Slip flow forced convection in a microchannel with semi-circular cross-section

This paper offers theoretical results for fully developed slip-flow forced convection through a microchannel of semicircular cross-section. Numerical results are also presented to study the developing region. Velocity slip and temperature jump boundary conditions are applied at the uniformly heated walls. The results from the two different sources are cross validated and those pertaining to the limiting case of no-slip flow are found to be in good agreement with those available in the literature.     

Numerical simulation of roughness effects on flow and heat transfer in microchannels at slip flow regime

A numerical simulation for studying fluid flow and heat transfer characteristics in microchannels at slip flow regime with consideration of slip and temperature jump is studied. The wall roughness is simulated in two cases with periodically distributed triangular microelements and random shaped micro peaks distributed on the wall surfaces. Various Knudsen numbers have used to investigate the effects of rarefaction. The numerical results have also checked with available theoretical and experimental relations and good agreements has achieved. It has been found that rarefaction has more significant effect on flow field in microchannels with higher relative roughness. The negative influence of roughness on fluid flow and heat transfer found to be the friction factor increment and Nusselt number reduction. In addition high influence of roughness distribution and shape has been shown by a comparison of Poiseuille and Nusselt numbers for tow different cases.     

Effects of thermal property variations on the liquid flow and heat transfer in microchannel heat sinks

Three-dimensional conjugate numerical simulations using the inlet, average and variable thermal properties respectively were performed for the laminar water flow and heat transfer in rectangular microchannels with D_h of 0.333 mm at Re of 101–1775. Both average and variable properties are adopted in data reduction. The calculated local and average characteristics of flow and heat transfer are compared among different methods, and with the experiments, correlations and simplified theoretical solution data from published literatures. Compared with the inlet property method, both average and variable property methods have significantly lower f_app, but higher convective heat transfer coefficient h_z and Nu_z. Compared with the average property method, the variable property method has higher f_appRe_ave and lower h_z at the beginning, but lower f_appRe_ave and higher h_z at the later section of the channel. The calculated Nu_ave agree well with the Sieder-Tate correlation and the recently reported experiment, validating the traditional macroscale theory in predicting the flow and heat transfer characteristics in the dimension and Re range of the present work.     

Slip-flow and heat transfer of gaseous flows in the entrance of a wavy microchannel

The present work investigates the developing fluid flow and heat transfer through a wavy microchannel with numerical methods. Governing equations including continuity, momentum and energy with the velocity slip and temperature jump conditions at the solid walls are discretized using the finite-volume method and solved by SIMPLE algorithm in curvilinear coordinate. The effects of creep flow and viscous dissipation are assumed. The numerical results are obtained for various Knudsen numbers. The results show that Knudsen number has declining effect on both the C_f.Re and Nusselt number on the undeveloped fluid flow. Significant viscous dissipation effects have been observed for large Knudsen number. Also, viscous dissipation causes a singular point in Nusselt profiles.     

Gap conductance in contact heat transfer

At low contact pressures, conduction across the gas gap is the predominant mode of heat transfer in a joint. Experimental results are presented for the solid spot and the gap conductance for a range of surface finishes and several interstitial gases and gas mixtures. The mean separation distance is then estimated as the difference between the effectiive gap thickness and the temperature jump distance. It is seen that a simple relation exists between the mean separation distance and surface roughness for all the gases and gas mixtures. This correlation satisfies 85% of data points to within ±4%.     

Second-law analysis of flow and heat transfer inside a microannulus

The purpose of this work is to investigate the entropy generation in a microannulus flow. Fully developed laminar flow is considered with uniform heat flux at the walls. The viscous dissipation effect, the velocity slip and the temperature jump at the wall are taken into consideration. The velocity and temperature profiles are obtained analytically and used to compute the entropy generation rate. The effects of Kn, Br, Br/Ω and r? on velocity, temperature profiles, entropy generation rate and average entropy generation are discussed. The present analytical results for the case with and without the viscous dissipation effect are compared with those available in the literature and an excellent agreement is observed. Entropy generation is shown to decrease with an increase in Kn while increasing Br, Br/Ω and r? results in increasing entropy generation.     

Unsteady flow and heat transfer of a ferrofluid between two rotating,vertical moving and stretching disks

Unsteady flow and heat transfer of a magnetic fluid between two rotating disks is investigated. Both the disks are stretchable and the lower disk moves in the vertical direction. A new approach of similarity transformation is adopted to transform the equation of continuity, momentum, and the energy equation into ordinary nonlinear coupled differential equations. The numerical solution of the converted nonlinear differential equations is obtained using the finite element method. The effects of magnetization force, rotational viscosity, Prandtl number, and Eckert number on the velocity and temperature distributions are studied. The impact of stretching, movement, and rotation of the disk is also considered in this computational study. The skin friction coefficients and heat transfer rate on the lower disk for different physical parameters are calculated. Different types of motion of the disks and the magnetization force are crucial aspects in the stress distribution and heat transfer rate near the lower disk.     

