Micro-structured surfaces have a significant impact on the flow boiling process in microchannels, but few numerical studies have been carried out due to their complex nature. In this study, the numerical investigation of flow boiling on micro-fin, micro-cavity, and smooth surfaces in a microchannel was conducted, with water serving as the working fluid. The volume-of-fluid (VOF) method, the phase change model, and solid-fluid thermal coupling were adopted in an OpenFOAM solver to perform the computation. The enhancing effects and detailed mechanisms of the micro-fin and micro-cavity surfaces on the heat transfer process are discussed, and the influences of wettability on these surfaces are investigated. With a contact angle of 60°, the heat transfer coefficient of the micro-fin surface was 61.92% larger, and the overall thermal resistance was 36.64% lower than that of a smooth surface, respectively. The confined bubbles on the micro-fin surface had a much smaller dryout area on the heated wall due to the capillary wetting effect. Moreover, the micro-fin surface with the rising nucleate bubbles can induce vortexes, which strengthens the convective heat transfer. As for the micro-cavity surface, it had a moderate heat transfer enhancement with a 17.16% larger heat transfer coefficient and a 13.55% lower thermal resistance when compared with a smooth surface. When the wettability of a heating surface is enhanced, the dryout area is minimized. Thus, the heat transfer performance of the smooth and micro-cavity surfaces is enhanced. The enhancement resulting from modified surface wettability has less of an effect on the micro-fin surface because the micro-fin array serves a similar function to minimize the dryout area.

The conjugate heat transfer of bubble growth during flow boiling in microchannel has a significant effect on the flow field and heat transfer performance but few studies analyzed it before. In this study, the vol -ume of fluid (VOF) method, Hardt's phase-change model, conjugate heat transfer between solid and fluid domains are adopted within an OpenFOAM solver to investigate the bubble growth and heat transfer per-formance in a microchannel with changed wall thickness from 5 mu m to 160 mu m and materials including silicon, aluminum, and copper. The results reveal that even if uniform heat flux is applied to the bottom wall, heat flux is not uniform at the solid-fluid interface due to the phase-change process in the channel. Conjugate heat transfer between the fluid and solid domain plays an important role in transferring the uniform heat flux from the bottom wall to the solid-fluid interface and homogenizing the solid-region temperature distribution, which cannot be ignored in the simulation of the phase-change phenomenon. When using different wall thicknesses, the bubble growth period differs by over two times. An optimum thickness exists for each material because the increasing wall thickness leads to a faster bubble growth rate but higher thermal resistance. With the same bottom wall thickness, the solid material with higher thermal diffusivity owns a faster bubble growth rate, thus a higher heat transfer coefficient. The optimum thickness decreases with increasing thermal diffusivity of the solid-domain material. (c) 2021 Elsevier Ltd. All rights reserved.

The flow boiling instability caused by confined bubbles generated during the flow boiling process in parallel microchannels leads to flow reversal and earlier critical heat flux (CHF), which is a major problem with the extensive application of two-phase flow in microchannel heat sinks. In this study, single bubble growth in one of the two nearby microchannels is investigated through numerical methods. The VOF method, Hardt's phase-change model, conjugate heat transfer between solid and fluid domains were adopted within a self-developed OpenFOAM solver. After the growing bubble being confined by the sidewalls, the flow path of the channel is blocked thus the inlet liquid flow tends to flow into the other channels instead of pushing the confined bubble, which caused the confined bubble to extend towards upstream and flow reversal. By increasing the mass flux, the flow reversal is suppressed, but the flow boiling heat transfer enhancement is minimized due to the less evaporation area. The method of combination of microchannel and microgap (CMC) has little effect on flow reversal until the bubble enters the microgap. And its thermal performance is worse than the original heat sink. The confined bubble in the finned microchannel (FMC) heat sink would flow into the nearby channel through the secondary channel between the fins thus prevent flow reversal. Furthermore, the evaporation area between the bubble and solid walls increases, thus significantly lower thermal resistance (for about 40%) can be obtained. Considering the overall flow stability and heat transfer performance, the finned microchannel is recommended to be fabricated in industrial applications.

The manifold microchannel (MMC) heat sink for high-heat-flux removal in next-generation microelectronic system has received a significant attention recently. A numerical study is performed to analyze thermal performance and pressure loss of subcooled flow boiling in an MMC unit cell model. On the basis of OpenFOAM package, a new solver is developed for solving subcooled flow boiling and solid-fluid heat transfer. The simple coupled volume of fluid with level set (S-CLSVOF) method is used to capture the liquid-vapor interface during phase change. After validating the numerical approach with experimental data, effects of microchannel width w(c) and fin width w(f) on average chip wall temperature and inletto-outlet pressure drop are discussed. Seven MMC samples with different size of channel widths and fin widths are studied at inlet volume flow rates of 19, 31 and 42 mL/min as wall heat flux is fixed at 400 W/cm(2). The results indicate that decreasing w(c) and w(f) will lead to low average wall temperature on the heated wall, but pressure drop between inlet and outlet surfaces will rise dramatically. When the total number of channels of the MMC heat sink remains unchanged, increasing w(c) leads to decreasing w(f); the thermal resistance of the MMC heat sink is gradually increased while the pressure drop is reduced (e.g. +0.16 degrees C/W, -656 kPa at 42 mL/min). (C) 2020 Elsevier Ltd. All rights reserved.

