of ice slurry in pulsating flow is compared with that in steady state. It is found that the heat transfer efficiency of ice slurry can be significantly improved by pulsating flow. The maximum increment of cycle-averaged heat transfer coefficient reaches 34.32%. Moreover, the degrees of heat transfer enhancement increase with increasing pulsating frequency, relative amplitude of velocity, ice particle diameter and ice volume fraction, and decrease with increasing cycle-averaged velocity. And the heat transfer is more efficient to ice slurry with large particles and ice volume fraction under the pulsating flow conditions. The pulsating frequency is suggested to be not more than 10π Hz. Finally, the correlation for predicting cycle-averaged Nusselt number of ice slurry in pulsating flow is established, and obtaining the reasonably good prediction results. [ABSTRACT FROM AUTHOR]

· Behavior of local heat transfer coefficient in plane diverging channels is described. • Local Stanton number should be based on flow parameters at diverging channel inlet. • Empirical equations for local Stanton number are based on Re and Kays parameter. • Kinematic structure and TKE in plane diverging channels are estimated experimentally. • Correlation between local heat transfer and local Reynolds stresses is revealed. Heat transfer in diverging channels is essentially different from the one in channels of constant cross section. The mechanism of heat transfer enhancement in diverging channels compared to the channels of constant cross section is based on increased turbulence intensity. The present paper suggests new approach to the analysis and generalization of heat transfer data. Heat transfer on the wall of a diverging channel should be considered similarly to heat transfer from a plate instead of heat transfer in a channel. The data on heat transfer coefficient on the wall of a plane diverging channel at different expansion angles can be generalized by the Stanton number as a function of the Reynolds number and Kays acceleration parameter, St∼ f (Re, K). This function has characteristic sections along the wall that are typical of laminar, transitional and turbulent boundary layers. Stanton and Reynolds numbers based on the distance from the inlet and the velocity at the diverging channel inlet can be employed for data generalization in all the considered ranges of geometries and flow regimes. Kinematic structure of flow is examined to reveal hydrodynamic mechanisms behind heat transfer formation. Optical measurements provide the profiles of velocities and turbulence parameters in characteristic sections of the channel. Mechanism of turbulent structure formation in diverging channels is examined. Correlation between hydrodynamics and heat transfer is analyzed. Correlation between local coefficients of heat transfer and near-wall Reynolds stress is revealed. [ABSTRACT FROM AUTHOR]

• The heat transfer coefficients between dense particle flow and vertical heated plates were experimentally studied. • A numerical model and an analytical model were developed for the wall-to-particle heat transfer. • A semi-empirical correlation for the overall wall-to-particle heat transfer coefficient was derived. • The reduction of the width of flow channel and the mean particle diameter can improve the wall-to-particle heat transfer. The heat transfer coefficient between the gravity driven dense particle flow and vertical heated plates is one of important parameters for the design and optimization of shell-and-plate moving packed bed heat exchangers. This paper gives extensively study on heat transfer between gravity driven dense particle flow and vertical heated plates by combined experimental, numerical, and analytical methods. The wall-to-particle heat transfer coefficients between silicon carbide particles and heated plates are experimentally tested in the designed heat transfer apparatus by varying the mass flow rates of particles with three mean particle diameters (0.207 ∼ 0.364 mm) and different widths of particle flow channel (4 ∼ 10 mm). A numerical calculation model and an analytical model are also established based on plug flow assumption, and compared with the experimental data. The results show that both of the two models exhibit a good agreement with the experimental results when estimating the overall wall-to-particle heat transfer coefficient. However, the numerical calculation model can obtain a more accurate description of the temperature profile than the analytical model attributes to the consideration of the effect of vertical heat diffusion. At last, a new semi-empirical correlation for estimation of overall wall-to-particle heat transfer coefficient was derived based on the analytical solution. Within the test scope of this study, the mean error and maximum error of this new correlation is 3.2% and 9.8%, respectively. The results also show that the reduction of the width of flow channel and the mean particle diameter contributes to the improvement of the wall-to-particle heat transfer coefficient. [Display omitted] [ABSTRACT FROM AUTHOR]

