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The main objective of the present paper is to investigate the effect of using connecting holes on the hydrothermal performance and irreversibility behavior of laminar forced convection flow of a nanofluid (NF) inside a microchannel heat sink (MCHS). The biologically synthesized silver/water NF is employed as coolant. The influences of the volume concentration of NF (φ), the Reynolds number (Re) and the number of connecting holes on the performance features of MCHS and NF are considered numerically. It was observed that the CPU temperature is reduced and became more uniform due to boosting of Re and φ , and the consequence augmentation of the convective heat transfer. However, the pumping power and frictional irreversibility get raised as Re and φ are increased. The figure of merit is above unity for all Re and φ which indicate that the utilization of connecting holes is a suitable option to improve the MCHS performance from the first-law point of view. The heat transfer irreversibility, however, increased by using the connecting holes. Also, the merit of NF over the pure water is observed for Re of 500 and 2000 and φ of 0.1% and 1%. [ABSTRACT FROM AUTHOR]

Heat exchangers made of corrugated flow passages generally have better thermo-hydraulic performance compared to parallel flow passages. The corrugation angle (β), corrugation pattern, and the ratio of depth to pitch (h ch /P ch) are critical geometrical parameters influencing the heat transfer and pressure drop in corrugated flow passages. This paper experimentally investigates heat transfer and pressure drop characteristics of triangular-shaped cross-corrugated flow passages for the range of 25° < β < 75° and 0.13 < h ch /P ch < 0.36. Experiments are performed using representative compact heat exchangers in a small-scale test facility. This study also reports the effects of plate geometry on heat transfer and pressure drop and provides correlations for the average Nusselt number and friction factor. A detailed comparison of test data with relevant literature is also presented. Results of this study will be useful to manufactures and designers for developing high-performance heat exchangers. [ABSTRACT FROM AUTHOR]

In this paper, 3D analysis of forced convection of Newtonian and non-Newtonian nanofluids through a horizontal pipe using single- and two-phase mathematical models is performed. The models include the Newtonian single-phase model, non-Newtonian single-phase model, Newtonian two-phase mixture model and finally non-Newtonian two-phase mixture model. The geometry of the problem is a horizontal pipe that is filled with Al 2 O 3 /water nanofluid. The effects of particle volume fraction and the Reynolds number on the heat transfer and hydrodynamic characteristics of the flow are examined. Results indicate that for the volume fraction of 0.01, the non-Newtonian mixture model gives more accurate results in comparison with the other models. By increasing the solid volume fraction, the mixture models give a higher heat transfer coefficient in comparison with that of the corresponding single-phase model; however, by comparing the results of Newtonian and the corresponding non-Newtonian models, no meaningful relationship can be obtained. Furthermore, for all the investigated models, the Nusselt number is increased with increasing the Reynolds number. [ABSTRACT FROM AUTHOR]

Packed beds with methane steam reforming reaction (MSR) can be used as thermochemical storage devices. The packing configuration can have a significant effect on the flow distribution which will further influence the heat/mass transfer, dispersion and reaction characteristics. In the present paper, the hydraulic, dispersion, heat/mass transfer and the MSR reaction properties are investigated in packed beds with the simple-cubic (SC) packing, the body-centered cubic (BCC) packing and face-centered cubic (FCC) packing forms. ANSYS FLUENT integrated with user defined scalars (UDS) and user defined functions (UDF) are used to perform the simulations. Results show that firstly, the residence time distribution (RTD) can be used to evaluate the dispersion property in packed beds where a narrower RTD curve reflects a more uniform velocity distribution with less dispersion. Secondly it is found that, regions with backflow are not beneficial for heat/mass transfer and chemical reactions from two aspects. One is that the heat transfer between the main fluid and the particle surfaces is weak and low temperature of the particles will reduce the reaction rates. The other is that the poor species transport will prevent the reactant to flow into the reaction region and the products cannot be taken away by the main flow timely, which will have a negative effect on the reaction. Thirdly, it is revealed that, the dimensionless variation of the residence time distribution (DVRTD) can reflect the heat/mass transfer and reaction performances, where a smaller DVRTD corresponds to better heat/mass transfer and higher reaction rates. [ABSTRACT FROM AUTHOR]

In this paper, the pressure loss and tip-wall heat transfer are presented in an idealized U-bend channel with novel turning vanes. The cross-sectional area variation, flow separation, mainstream and secondary flow, pressure distribution in different turning vane(s) channels are studied. Moreover, the mechanism of heat transfer enhancement is analyzed by studying the distribution of parameters. Computational fluid dynamics technology is used for solving the steady RANS equations. Results revealed that the double-layer turning vanes channel overcomes the unfavorable factors such as flow separation, recirculation and impingement in the bend region, reducing the pressure loss in the channel by 70%. The double-layer, dome shaped turning vanes channel is a novel cooling structure based on the double-layer turning vanes. The upper wall of the upper layer turning vane is improved into a three dimensional domed structure similar to the hemispherical shell. By main flow and secondary flow acceleration and the special flow pattern caused by the surface of the dome-shaped turning vane, the pressure distribution is more uniform and more coolant flows attached to the tip wall at high velocity. The tip-wall heat transfer performance is improved by 21.77%, and Performance Evaluation Criteria increased by 71.64%. The traditional internal cooling channel guiding devices only play a single role in reducing pressure loss or improving heat transfer. While the double-layer, dome-shaped turning vanes combine can not only improve the heat transfer but also reduce the pressure loss. [ABSTRACT FROM AUTHOR]

High temperature thermal contact resistance is the research focus of many applications involving thermo-mechanical issues including spacecraft thermal protection, nuclear reactor cooling and heat treatment process. In this paper, a finite element contact model was developed with the rough surfaces, which were generated from the optically measured surface topography of HTA and C/ZrB 2 -SiC. The mechanical deformation was conducted account for the elastic-plastic mechanics, then the thermal analysis was completed in terms of the interface heat transfer condition with the solid spots conduction, far-field and near-field radiation. The results show that thermal contact resistance decreases from 1611 to 488 mm2K/W when the interface pressure increases from 0.1 to 0.6 MPa, as the solid spots conduction is sensitive to pressure. Also, thermal contact resistance decreases from 1611 to 340 mm2K/W when the interface temperature increases from 630 to 1100 K, due to the enhancement of heat transfer by radiation. And the thermal contact resistance account for solid conduction and radiation is reduced to 1/25 of that only considering solid conduction. Additionally, the accuracy of present method was effectively validated by comparing the numerical results with experimental data. [ABSTRACT FROM AUTHOR]