微通道内气体-近壁颗粒流动传热数值模拟研究

数值模拟了微通道受限空间内气体-近璧颗粒流动与传热过程,所建模型考虑微尺度气体的可压缩与交物性特征,且在通道和颗粒壁面采用速度滑移和温度跳跃边界条件以考虑滑移区气体动量/能量非连续效应.在此基础上,计算分析了克努森数(Kn)和颗粒偏移比对颗粒表面拖曳力系数(CD)以及传热努塞尔数(Nu)的影响规律.研究结果表明:受气体...     

Finite element analysis of the flow and heat transfer of solid particles in moving beds

A numerical analysis for the flow and heat transfer of solid particles in moving beds of heat exchangers is presented. The solid particles pass through a bundle of heat source tubes as the result of the gravitational force. Heat energy is transferred through direct contact of particles with the heat source tubes. A viscous-plastic fluid model and a convective heat transfer model are employed in the analysis. The flow field dominantly determines the total heat transfer in the heat exchanger. As the velocities of solid particles around the heat source tubes increase, the heat transfer from the tubes also increases. Examples are presented to show the performance of the numerical model. The effect of flow on heat transfer has also been studied. © 1998 John Wiley & Sons, Ltd.     

Thermal modeling and design for microchannel cold plate with high temperature uniformity subjected to multiple heat sources

A compact thermal model was established to model thermal characterization of a microchannel cold plate for high temperature uniformity in multiple heat sources. Analytical calculation of the compact model was presented. Experiments were also conducted to validate the model. The comparison reveals that the measured temperatures are close to the ones predicted by the compact thermal model. The cold plate can help multiple heat sources achieve high temperature uniformity, and the maximum temperature difference among the heat sources is 1.3 °C in the present experimental cases. Moreover, a straight minichannel cold plate was designed and tested under the same flow rate. The measured results show that there exists relatively large temperature gap among the heat sources, and the maximum value is 6.7 °C.     

Chemical reaction and Dufour effects on nonlinear free convection heat and mass transfer flow near a vertical moving porous plate

A theoretical analysis is made for steady fully developed free convection and mass transfer flow near an infinite vertical moving porous plate by taking into consideration the first‐order chemical reaction and Dufour effects. The mathematical model responsible for the present physical situation is based on the nonlinear density variation with temperature as well as nonlinear density variation with concentration. Exact solutions are derived for heat mass and momentum equations under relevant boundary conditions. The dimensionless velocity, temperature, and concentration are presented in terms of exponential functions. The impact of controlling parameters such as Dufour number (diffusion thermo effect), chemical reaction parameter, Prandlt number, Schmidt number, on velocity, temperature, Nusselt number, and skin friction are discussed with the aid of line graphs, contours, and tables. The analysis of the result shows that Nusselt number, skin friction, and velocity increases with increase in Dufour number. Furthermore, velocity and skin friction are higher in case of nonlinear convection in comparison to linear convection.     

Slip flow and heat transfer in microbearings with fractal surface topographies

The effects of random surface roughness on slip flow and heat transfer in microbearings are investigated. A three-dimensional random surface roughness model characterized by fractal geometry is used to describe the multiscale self-affine roughness, which is represented by the modified two-variable Weierstrass–Mandelbrot (W–M) functions, at micro-scale. Based on this fractal characterization, the roles of rarefaction and roughness on the thermal and flow properties in microbearings are predicted and evaluated using numerical analyses and simulations. The results show that the boundary conditions of velocity slip and temperature jump depend not only on the Knudsen number but also on the surface roughness. It is found that the effects of the gas rarefaction and surface roughness on flow behavior and heat transfer in the microbearing are strongly coupled. The negative influence of roughness on heat transfer found to be the Nusselt number reduction. In addition, the effects of temperature difference and relative roughness on the heat transfer in the bearing are also analyzed and discussed.     

Conduction and entrance effects on laminar liquid flow and heat transfer in rectangular microchannels

The paper presents both three and two-dimensional numerical analysis of convective heat transfer in microchannels. The three-dimensional geometry of the microchannel heat sink followed the details of the experimental facility used during a previous research step. The heat sink consisted of a very high aspect ratio rectangular microchannel. Two channel spacings, namely 1 mm and 0.3 mm (0.1 mm), were used for three-dimensional (two-dimensional) numerical model, respectively. Water was employed as the cooling liquid. The Reynolds number ranged from 200 to 3000. In the paper, thermal entrance effects and conduction/convection coupling effects are considered both for the test case of uniform channel inlet conditions and the complete geometry of the experiment. Finally, the comparison between measured and computed heat flux and temperature fields is presented. Contrary to the experimental work, the numerical analysis did not reveal any significant scale effect on heat transfer in microchannel heat sink down to the smallest size considered (0.1 mm).     