A numerical investigation was conducted to determine the effects of fin shape on heat transfer coefficients during in-tube condensation of R410A. Four micro-fin tubes of different geometries were selected for this study, including a straight-finned one and three helical tubes, and two different helical angles of 18 degrees and 31 degrees were included to explore the influence of helical angles. The numerical results were verified by previous experimental results. It was found that there was a positive relationship between heat transfer coefficients and mass velocities and vapor quality, while the helical angle seemed to improve the heat transfer coefficients especially for medium vapor quality. A nearly symmetrical liquid film distribution was found along the circumference for all three micro-fin tubes due to the centrifugal force, while the liquid tends to accumulate at the bottom of the straight-finned tube.

A fundamental numerical investigation on hydrodynamics and heat transfer performance of annular flow boiling in a rectangular microchannel with large width-to-depth ratio is performed in the paper. The saturation temperature recovery model based on the volume of fluid (VOF) method is implemented in OpenFOAM package to explore the heat transfer characteristics in annular flow regime with a three-dimensional computational domain. Validation of our methods has been conducted by comparing numerical results with experimental data. Effects of operating conditions including inlet mass flux, wall heat flux and inlet vapor quality are further discussed. It is found that the increase in wall heat flux or inlet quality will lead to the decrease of liquid thin film thickness between the interface and the heating wall. As the interfacial temperature is fixed at the saturation temperature, thinner liquid film will result in larger local heat transfer coefficient and lower wall temperature, which improves the heat transfer performance of the narrow microchannel. However, increasing the inlet mass flux will decrease the wall heat transfer coefficient, which is in accordance with the experimental result. (C) 2019 Elsevier Ltd. All rights reserved.

An experimental investigation was conducted to evaluate heat transfer performance of several enhanced surface tubes during in-tube evaporation and condensation of R410A; results were then compared to the results of a smooth tube. Tubes considered in this evaluation included: smooth; herringbone and helix micro-grooves; herringbone-dimple and hydrophobic; all the tubes evaluated have the same external diameter of 12.7 mm. Experimental condensation and evaporation results were acquired at saturation temperatures of 318 K and 279 K, respectively. The mass velocities varied in the range of 40-230 kg m(-2)s(-1); vapor quality decreased from 0.8 to 0.2 for condensation and 0.2 to 0.8 for evaporation. Moreover, heat fluxes increased with mass velocity. Condensation heat transfer coefficients (HTCs) are enhanced by 40%-73%, with the dimpled herringbone-finned tube (EHT-HB/D) exhibiting the highest HTC among five tested tubes. The herringbone grooves can help lift the accumulated condensate along the circumference in addition to producing the drainage effects; dimples produce condensate turbulence and the droplet entrainment. For the evaporation, hydrophobic herringbone tube (EHT-HB/HY) provides the best thermal performance; its HTCs are 4%-46% larger than those of the smooth tube. This enhancement may be attributed to the expanded heat transfer area and the increased nucleation sites.

A turbulent transition model has been applied to fluid flow problems that can be laminar, turbulent, transitional, or any combination. The model is based on a single additional transport equation for turbulence intermittency. While the original model was developed for external flows, a slight modification in model constants has enabled it to be used for internal flows. It has been successfully applied to such flows for Reynolds numbers that ranged from 100 to 100,000 in circular tubes, parallel plate channels, and circular tubes with an abrupt change in diameters. The model is shown to predict fully developed friction factors for the entire range of Reynolds numbers as well as velocity profiles for both laminar and turbulent regimes.

An experimentally validated computational model is used to determine quantitative information on convective heat transfer coefficients on all of the external surfaces of a generic residence. The motivating application for this investigation is the need for overall convective heat transfer coefficients to enable accurate design and analysis for devices such as roof top solar panels. The numerical solutions provide detailed distributions of the local heat transfer coefficient on all of the surfaces of the residence that are otherwise difficult to obtain by experimental measurements alone. The unsteady fluid flow behavior around residential buildings and its affect on heat transfer were found to be significant at higher wind speeds.

A device has been designed, constructed, and tested for heating fluids using solar energy. The device heats water to levels to kill pathogens by a parabolic reflecting surface that concentrates solar energy along an axis. Among the components that increase the thermal performance of the system is a thermally actuated valve, which controls the temperature and the thermal exposure duration of the fluid to cause deactivation of targeted pathogens. Also, a novel fluid-to-fluid heat exchanger arranged in counter flow is used. Experiments were performed with a water solution containing non-pathogenic Escherichia coli K-12 MG1655 (E. coli) bacteria. The results showed that the system is capable of pasteurization to levels where no living pathogens were detected in the heated fluid. The experiments were carried out over a wide range of temperatures and exposure durations to test the device and the underlying mathematical model. E. coli log reductions greater than 1 were achieved in all cases and it is shown that arbitrary values of reduction can be achieved with appropriate temperature/time settings.