A methodological approach has been adopted in order to investigate the magnetohydrodynamic (MHD) flow of (Cu-SiO 2)/water hybrid nanofluid keeping in view the peristaltic motion. An endeavor is made to obtain accurate results of velocity slip and convective boundary conditions while studying four different types of geometric shapes (sphere, bricks, cylinder and platelets). The formulated equations are theoretically examined. In the manuscript, flow phenomenon of relaxation and contraction in a geometry of finite length of a non-uniform tube with regards to Cu/water and (Cu-SiO 2)/water hybrid nanoparticles are deliberated. Equations related to realistic boundary conditions i.e. dimensionless control equations, are also studied in the manuscript. In order to validate theoretical findings of the work, graphical discussion on supplementary nanoparticles form a part of this manuscript. Effects of these nanoparticles on velocity, temperature distribution and transfer of heat is also discussed in the same manuscript. The analysis shows that conductivity of the nanoparticles is directly proportional to the volume of the nanoparticles. The results also show that there is significantly increase in temperature and velocity due to increase of heat absorption which also results in increases in elevation for hybrid nanofluid. The above discussion implies that convective heat transfer parameter has higher impact in case of (Cu- SiO 2)/water hybrid nanofluid as compare to Cu-water/nanofluid. Streamlines pattern of Peristaltic transport is also discussed in the paper. This manuscript also draws the comparison between the results of Cu-water and (Cu- SiO 2)/water hybrid. The emphasis of the manuscript attentions to different shapes of nanoparticles and effects of convective heat transfer with regards to steady physiological delivery of MHD with nanofluid and hybrid nanofluid in a tube. Some noticeable facts are highlighted as under: • There is an incline in temperature as there is incline in heat absorption parameters, whereas, it shows a decline in the case of hybrid nanofluids. • Heat absorption parameters also effects the behavior of the velocity. It has been observed that, with an increase in the value of heat absorption parameter, there is an increase in the velocity for both fluids. • Hartmann number is inversely proportion to the velocity distribution. • The results show that there is a drop in the velocity distribution for both fluids for various higher terms of E1, E2. • It is clearly demonstrated that decrease can be observed in the size of bolus in the case of Cu-water, whereas, there is an increase in the size of the bolus in case of for hybrid nanoparticles. [ABSTRACT FROM AUTHOR]

These notes aim to increase the awareness of the concept of Bejan Number among the readers of International Communications in Heat and Mass Transfer. They refer to the paper of Zimparov et al. because the authors, in their outstanding commentary on Bejan number neglect both the possible confusion which may arise between Hagen Number and Bejan number and the more recent outcomes which opens a new direction for future research. [ABSTRACT FROM AUTHOR]

This paper studies the magnetohydrodynamic (MHD) flow of unsteady Jeffrey nanofluid due to vertical stretchable cylinder. Joule heating and thermal radiation outcomes are additionally addressed. The Buongiorno model is utilized to describe the characteristics of nanofluid. The mathematical form of the problem consists of momentum, continuity, concentration, and energy equations. Suitable transformations are employed to obtain the dimensionless form. Numerical solutions for velocity, energy and concentration equations are obtained by the finite difference method (FDM). Impacts of various parameters (i.e. Prandtl number, Eckert number, Hartmann number, Grashof number, Deborah numbers, Brownian parameter, stretching parameter and Schmidt number,) on velocity, concentration, temperature, Nusselt and Sherwood number numbers are scrutinized graphically. Results show that velocity decays for higher Grashof number and stretching parameter. Temperature and concentration decreased for higher Brownian parameter, Prandtl and Schmidt numbers. [ABSTRACT FROM AUTHOR]

In order to improve the heat recovery efficiency of high-temperature solid particles, a two-dimensional heat transfer model of double vacancy particle bed was established. This paper investigated the effects of particle size and thermal conductivity on the heat transfer process of double vacancy particle bed. With the increase of the thermal conductivity from 0.4678 W/m·K to 46.78 W/m·K, in the area near the vacancy, the particle isotherm is significantly affected, the influence area of double vacancy in X direction increases from 1.5 to 4.5 times of particle size, the influence area of double vacancy in Y direction increases from 2.2 to 7.2 times of particle size, and the wall heat flux increases from 0.53 kW/m2 to 36.28 kW/m2. The particle size has little effect on the overall temperature distribution of the particle bed, the maximum influence area in the X direction is approximately the same, all are 4.5 times particle size, and the maximum influence area in the Y direction is increased from 7.1 to 7.4 times particle size, but as the particle size increases, the wall heat flux increases obviously. The results will further improve the thermal resistance network theory. [ABSTRACT FROM AUTHOR]

In this paper, experimental and numerical studies of natural convection heat transfer of Cu-water nanofluid in a cubic enclosure under the effects of both the time-unvarying and alternating magnetic fields are presented. The effect of the nanofluid volume fraction on the natural convection heat transfer in the presence of a constant magnetic field is studied for different Grashof numbers. Moreover, the influence of an alternating (in time) magnetic field in the shape of a rectangular wave with different frequencies on the heat transfer rate is investigated. The results show that in the absence of a magnetic field, adding Cu-nanoparticles to water enhances the heat transfer performance. The Nusselt number increases with increasing the volume fraction of the nanofluid. On the other hand, if the enclosure is subjected to a strong enough magnetic field, the negative effect of the elevated electrical conductivity by adding the nanoparticles dominates the effect of the increased thermal performance by using nanofluid. The increased electrical conductivity raises the magnetohydrodynamic effect, which acts against the buoyancy force. As a result, increasing the nanoparticles volume fraction reduces the heat transfer performance if the magnetic field strength is beyond a Hartmann number that is greater than a threshold value. [ABSTRACT FROM AUTHOR]