Subcooled flow boiling heat transfer in narrow passages

Subcooled flow boiling heat transfer coefficients for refrigerants R11 and HCFC123 in smooth copper tubes of small diameter have been investigated experimentally. The parameter ranges examined are: tube diameters of 0.92 and 1.95 mm; heat fluxes 11-170 kW m⁻²; mass fluxes 110-1840 kg m⁻² s⁻¹. The range of liquid Reynolds numbers encompassed by the data set is 450 to 12,000.The data in the subcooled and saturated regions are well represented by the simple addition of convective and nucleate boiling heat transfer contributions

     

Numerical studies on flow and heat transfer in membrane helical-coil heat exchanger and membrane serpentine-tube heat exchanger

The flow and heat transfer characteristics of synthesis gas (syngas) in membrane helical-coil heat exchanger and membrane serpentine-tube heat exchanger under different operating pressures, inlet velocities and pitches are investigated numerically. The three-dimensional governing equations for mass, momentum and heat transfer are solved using a control volume finite difference method. The realizable k-ε model is adopted to simulate the turbulent flow and heat transfer in heat exchangers. There flows syngas in the channels consisting of the membrane helical coils or membrane serpentine tubes, where the operating pressure varies from 0.5 to 3.0 MPa. The numerically obtained heat transfer coefficients for heat exchangers are in good agreement with experimental values. The results show that the syngas tangential flow in the channel consisting of membrane helical coils is significant to the heat transfer enhancement to lead to the higher average heat transfer coefficient of membrane helical-coil heat exchanger compared to membrane serpentine-tube heat exchanger. The syngas tangential velocity in the membrane helical-coil heat exchanger increases along the axial direction, and it is independent of the gas pressure, increasing with the axial velocity and axial pitch rise and decreasing with the radial pitch rise.     

Microchannel heat transfer and dispersion of nanoparticles in slip flow regime with constant heat flux

If the hydrodynamic diameter of a channel is comparable with the mean free path of the gas molecules moving inside the channel, the fluid can no longer be considered to be in thermodynamic equilibrium and a variety of non-continuum or rarefaction effects can occur. To avoid enormous complexity and extensive numerical cost encountered in modeling of nonlinear Boltzmann equations, the Navier–Stokes equations can be solved considering the concepts of slip flow regime and applying slip velocity boundary conditions at the solid walls.     

Numerical simulation of fluid flow and heat transfer in a microchannel heat sink with offset fan-shaped reentrant cavities in sidewall

The paper is focused on the investigation of fluid flow and heat transfer characteristics in a microchannel heat sink with offset fan-shaped reentrant cavities in sidewall. In contrast to the new microchannel heat sink, the corresponding conventional rectangular microchannel heat sink is chosen. The computational fluid dynamics is used to simulate the flow and heat transfer in the heat sinks. The steady, laminar flow and heat transfer equations are solved in a finite-volume method. The SIMPLEX method is used for the computations. The effects of flow rate and heat flux on pressure drop and heat transfer are presented. The results indicate that the microchannel heat sink with offset fan-shaped reentrant cavities in sidewall improved heat transfer performance with an acceptable pressure drop. The fluid flow and heat transfer mechanism of the new microchannel heat sink can attribute to the interaction of the increased heat transfer surface area, the redeveloping of the hydraulic and thermal boundary layers, the jet and throttling effects and the slipping over the reentrant cavities. The increased heat transfer surface area and the periodic thermal developing flow are responsible for the significant heat transfer enhancement. The jet and throttling effects enhance heat transfer, simultaneously increasing pressure drop. The slipping over the reentrant cavities reduces pressure drop, but drastically decreases heat transfer.     

Numerical simulation of heat transfer in a gas solid crossflow moving packed bed heat exchanger

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Study of heat and mass transfer in integrated thermal management controller (ITMC) employed in waste heat recovery application

In the waste heat recovery process, heat source temperature control and thermal management are always required to ensure safety and high efficiency of the waste heat recovery system. To this aim, the conventional method is to establish a series of independent heat transfer units and adopt a complex control strategy, which makes the system very complex and only applicable for a specific object. The concept of “integrated thermal management controller” (ITMC) is presented in this work to provide a novel method to solve the above problems. A two-dimensional heat and mass transfer model is developed to analyze and predict the operation performance of the ITMC. The numerical analysis indicates good heat source temperature control and thermal management performance of the ITMC. In addition, an experimental prototype is established, and test data are presented, which agree well with the numerical results and verify the correctness of the model.