• Flow boiling in micro channels with and without inlet restrictions was tested. • Heat transfer characteristics of studied geometries is generally similar. • The dominant heat transfer mechanism for subcooled flow is forced convection. • The dominant heat transfer mechanism for saturated flow is nucleate boiling. • Average heat transfer coefficient fell by up to 34.0% when restrictions were used. Flow boiling in micro channels is considered to be a promising solution for electronics cooling. However, its development is impeded by the lack of full understanding of mechanisms responsible for heat exchange and its susceptibility to instabilities. A common way of flow stabilization is employing restrictions at channel inlet, but their impact on micro channel thermal performance remains an open question. Therefore, this paper focuses on experimental determination of the dominant heat transfer mechanism during flow boiling of R245fa in 10 parallel rectangular channels with hydraulic diameter of 1 mm without inlet restrictions and with 0.5 mm wide and 2.5 mm long restrictions. Studied heat fluxes vary between 30 and 50 kW / m 2 , mass fluxes between 400 and 1000 kg / m 2 s , saturation temperatures between 49 and 82 ∘ C, inlet subcoolings between 13 and 19.4 K. For the subcooled flow the dominant heat transfer mechanism was forced convection with heat transfer coefficient rising with mass flux, vapor quality and saturation temperature. For the saturated flow nucleate boiling was the dominant mechanism with heat transfer coefficient increasing with heat flux and saturation temperature and decreasing with mass flux and vapor quality. Geometry with inlet restrictions in general exhibited similar thermal behavior, but in the subcooled zone a more pronounced positive dependence on saturation temperature and a negative dependence on mass flux especially at lower temperatures were observed. This indicates that restrictions augment nucleate boiling contribution to the forced-convection-dominated heat exchange in the subcooled zone. Restrictions yielded similar or slightly lower heat transfer coefficients. Comparing the data against 24 heat transfer models showed that the model of Thiangtham et al. was the most accurate with MAPE 23.7% and 20.2% for geometry without and with restrictions respectively. [ABSTRACT FROM AUTHOR]

• Local temperature of airflow was obtained in forced and mixed convection conditions. • Heat transfer was quantified in flow visualization experiment with IR thermometry. • Turbulence model assessment was conducted with linear and nonlinear EVMs. • Changes in the heat transfer phenomena were explained inside a rectangular riser. • Flow laminarization preceding near the corner under mixed convection was discussed. A very high-temperature gas-cooled reactor (VHTR) is an attractive option for hydrogen production due to its high operating temperature. The reactor cavity cooling system (RCCS), a passive safety system of the VHTR, surrounds the reactor vessel with rectangular riser channels and removes decay heat from it. Under emergency operation conditions such as a decrease in the flow rate of the RCCS, the flow condition inside the RCCS riser can become a mixed convection condition accompanying heat transfer deterioration. To design and operate the VHTR with reliable safety, accurate predictions for various convective heat transfer conditions in RCCS riser channels are required. In this study, by comparing the results of an air flow visualization experiment, the predictability of the turbulence models was assessed for the convective heat transfer and complex flow characteristics near the corner region of the rectangular duct confirmed from the experimental study. In this paper, the heat removal rate through the test section of the visualization experiment was quantified under various convective heat transfer conditions, and local temperature measurement data from the experiment were presented to describe the heat transfer deterioration phenomena inside a rectangular riser in turbulent mixed convection. Linear and nonlinear eddy viscosity models were evaluated by comparing the heat transfer coefficient, local temperature, local flow structure including secondary flow, and turbulence quantities obtained from the experiment. Furthermore, improvements to turbulence models for enhancing the prediction of mixed convection heat transfer in rectangular riser ducts were proposed. [ABSTRACT FROM AUTHOR]