Various methodologies have been proposed in literatures to simulate pure steam filmwise/dropwise condensation and filmwise condensation in the presence of non-condensable gas (NCG). However, simulation methodology for dropwise condensation in the presence of NCG is rarely to be studied. In this paper, theories related to chemical gas-liquid absorption are referred, coupling with the widely used pure steam dropwise condensation model, a new methodology for heat transfer calculation of dropwise condensation in the presence of NCG is finally developed. After comparing experimental data from literatures with simulation results based on the proposed model, the new developed methodology gets validated. Heat transfer performance is found to be coordinately controlled by capacities of absorption mass transfer and surface condensation. Then, the effect of two practical easy to be controlled operation variables of Re number and sub-cooling degree on dropwise condensation process in the presence of NCG are investigated. Results indicate that sub-cooling degree is more sensitive than Re number, and the key to improve total heat flux is to accelerate the rate-determining step. Finally, 133° is found to be the optimal contact angle, under which the maximum heat flux could be achieved. [ABSTRACT FROM AUTHOR]

The current paper aims at preparing novel anti-freeze hybrid nanofluids in EG as based fluid via dispersion of synthesized nanocomposites mesoporous silica adjusted with Cu NPs (Cu/SBA-15). FTIR, SEM, TEM, EDX, XRD, and XPS were used for characterizing the developed nanocomposite. The present study mainly aims at measuring the thermal conductivity of SBA-15 decorated with Cu nanoparticles EG-based hybrid nanofluid at 25–50 °C and a nanocomposites weight fraction concentration of 0.017% to 0.064%. The research findings indicated thermal conductivity enhancement of the hybrid nanofluid at optimum temperatures and concentrations of the nanocomposite. It is suggested these hybrid nanofluids are used as a promising choice in thermal engineering systems. Ultimately, the empirical data were utilized for indicating a correlation as a function of the nanocomposite concentration and temperature and calculating the thermal conductivity ratio of modified SBA-15 with Cu NPs. [ABSTRACT FROM AUTHOR]

Swirl cooling is an excellent candidate for internal cooling of turbine blade leading edge. In this paper, a novel swirl chamber with distributed multi exit slots is proposed, for which different outlet configurations are considered. Flow and heat transfer characteristics are numerically investigated and compared with those of conventional chamber. Then dimples/protrusions are innovatively introduced into the swirl chamber, with various arrangements. The influence of temperature ratio is further discussed. The results show that, a typical flow structure of "inner vortex + outer vortex" is formed in the jet regions. In novel swirl chambers, the extraction effect of exit slots avoids the formation of crossflow, thus the friction can be effectively reduced while the heat transfer is strengthened, for which a 46.9% improvement of overall thermal performance (TP) is achieved. The dimple/protrusion can realize the local flow control, and produce a 54.8% improvement in TP. The nozzle-side arrangement of dimple/protrusion is better than the slot-side arrangement, and the protrusion is superior to the dimple. With the increase of temperature ratio, the TP shows the variation trend of increasing first and then decreasing. This research will provide important data support for the internal cooling design of turbine blade leading edge. • A novel swirl chamber with distributed multi exit slots is proposed. • Dimple/protrusion is introduced into the swirl chamber, with various arrangements. • Effect of temperature ratio on thermal performances of swirl chambers is discussed. • Extraction effect of exit slots effectively restrains the development of crossflow. • A 54.8% improvement in TP is achieved by the protrusion swirl chamber. [ABSTRACT FROM AUTHOR]

This paper proposes an innovative thermal control strategy for obtaining optimum layout of Orb-shape Membrane Energy Gathering Array (OMEGA) space solar power station radiator based simultaneous shape and topology optimization. Firstly, a parameter in term of maximum heat dissipation per mass is introduced for the optimal configuration using the Particle Swarm optimization algorithm. Then, analogizing the butterfly wing vein and branch points, a better layout of heat transfer network is inspired to conduct. Based on this, the virtual temperature points are added for completely constructing bionic butterfly wing heat transfer network and the calculation method is derived and discussed in detail. Finally, the derived method is realized through topology optimization method and the validity is verified through comparison of thermal analysis results. Unlike other radiators design, the proposed method optimizes layout of radiator from both two aspects of configuration outside and bionic butterfly wing heat transfer network inside, which is meaningful for the future application. [ABSTRACT FROM AUTHOR]

This paper reports the effect of a single vacancy on heat transfer of particle pile. A three-dimensional steady-state heat transfer model of particle pile with a single vacancy was established. The effects on the temperature distribution, heat transfer mode and heat flux characteristics of the particle pile were studied. In the area close to the vacancy, the particle temperature distribution is significantly affected, and the average temperature line is significantly deformed. The heat conduction between particles along the Y-direction is the dominant heat transfer mode around the vacancy. Compared with a particle pile without vacancy, the total heat flux at the hot wall decreases by 0.63% and the apparent thermal resistance of the particle pile increases by 0.71%. The size of the region affected by a vacancy on heat transfer was determined. At the X-direction, the size of the region affected by a vacancy is 1.79 times of particle size. Upstream position of vacancy at the Y-direction, the size of the region is 4.03 times of particle size. Downstream position of vacancy at the Y-direction, the size of the region is 3.81 times of particle size. The research results can be used to improve the thermal resistance network theory. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study on the heat transfer characteristics of a porous brick roof filled with phase change materials (PCMs) in a typical day of June 2019 in Shanghai. Paraffin wax with different phase transition temperatures T m was selected as the PCM in the porous brick. A graphics processing unit (GPU)-accelerated multiple-relaxation time lattice Boltzmann method (MRT-LBM) was proposed owing to its good flexibility and parallel computing features. It was found that a lower T m resulted in a larger liquid fraction, and the PCMs with T m = 25 °C and 26 °C melted completely during the daytime. In the afternoon, the temperature gradient in the porous roof filled with PCM exceeded that of the roof filled with air, implying that PCMs have a significant thermal buffering capacity. In comparison with the traditional porous brick roof filled with air, the porous brick roof filled with PCM of T m = 27 °C performed best, and the heat flux during air-conditioning operation period was 67.8 kJ/m2 and reduced by 73.4%. Therefore, the porous brick roof filled with PCM exhibited a significant improvement in the thermal inertia of the roof, and the energy consumption by the air-conditioning system could be reduced accordingly. [ABSTRACT FROM AUTHOR]