This paper presents fundamental research on the hydrodynamics and heat transfer surrounding a single elongated bubble during flow boiling in a circular microchannel. A continuum surface force (CSF) model based on the volume of fluid (VOF) method is combined with the thermocapillary force to explore the effects of thermocapillarity for flow boiling in microchannels. To validate the self-defined codes, a two-phase thermocapillary-driven flow and a Taylor bubble growing in a capillary tube are studied. Results of both test cases show good convergence and agreement with data from the earlier literature. The bubble motion and the local heat transfer coefficient (HTC) on the heated wall with respect to time are discussed. It is found that for large Marangoni number (case 3), variation of surface tension has affected the bubble shape and temperature profile. The thermocapillary effect induces convection in a thin liquid film region, which augments the HTCs at specified positions. The numerical investigation also shows that the average HTC increased by 6.7% in case 3 when compared with case 1. Thus, it is very important to study further the effects of themocapillarity and the Marangoni effect on bubble growth in microchannels.

Wavy fins have been considered as an alternative of the straight fins in compact heat exchangers (CHEs) for better heat transfer performance, which can be augmented by considering vortex generators (VGs). This work is related to numerical investigation and optimization of corrugation height of fin and angle of attack of delta winglet type VGs in a wavy fin-and-tube heat exchanger. For this purpose, three-dimensional (3D) Reynolds-averaged Navier-Stokes analysis and a multi-objective genetic algorithm (MOGA) with surrogate modeling are performed. Numerical simulation is carried out to study the effect of delta winglets with varying the corrugation height of wavy fin in three rows of tubes with staggered tube arrangements. The corrugation height (H) and angle of attack (α) vary from 0.3 mm to 1.8 mm and 15 deg to 75 deg, respectively. Results are illustrated by investigating the flow structures and temperature contours. Results show that increasing the corrugation height of wavy fin and angle of attack of delta winglets enhances the heat transfer performance of heat exchanger while friction factor is also increased. Employing delta winglets has augmented the thermal performance for all corrugation heights and superior effect is observed at a higher corrugation. To achieve a maximum heat transfer enhancement and a minimum pressure drop, the optimal values of these parameters (H and α) are calculated using the Pareto optimal strategy. For this purpose, computational fluid dynamics (CFD) data, a surrogate model (neural network), and a multi-objective GA are combined. Results show that optimal orientation of delta winglets with respect to corrugation height can improve both the thermal and hydraulic performance of the heat exchanger.

A numerical-based model was developed and implemented to determine the spatial and temporal temperature distributions within skin tissue resulting from thermal contact with a heated and high thermal conductivity metallic medium. In the presence of wet tissue, boiling is likely to occur, thereby affecting the probability of inducing burns. This investigation deals with how contact between a hot, highly conductive metallic material and skin gives rise to burns. In particular, the study focuses on the likelihood that metals typically used in cooking or industrial applications may cause burns. Insofar as the surfaces under consideration are above the boiling temperature of water, a mathematical model including phase change was developed. That model allowed different thermophysical properties to be respectively employed for dry and wet tissues. Multiple processes and their governing parameters were investigated to assess their impact on burn severity, including the temperature of the metal, the duration of contact, the contact resistance between the surface and the skin, the temperature range over which phase change occurred, and the cooling environment after the exposure. It was discovered that the most important parameters are the surface temperature and exposure duration. The other conditions/parameters had lesser impacts on the results.

1.High-Perfomance Computing for Fluid Flow and Heat Transfer 2.Unstructured Finite Volume Methods for Multi-Mode Heat Transfer 3.SpectralElement Methods for Unsteady Fluid Flow and Heat Transfer in Complex Geometries:Methodology and Applications 4.Finite-Volume Method for Radiation Heat Transfer 5.Boundary Element Methods for Heat Conduction 6.Molecular Dynamics Method for Microscale Heat Transfer 7.Numerical Methods in Microscale Heat Transfer:Modeling of Phase-Change and Laser Interactions with Materials 8.Current Status of the Use of Parallel Computing in Turbulent Reacting Flows:Computations Involving Sprays, Scalar Monte Carlo Probability Density Function and Unstructured Grids 9.Overview of Current Computational Studies of Heat Transfer in Porous Media and Their Applications-Forced Convection and Multiphase Heat Transfer 10.Overview of Current Computational Studies of Heat Transfer in Porous Media and Their Applications-Natural and Mixed Convection 11.Recent Progress and Some Challenges in Thermal Modeling of Electronic Systems 12.Index