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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]