Epistemic uncertainties are one of the important class of uncertainties that occur in heat transfer problems. These uncertainties are caused due to experimental error, improper or insufficient knowledge of operating conditions, and imprecise or vague information about the involved constants, coefficients, and parameters. As such, in this paper, a convection-diffusion heat transfer problem is considered with epistemic uncertainties viz. thermal conductivity, ambient temperature, convective heat transfer constant, and source. The uncertainties are taken as intervals with a 5% error to the exact value. To compute interval uncertainties effectively, a transformation-based technique is proposed that converts intervals into single variable linear functions. Then to solve the governing differential equation with uncertainties, the transformation-based method is used with Interval Finite Element Method (IFEM), and accordingly, the problem is modeled. Finally, the uncertain distribution of temperatures is quantified for a rectangular plate with nonhomogeneous boundary conditions, and the sensitiveness of the uncertainties are investigated. [ABSTRACT FROM AUTHOR]

New and efficient mixing heat technology is increasingly demanded for many industrial fields. This paper proposed a novel technique of enhancing convective heat transfer by dynamic controlling rib and investigated several cases of vibration rib controlling flow. The study results of heat transfer coefficient, temperature and turbulent intensity indicate that dynamic flow control is an efficient heat transfer enhancement technique. The dynamic flow control breaks through the current enhancement heat transfer technologies such as general stationary rib and other vortex generators, and achieves a better heat transfer effect. Comparing traditional rib technologies, vibration rib does not only increase heat transfer performance but leads a lower flow resistance. More importantly, it can be dynamically adjusted to achieve better flow turbulence. As a conclusion, the enhancement heat transfer performance can be timely dynamically adjusted to meet the different heat transfer demand by changing timely the frequency and amplitude of vibration rib, which achieves the optimization of whole system thermal protection. New enhancing heat transfer technique effectively improve the comprehensive heat transfer performance, and provides a reference for intelligent thermal control protection system [ABSTRACT FROM AUTHOR]

This paper numerically qualifies the effect of rib orientation on heat transfer and flow performances of mist/steam in square channels with inclined ribs. Four types of rib orientations are investigated. The study is conducted in contrast to steam with Reynolds number ranged from 10,000 to 60,000. Secondary flow patterns and droplet trajectories are analyzed to reveal the underlying reasons bringing about heat transfer enhancement due to mist/steam. The results indicate that rib orientation greatly determines secondary flow pattern in channels, and the distribution of secondary flow triggered by N type ribs is opposite to that by P type ribs. Such secondary flow forces droplets at different locations to assemble near different side walls. The places where water drops gather in a large quantity provide splendid heat transfer enhancement. And this effect gets more obvious at higher Reynolds number. For mist/steam, both heat transfer coefficient and overall heat transfer performance in the first passage are remarkably stronger than that in the second one. In general, NN channels and PP channels filled with mist/steam have the best heat transfer performance among the investigated cases. [ABSTRACT FROM AUTHOR]

The paper presents a numerical study on an impingement cooling system which employs a self-excited oscillating air jet discharging into a confined space. The effect of nozzle width was examined on the flow structure and heat transfer at different inlet flow rates and impinging distances. The unsteady incompressible flow was solved by the finite volume based tool OpenFOAM code. The k-ω (SST) model was selected to capture the turbulence effects. The results indicated that reducing the nozzle width increased the frequency of the oscillation at a constant flow rate. On the other hand, the Nusselt number at the impingement wall increased linearly by reducing the nozzle width. For the ratio of nozzle width to cavity width greater than 0.195, the oscillation of the flow completely vanished. In these cases, the self-excited oscillating impinging jet was converted into a conventional stationary impinging jet. The results showed that the value of the average Nusselt number of the conventional stationary jet was lower than that of the self-excited oscillating ones. Also, small variations in the impingement distance slightly changed the mean Nusselt number at small nozzle widths. The results suggest that the oscillation of the flow enhances the cooling performance of the system. [ABSTRACT FROM AUTHOR]

High-efficiency trailing edge cooling technology is essential for the safe and stable operation of turbine blades. In this paper, trapezoidal trailing edge channels with lateral slots are modeled, and a variety of dimples with different sizes and arrangement densities are placed on the end walls. Flow characteristics, wall heat transfer distributions and overall heat transfer performance under stationary and rotating states are numerically investigated and effects of dimple arrangement, rotation number (Ro) and channel orientation (β) are comprehensively analyzed. The results show that, under rotating state, the Coriolis force leads to the flow separation, while dimples promote the generation and development of three-dimensional spatial vortices, and effectively suppresses the expansion of separation area. Dimples greatly improve the rotating cooling performance, and there exists a critical point of arrangement density, in which the maximum heat transfer level is obtained. A 108.2% improvement in relative Nusselt number of trailing surface can be achieved by increasing rotation number in dimple channels. And the operation condition of Ro = 0.8 and β = 15° is recommended for the cooling of trapezoidal trailing edge. Compared with rotation number, the channel orientation has a stronger effect on the flow and heat transfer performances. [ABSTRACT FROM AUTHOR]

This paper designs the treelike networks with the wedge-shaped channel from the idea of bionics. Pool boiling heat transfer experiments of FC-72 under different liquid subcoolings were conducted on the networks with different branch levels. The bubble phenomenon was recorded to reveal the boiling mechanism. The bubble pumpless movement towards the diverging direction in the wedge-shaped channel under the action of capillary force was observed. Under the saturated condition, the surfaces are easier to be completely covered by bubbles and the effect of treelike networks is hindered, which results in similar heat transfer performance. With the increase of liquid subcoolings, the bubble fraction decreases and the effect of the branch level comes into effect. On the surface with high branch level, the bubbles are transported immediately at small radius by the narrow wedge-shaped channel. The moving bubbles disturb the fluid and coalesce with other bubbles, which increases the heat transfer coefficient. There are also more liquid supply pathways between small bubbles, which increases the critical heat flux. The importance of this research is to provide a solution to the difficulty of bubbles vertical departure caused by the lack of buoyancy under microgravity. [ABSTRACT FROM AUTHOR]