• Successive droplet impingement cooling on hydrophilic (uncoated) and superhydrophobic (coated) surfaces is investigated experimentally. • A new hybrid heat transfer solution using the integral transform technique is explained and used. • Single droplet impingement onto the uncoated surface showed higher heat transfer rates than for the coated surface. • Successive droplet impingement onto the coated heated surface allows surface refreshing with potential benefits for spray cooling. New high-performance electronic components generate a massive amount of heat per unit of area, and spray cooling is a field of interest for this application. Spray cooling achieves higher heat transfer coefficients than commonly used forced air convection cooling systems. In this study, we performed multiple droplet impingement experiments over heated surfaces in the non-boiling regime, a scenario that is closer to the phenomena of spray cooling than individual droplet impact. This paper focuses on the effects of droplet interactions with surfaces exhibiting distinct wettabilities. A surface thermocouple was used concomitantly as the impact surface and the temperature sensor. The surface thermocouple was originally hydrophilic (uncoated), which later we treated with a low cost commercial superhydrophobic coating (coated). High-speed imaging showed the droplet motion during impact. We proposed a one-dimensional inverse heat conduction semi-analytical solution, adopting a time-dependent droplet heat flux as one of the boundary conditions to represent the droplet cooling effect. We compared the heat transfer performances of both uncoated and coated surfaces for different droplet velocities but same initial temperature. The results showed higher heat transfer rates on uncoated surface relative to the coated one. Also, higher droplet velocities showed higher heat transfer mainly due to the increase in the droplet spreading diameter in contact with the solid. Successive droplets showed lower heat flow for the uncoated surface because of the low thermal effusivity and thickness of the heated solid, where most of the heat is removed by the first droplet. The first droplet thermal resistance and damping effects for the subsequent droplets, reduced the overall cooling performance. On the other hand, for the coated surface we found that the consecutive droplets continued to transfer heat with a similar rate as the previous one, which is desirable for spray cooling. However, the reduced overall heat transfer performance for the coated surface can be explained by the air pockets within the coating structure. [ABSTRACT FROM AUTHOR]

• Numerical study to investigate the effect of geometry and fluid properties on the convective heat transfer coefficients of reticulated porous media. • Reticulated porous structures are modelled using the theoretical Kelvin structure model. • A new relationship is proposed to calculate the heat transfer coefficient in reticulated Kelvin structures and in reticulated porous structures in general. • The proposed correlation is validated with literature data. This paper presents numerical study to calculate convective heat transfer coefficients in reticulated porous media. In this study, the reticulated porous structures are modelled based on theoretical Kelvin model, representing the real porous structures. The geometry of these structures are generated with the help of in-house code and visualisation tool kit (VTK) libraries. The ideal and randomized Kelvin structures are generated for different PPI & porosities. These structures are utilized to calculate the fluid flow and heat transfer for different fluids of different Prandtl numbers (air, water & sea water). By varying the geometrical parameters, the influence of geometries on heat transfer between the flowing fluid and solid phase of open-cell foams are investigated. For this reason, the momentum and energy equations for forced convection in reticulated structures are solved using standard CFD-FVM approach. Based on the simulation outcomes, a new correlation is proposed to calculate the heat transfer coefficients in the reticulated porous structures. The proposed correlation is validated by comparing it with numerical and experimental data of real reticulated porous structures available in the literature. The effects of the Colburn j factor and performance factor are also computed to obtain the best outcome. [ABSTRACT FROM AUTHOR]

• A honeycomb-like superhydrophobic layer was fabricated on four metallic substrates. • Heat transfer is enhanced by ∼6 times through realizing the droplet-jumping condensation. • Jumping-droplet condensation is independent of the substrate thermal conductivity. • The magnitude of constriction resistance is large during flooding condensation transition. • Droplet departure frequency in flooding is 100% higher than that in the dropwise mode. The efficiency of dropwise condensation heat transfer has been improved significantly by the development of superhydrophobic surfaces, which are usually fabricated by coating a thin superhydrophobic layer on different kinds of substrates (e.g., different metals/alloys). However, the effects of the thermal conductivity of the substrate on condensation heat transfer over such superhydrophobic surfaces remain unclear. In this paper, we investigated experimentally the condensation heat transfer and droplet dynamics on honeycomb-like superhydrophobic (HCLS) surfaces with hierarchical microporous structures, fabricated by an electrochemical method on four metallic substrates with different thermal conductivities, i.e., copper, brass H96, brass H65 and stainless steel SS316. We observed spontaneous droplet jumping on all these HCLS surfaces at a relatively low degree of subcooling. We also demonstrated the interplay between the substrate thermal conductivity and condensation heat transfer performance during the flooding condensation transition (FCT) by numerical simulations. In the jumping-droplet condensation and flooding condensation regimes, the condensation heat flux and heat transfer coefficient were found to be almost unchanged for different substrates. However, the substrate with a higher thermal conductivity leads to a higher condensation heat transfer coefficient as well as a higher droplet departure frequency at moderate degrees of subcooling, indicating the synergistic effect between the substrate thermal conductivity and droplet dynamics during FCT. The substrate thermal conductivity-mediated droplet dynamics enables a way for further enhancing condensation heat transfer on the HCLS surfaces. [ABSTRACT FROM AUTHOR]