This paper presents a detailed discussion on the hysteresis phenomenon caught during the flow boiling heat transfer of R245fa on a Carbon/Carbon surface. Due to the partial hydrophilicity of the Carbon/Carbon surface, a hysteresis in flow boiling heat transfer is observed and studied to understand the underlining heat transfer mechanisms on the basis of the surface activation. A combined experimental approach is developed by monitoring the local wall temperature and contemporarily recording the two-phase flow patterns using a high-speed video camera. The observed dewetting/rewetting phenomenon is linked to the activation process of the Carbon/Carbon surfaces giving new insights on the boiling behavior of hydrophilic surfaces. The results show that after activation the heat transfer coefficients increases up to 20%. The experimental results are compared against the estimations of a model recently proposed by the present Authors and based on R134a measurements. The proposed model is able to predict the heat transfer coefficients within ±20%. • R245fa flow boiling on a Carbon/Carbon surface is studied. • A hysteresis of boiling heat transfer is observed and studied. • A combined experimental methodology is applied to study the surface activation process. • The dewetting/rewetting phenomenon during flow boiling was linked to the activation process. [ABSTRACT FROM AUTHOR]

The present paper probed the problem of thin liquid film flow and heat transfer in the Carreau fluid past a permeable stretching sheet. The similarity transformation reduced the partial differential equations into a system of ordinary differential equations, which was then solved numerically by a collocation method. The influence of the governing parameters towards the model was discussed and presented graphically. The non-uniqueness solutions are observable as the governing parameters vary. [ABSTRACT FROM AUTHOR]

In this paper, heat transfer coefficients and critical heat fluxes of the R134a fluid in two-phase flow was studied in a 27-channel, 421 μm hydraulic diameter and 40 mm long microchannel experimental set-up. Data were collected at constant mass flux of 1000 kg m−2 s−1, saturation inlet temperature range of 20–28 °C and at inlet vapor quality range of 0.01–0.20. The heat flux was initiated at 250 kW m−2 and gradually increased until reaching the critical heat flux (CHF). The effect of inlet vapor quality, saturation inlet temperature and heat flux on heat transfer coefficient and critical heat flux were investigated in detail and presented graphically. It can be observed that the CHF decreases with the increasing of saturation inlet temperature and inlet vapor quality. However, the heat transfer coefficient at the CHF point increased with the increasing of saturation temperature and the heat transfer coefficient at the CHF point was decreased with the increasing of vapor quality. The test results from the correlations, suggested in well-known articles, were compared with the experimental data, where the Kosaz et al. proposed correlation was showing well fit with the experimental data having absolute error of 9.86%. In this investigation, the new correlations for the heat transfer coefficient for the heat transfer coefficient and the critical heat flux have been proposed by taking into account the effect of the vapor quality. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, a numerical model of nanosecond laser ablation of crystalline silicon has been established. Based on the highly nonlinear model of heat transfer and phase change in crystalline silicon after absorbing laser light, heat transfer equation is solved by using finite element method implemented in ANSYS. The simulation of ablation depth of crystalline silicon is obtained under different conditions of laser fluence and pulse overlap. Comparing with the ablation morphology obtained from SEM observations, the computational results and experimental data show good agreement. [Copyright &y& Elsevier]

This paper intends to analyze computationally the rarefied gaseous flow and performance of heat transfer for a steady-state laminar 2D free convection within tilted enclosures where the sided walls are of wavy shape. Low-pressure or rarefied flows are flows with Knudsen number greater than zero. These kinds of flows are of incredible significance in different applications. Boussinesq estimation is adopted to represent the buoyancy effects. Grid independence tests are carried out to ensure accuracy. Moreover, the current code is verified with numerical and experimental results existing in the literature. Flow and thermal fields for simulated cases are presented. The outcome of this research indicates that there is a direct relationship between the average Nusselt number and the Rayleigh number, and a converse relationship between the average Nusselt number and Knudsen number. Additionally, the numerical simulations reveal that increasing the amplitude of the two-sided wavy walls of the enclosure slightly mends the transfer of heat. Besides, it is determined that the tilt angle slightly affects the heat transfer performance of such flows; and it is evident from the simulations that horizontal or nearly horizontal orientations of such cavities give the maximum transfer of heat. Finally, a correlation representing the relationship of the average Nusselt number in terms of all the examined parameters is introduced. [ABSTRACT FROM AUTHOR]

In this paper, the annular condensation of vapor Argon through a nanochannel is studied using the molecular dynamics method. Simulation is performed in a computational domain with two solid platinum walls for different saturation conditions with focusing on the flow characteristics and heat transfer. The density, surface tension, and velocity profiles are obtained for various axial locations. Also, the effect of different wall temperatures on the density profile and heat transfer of condensation flow is investigated. It is found that the wall temperature does not have a significant effect on the vapor and liquid density, while the heat flux value is dependent on the wall temperature. Finally, thermal conductivities for the two-phase flow of Argon are estimated at different saturated temperatures, and an equation is presented for calculating the thermal conductivity of the liquid-vapor Argon in terms of the thermal conductivity of each phase. The results show that the formula can predict the thermal conductivity of two-phase flow of Argon quite well. [ABSTRACT FROM AUTHOR]

This paper addressed the scarcity of experimental studies on the duo effect of magnetic excitation and hybrid nanofluid on the thermo-convection characteristics in enclosures. Novel Fe 2 O 3 -MWCNT (80:20)/deionized water (DIW) nanofluids with volume concentrations (φ) of 0.05–0.4 vol% were employed in a rectangular cavity exposed to magnetic excitation (4.89–21.95 mT) to examine the thermo-convection heat transfer behavior. The prepared hybrid magnetic nanofluids (HMNFs) were characterized for their stability, viscosity, thermal conductivity, and morphology. The DIW and stable HMNFs were charged into the cavity and differentially heated at ΔT of 20–35 °C under steady conditions. The results showed that without magnetic excitation, the average Nusselt number (Nu av) were observed to augment as Rayleigh number (Ra) and ΔT increased. Nu av and average heat transfer (Q ̇ av) were augmented for φ ≤ 0.2 vol% but they depreciated with φ > 0.2 vol% when compared with DIW. As the viscosity of HMNFs and φ increased heat transfer was noticed to depreciate. At ΔT = 35 °C, highest Nu av (11.33%) and Q ̇ av (11.21%) were achieved with φ = 0.05 vol%. With the magnetic excitation of 4.89–21.95 mT applied to the bottom, side (vertically and horizontally) and top (perpendicular and parallel to ΔT direction) walls of the cavity containing 0.05 vol% HMNF at ΔT = 35 °C, Q ̇ av was enhanced by 0.86%–1.46% and 1.72%–5.02% for the bottom and side walls when compared with the case of no magnetic excitation. Highest Q ̇ av (2.69%–5.02%) was recorded when the magnetic excitation was applied vertically on the side wall. Conclusively, the obtained results were observed to be strongly related to φ, ΔT, strength of magnetic excitation, use of HMNFs, position, and configuration of magnetic excitation. [ABSTRACT FROM AUTHOR]

In order to design a new vortex generator (VG) and effectively induce and properly control the vibrations of elastic coil pipes (ECPs) in heat exchanger (HE), the effects of the fluid flow parameters, vortex element size and external flow field on vortex flows in a fluid domain with variable cross-section were analyzed. The vibration and heat transfer characteristics of the ECPs were studied based on the new VG. The numerical results show that the fluid flow parameters, vortex element size and external flow field have influence on the frequency and intensity of vortex flow. The VG designed in this paper can generate vortex flows with basically the same frequency and intensity at each branch outlet. Under the action of the new VG, the vibration amplitudes of the ECPs are significantly enhanced and the vibration uniformity is improved. Meanwhile, the new VG can increase the heat transfer coefficients of the ECPs and improve the consistency of the heat transfer performances of the ECPs. [ABSTRACT FROM AUTHOR]

This paper explores machine learning approach as a heat transfer correlation. Machine learning significantly reduces the effort to develop multi-variable heat transfer correlations, and is capable of readily expanding the parameter domain. Random forests algorithm is used to predict the convection heat transfer coefficients for a high-order nonlinear heat transfer problem, i.e., convection in a cooling channel integrated with variable rib roughness. For 243 different rib array geometries, numerical simulations are performed to train and test ML model based on six input features. Machine learning model predicts closely to numerical simulation data with high determination of coefficient (R2), e.g., R2 > 0.966 for the testing dataset. The capability and limitation of random forests algorithm are discussed with validation dataset. [ABSTRACT FROM AUTHOR]

This paper presents new insight concerning the formulation and definition of one of the two Bejan numbers (Be) derived from the first and second law analysis, namely this one derived from the first law of thermodynamics. The systematic analysis performed reveals the role and physical meaning of the Bejan number (Be) as criterion of similarity or dimensionless variable. It is shown that the unique Bejan number (Be) can be derived from a classic fluid mechanics, whereas the other extensions of Bejan number (Be) for heat transfer or mass transfer are combinations of unique Bejan number (Be) with Prandtl (Pr) or Schmidt (Sc) numbers respectively. [ABSTRACT FROM AUTHOR]

Nano-encapsulated phase change materials (NEPCMs) are known to enhance the thermal characteristics of fluids; therefore, there is a rising interest in employing these materials in future thermal control systems. This paper investigates hydrodynamic and thermal characteristics of nanofluids, NEPCMs mixed in the host liquid, in glass balls as a porous structure. The geometry is a two-dimensional porous square cavity in which the left boundary is hot, the right boundary is cold, and the horizontal ones are considered to be insulated. The NEPCMs are composed of polyurethane (PU) as shell and nonadecane as a core. The impact of different non-dimensional parameters, such as Darcy number, 10−5 ≤ Da ≤ 10−1, porosity, 0.4 ≤ ɛ ≤ 0.9, Stefan number, 0.2 ≤ Ste ≤ 100, fusion temperature, 0 ≤ θ f ≤ 1, and volume fraction of the NEPCMs, 0 ≤ ϕ ≤ 0.05, is studied on the flow and heat transfer characteristics. It is shown that the volume fraction of NEPCMs is directly proportional to the strengthening of the heat transfer rate in such a way that applying 5% volume fraction of NEPCMs could enhance the heat transfer up to 20.1% and 14.1% at θ f = 0.5 in comparison to the cases of pure fluid and NEPCM mixture with no core-phase change, respectively. The effect of non-dimensional fusion temperature on the rate of heat transfer is also found to be noticeable. The maximum average Nusselt number emerges at θ f = 0.5, which is the optimum fusion temperature. [ABSTRACT FROM AUTHOR]

In this paper, heat transfer is investigated in thin metal films subjected to a short-pulse laser. Due to the short temporal scales and high intensity of laser pulses during the heating process, the non-Fourier phenomenon can be observed in the temperature response of thin metal films. Therefore, the fractional non-Fourier model has been applied to investigate the heat transport in thin metal films to capture the thermal behaviors of nano-scale metal films. The fractional non-Fourier model is presented to investigate the heat transfer in single-layer gold film and gold‑chromium double-layer films under short-pulse laser. The governing fractional heat conduction equation is solved numerically using the finite difference method based on implicit scheme, and the obtained results are compared to the experimental data. The numerical results obtained from the fractional model represent its accuracy and suitability to predict the thermal behavior of the thin metal films irradiated by short-pulse laser. In addition, upon comparing the results of the fractional model with the results of other models which have been employed in this field, it can be seen that the fractional model has a higher capability than others to examine the non-Fourier effects during laser heating of thin metal films. [ABSTRACT FROM AUTHOR]

In this paper, a mathematic model of a closed loop OHP with two branches was established to reveal the motion law of internal working fluid and the mechanism of heat transfer enhancement excited by different heat source modes, that is, uneven pulsing heat source mode (UP-mode), the even pulsing heat source mode (EP-mode) and the alternate heat source mode (A-mode). The simulation result shows that the vapor plug pressure and the liquid slug displacement both present the characteristic of variable amplitude fluctuation under the three heating modes. The amplitude generally changes periodically which is different from that of the continuous external heat source mode, which make the parameters above show additional fluctuation characteristics on the original oscillation law. The specific variation law is affected by the factors such as the heating period and heating interval. The three heat source modes introduced can enhance the transfer heat capability of the OHP under appropriate condition. But if the configuration condition is not suitable, the three modes above will weak the transfer heat ability, even result in that work stability of OHP deteriorates and stops. [ABSTRACT FROM AUTHOR]