• Nanomaterial deposition on heating surface are used to attain reduction in wall superheat, enhancing nucleate boiling HTC and CHF. • Different working fluid is used such as de-ionized water, fluorocarbon fluid, hydofluoroethers and refrigeration-based fluid for enhancing CHF. • The surface modification enhanced the CHF due to the reduction in wall superheat temperature, increased nucleation site, porosity rise, enhanced surface wettability, small nano-pores/cavities generation, rise in bubble release frequency. • Nanoparticle size, nanofluid preparation techniques and concentration of nanofluid mainly influenced the CHF. Boiling is the most significant phase change phenomena for various applications such as cooling of electronic devices, heat pipes, cooling in nuclear reactor, different industrial application and plenty more. Due to its wide scope of application, any enhancement in this domain leads to remarkable economic and energy efficient consequences. Augmentations in boiling heat transfer performance are very crucial which establishes the various industrial applications further energy efficient. To reduce the energy dissipation in heat transfer equipment, boiling heat transfer augmentation is one of the significant techniques used by researchers. This paper demonstrates a thorough review and investigation of published articles describing the various surface modification method (nanomaterial coating, nano/micro porous coating, nano/micro structured surface) for pool boiling and flow boiling heat transfer enhancement as well as the effect of surface modification on wettability, porosity, surface roughness, nucleation sites are studied. Various augmentation methods are utilized to attain three apparent heat transfer desire: reducing wall superheat temperature, enhancing pool boiling heat transfer coefficient (HTC) and improving critical heat flux (CHF). [ABSTRACT FROM AUTHOR]

• Flow and heat transfer characteristics of a straight channel PCHE are analyzed. • The idea of thermophysical property-matching partitioned heat transfer enhancement is proposed. • The wavy channel structure is adopted to improve thermal hydraulic performance. • Effectiveness of the partitioned heat transfer enhancement is examined. Heat transfer enhancement within the precooler in supercritical CO 2 Brayton cycles is challenging due to the dramatic variations of thermophysical properties. The local flow and heat transfer characteristics of a straight channel PCHE are analyzed firstly in this paper and the idea of thermophysical property-matching partitioned heat transfer enhancement is proposed accordingly. Subsequently, three types of hybrid channel, where sinusoidal wavy structures are combined with straight channel in different ways, are designed to examine the effectiveness of the partitioned heat transfer enhancement idea. A comparative study on the thermal hydraulic performance is performed with different geometrical parameters under different working conditions. The results indicate that the partitioned heat transfer enhancement shows great advantages in reducing the total pressure drop under the given heat flux boundary conditions. Compared to the full-wavy and the other two hybrid channels, the overall thermal hydraulic performance of the type-C channel is the best. A maximum pressure drop reduction of 23% could be observed in the type-C channel (with the wavy section used in the low-temperature region) compared to that in the type-A channel (with the wavy section used in the high-temperature region). Such advantage will become more prominent when the outlet temperature of the precooler is closer to the pseudocritical temperature. Moreover, the averaged increment of local heat transfer coefficient in the wavy section is 1.56 kW/m2⋅K for the type-C channel when compared to the straight channel section. It's also found that the effects of gravity on the local heat transfer coefficient are almost negligible in the high temperature region, while heat transfer in the low temperature region can be affected by changing the placing direction of the precooler. The present study may provide a practical guidance on the development of highly efficient heat exchangers where the thermophysical properties of the working fluid are highly variable. [ABSTRACT FROM AUTHOR]