In this paper heat transfer in a channel with parallel plates has been studied using Dissipative Particle Dynamics with energy conservation (eDPD). To simulate and present different thermal flows in finite geometries, a computer program was developed. The thermal boundary condition of the wall is assumed to be a constant temperature. Each particle in contact with the wall achieves the wall temperature. Also periodic boundary condition in the direction of the flow of the solution domain and no-slip boundary condition on the walls have been considered. The no-slip boundary condition is possible by freezing the layers of particles on the walls and applying bounce-back reflection. The results show dimensionless temperature and velocity profiles perpendicular to the direction of the flow along the channel. Finally, in this research it is tried to endeavored the fluctuations near the wall and promote the method by proposing different weight functions. Four different weight functions were used and the results on the velocity and temperature profiles were investigated. All of the results in the present work were compared and verified with the previous results. [ABSTRACT FROM AUTHOR]

In this paper, the classical collocation method has been revisited and modified by using the Chebyshev polynomials for solving nonlinear differential equations. Linear and nonlinear terms are converted to algebraical equations with the aid of the matrix relations. Resulting equations are solved to get unknown coefficients of rational Chebyshev polynomials. We apply the proposed technique for solving nonlinear heat transfer equations. Obtained results reveal that the rational Chebyshev collocation method can be safely applied to different types of nonlinear ordinary differential equations arising in science and engineering. [ABSTRACT FROM AUTHOR]

Liquid metal natural convection heat transfer under a transverse magnetic field is a fundamental problem in a fusion reactor. Several previous experimental and numerical studies have addressed this problem, however, few studies have focused in the development of correlations giving the heat transfer rate. The most important result in this problem is the existence of a maximum value of the heat transfer rate greater than its value in the absence of the magnetic field. In this paper, we propose to develop a correlation giving the heat transfer rate of the NaK liquid metal through a vertical wall heated by a uniform heat flux and subjected to a transverse magnetic field perpendicular to the thermal gradient. The scale analysis method was used to obtain the general form of the heat transfer rate correlation, while the previous experimental data were used to determine the various parameters and weight coefficients introduced. The results show that the proposed correlation predicts well the heat transfer behavior of the NaK liquid metal in the presence of a transverse magnetic field. [ABSTRACT FROM AUTHOR]

To improve the efficiency of full-spectrum correlated- k -distribution (FSCK), a new method FSCK-RSM has been proposed based on response surface methodology (RSM) in this paper. The k -distributions of FSCK was used to fit the response surface model based on radial basis function and the radiative calculation efficiency was improved by avoiding multiple computations and interpolation in the FSCK-RSM. The thermal radiation heat transfer of five combustion gases (H 2 O, CO 2 , CO, C 2 H 2 and C 2 H 4) in a one-dimensional layer was investigated and the radiative sources calculated by the LBL, SNB, FSCK and FSCK-RSM methods were given at different distributions of temperature and gas concentration. The results showed that the needed amount of input data was reduced by 677 times using FSCK-RSM comparing to the FSCK and the maximum of the average normalized deviation for FSCK-RSM was 2.46% in the non-isothermal homogeneous medium. The model was finally used for radiation reabsorption calculations in planar C 2 H 4 /O 2 /N 2 /CO 2 flames with full coupling to heat transfer and multi-species chemistry. The computational time using the FSCK-RSM was found to be at most half of that using the FSCK method. This FSCK-RSM model was an effective method for addressing the radiation problems that occur in combustion systems. [ABSTRACT FROM AUTHOR]

This paper presents experimental and numerical studies of heat transfer enhancement of water in an annulus under ultrasonic waves and fluid flow. An experimental setup is designed and built from two concentric pipes and a bolted Langevin ultrasonic transducer with a frequency of 25.7 kHz stuck to the inner pipe. Two new solvers are developed in OpenFOAM software and a numerical simulation is carried out to model the effect of both acoustic streaming and fluid flow on heat transfer and flow field. There are good agreements between the experimental and numerical results. Experimental results show that increasing the Reynolds number decreases the effect of acoustic streaming as compared to the effect of fluid flow on heat transfer and flow field. As a result, heat transfer enhances about 87% and 25% for Reynolds numbers of 32 and 674, respectively, at the transmitted acoustic power of 100 W. Numerical simulation is used to find the reason of heat transfer enhancement under ultrasonic waves, and results show that the cross-flow generated by circulations due to acoustic streaming is responsible for heat transfer augmentation. Unlabelled Image [ABSTRACT FROM AUTHOR]

A good knowledge of the effective thermal conductivity (ETC) of geomaterials is required to achieve accurate study on heat transfer in a variety of earth-contact engineering applications. On account of composite-porous characteristic of geomaterials, the thermal conductivity of each component is the foundation while the porous structure is the key influence factor. Accordingly, a fractal capillary bundle model at pore scale is proposed in this paper to predict the ETC of saturated/unsaturated composite-porous geomaterials. The combination of fractal theory and capillary bundle model hypothesis is used to describe the random, disordered and extremely complicated microstructure of geomaterials. The analytical solution of the ETC is obtained by using the thermal-electrical analogy technique, which demonstrates that the proposed model is a function of variety of composition and microstructure parameters. Both the effects of these two types of parameters on the ETC of geomaterials are analyzed theoretically. Compared with other models, the present model represents specific physical meaning and shows acceptable accuracy and performance. The model predictions are further compared with the available experimental data for three types of concrete, and a good agreement between them can be found in most of saturation degrees (0.2 < S r < 0.8). [ABSTRACT FROM AUTHOR]

Heat storage systems are used as a method for transferring and controlling the cold load in hot seasons. In this paper, to analyze the phase changes of an ice-on-coil thermal storage tank, the process of ice formation in a cavity with refrigerant carrier tubes is carried out using numerical simulation. Moreover, performance of some heat transfer enhancement methods including usage of fins around tubes with various dimensions and numbers and various tube number and arrangement, are compared. In this study, the inlet temperature is considered to be 275 K, which the temperature decrease up to 10 K due to freezing is observed. According to the results, the fin height is a very effective parameter in increasing the freezing rate. Also, upon increasing the tube number with smaller diameter, ice is formed with a higher speed due to their uniform distribution. So, with the tube number increasing from 9 to 16 in these systems at the same time interval, the fluid temperature drops from about 275 to 269 K, as well as the volume fraction of liquid decrease from about 0.4 to 0.22. Finally, by changing the tube arrangement from rectangular to triangular, faster rate of ice formation are observed. [ABSTRACT FROM AUTHOR]

Heat transfer performance of downward-facing flat heaters is summarized in this paper. The combined effects of material, orientation, dimension and pressure are considered. According to the overall trends, the results show complicated tendencies in which the parameters are interrelated. With the heat transfer characteristics, some CHF trends that have been observed in previous studies are analyzed. Some transitional phenomena occur with orientation, and a related analysis is conducted using a dry-spot concept. Various environments like pressurized conditions are considered in this study, including real accidental conditions. For the material effect, a real material, carbon steel, used for the severe accident mitigation strategies is considered. The overall heat transfer performance at the most critical locations of the severe accident strategies can be extracted from the results. [ABSTRACT FROM AUTHOR]

In this paper, a combined internal and external blade cooling system is conducted numerically to study the effect of different nozzles arrangements on flow and heat transfer performance. The applied operating conditions are different values of Reynolds number ranging from 5000 to 25,000 and different temperature ratios varied from 0.5 to 0.85. Results indicate that Nusselt number increases with the increase of Reynolds number at constant temperature ratio for all cooling models. The staggered normal cooling model achieves the highest overall Nusselt number with a maximum increase of 14.3% compared to the other cooling models. Overall film cooling efficiency increases with the increase in the corresponding blowing ratios. The staggered normal cooling model achieves superior overall cooling performance compared with the other cooling models. A correlation for overall Nusselt number is derived as a function of Reynolds number and temperature ratio. [ABSTRACT FROM AUTHOR]

This paper numerically investigates the effect of unipolar charge injection on the onset of Rayleigh-Bénard convection (RBC) in a layer of insulating liquid contained between two parallel plates. A recent developed unified model on the basis of the mesoscopic lattice Boltzmann method (LBM) is employed to solve the entire macro control equations. Two different cases of charge injection from above and from below are considered. It is interesting to find that the charge injection advances the onset of convection regardless of the injection direction. Besides, the decrease of critical Rayleigh number, along with the increase of flow intensity and the enhancement of heat transfer resulting from charge injection remain the same for different injection directions. However, the results show that the charge injection directions have significant effect on temperature and charge density distribution, as well as the evolution of flow motion, especially for the case of larger electric Rayleigh number. In addition, the effects of Prandtl number, the mobility number and the injection number are also studied. [ABSTRACT FROM AUTHOR]

The efficient cooling of turbo-generators especially in the end-windings region directly affects the power output rating. Modeling the coolant flow passing through the stator end-winding of a high-powered generator, however, remains a challenge due to a large number of parts with irregular geometries as well as the intricacy of cooling flow paths. This paper focuses on a numerical investigation of flow and heat transfer in stator end-winding aiming at replacing the geometry of winding with a porous medium model. Using computational fluid dynamics analysis to examine the viscous and inertial contributions to the pressure drop of airflow in the stator end-winding region, a generalized porous medium model is developed for the real physical geometry of winding together with a correlation for interfacial Nusselt number. The results indicate that the inertial effect considerably prevails over the viscous effect, that is, viscous contribution becomes small and almost negligible. It is shown that not only the pressure drop characteristics match closely within both real and porous models, but also the heat transfer results agree as well which means that the replaced porous model can amply describe the macroscopic behavior of the winding. [ABSTRACT FROM AUTHOR]

This paper investigates convective heat transfer of nanofluid flowing through a copper nanochannel. Ar − Cu Nanofluid is used to study in the nanochannel. Simulations are performed by LAMMPS code and the effects of nanoparticles presence on convective heat transfer are investigated. For this purpose, Nusselt number for nanofluid is calculated and compared with base liquid. The results indicate that the presence of nanoparticles increases the Nusselt number, which is due to the change in the velocity and structure of base liquid atoms. Furthermore, the investigation of Nusselt number for nanofluid shows that it is affected by diameter and volume concentration of nanoparticles. In a same volume concentration, with the increase in diameter, the ratio of the surface to the volume of nanoparticles decreases. This reduces the fluid atoms mobility caused by the presence of nanoparticles. Also, decreasing this ratio reduces the amount of solid-like layer in vicinity of the nanoparticles, which decreases the heat transfer capability in the nanochannel. However, for constant diameter, the Nusselt number increases with increasing volume concentration, which is due to the increase in the number of nanoparticles and existence of more space for fluid atoms absorption. [ABSTRACT FROM AUTHOR]

Phase change materials (PCM) have been widely used as latent energy storage media due to their useful high energy density with small storage volume at nearly constant temperatures throughout solidification and melting. In this paper, experimental evaluation of the melting of a paraffin wax (RT58) PCM inside a hemicylindrical storage cell is outlined. The cell is heated on its straight vertical wall by means of flowing hot water, while the curved side of the cell is thermally insulated. The temperatures of the PCM at various locations are measured using thermocouples, while a digital camera is used to visualise the melting process. A set of experiments is performed to examine the influence of variation of hot water temperature on various melting characteristics including the temperature field of the PCM, the progress of the melting front, the melt fraction, the stored energy, and the Nusselt number. The experimental findings reveal that increasing hot water temperature leads to a reduction of melting time and increases in the stored energy and the Nusselt number. The melting process is expedited by 15.4 and 16.5% when the temperature of hot water increases from 80 °C to 85 °C, and from 85 °C to 90 °C, respectively. [ABSTRACT FROM AUTHOR]

In order to investigate the flow and heat transfer characteristics of sister holes cooling method, sister holes cooling structures have been applied on a flat plate with four primary to sister hole diameter ratios and three inclination angles under three blowing ratios. Single cylindrical hole film cooling structure is also established as a benchmark structure. The numerical results of the single cylindrical hole case and one sister holes case are validated with the experimental results available in literatures. The tangential velocity vectors and total vorticity distributions are studied and compared. The spanwise averaged film cooling effectiveness and the adiabatic temperature contours are studied and compared. Results show that the development of the kidney vortex induced by primary hole is impeded by the sister hole induced kidney vortex and thus the film cooling performance is promoted. The blowing ratio and diameter ratio heavily affect the flow and heat transfer characteristics of sister holes cooling cases, the influence of the inclination angle is much smaller. In this paper, the best film cooling performance of sister holes cooling cases is obtained under the smallest diameter ratio. [ABSTRACT FROM AUTHOR]