平行流热管管内流动与传热的数值模拟研究

为研究平行流热管的工作机理,本文基于Fluent软件中的VOF模型编写了蒸发冷凝相变的UDF程序,对不同功率下平行流热管管内两相流动和传热过程进行了数值模拟研究。模拟结果显示了初始阶段平行流热管管内的气液分布,启动阶段管内包括泡状流、弹状流、环状流等复杂流型的转变过程,稳定工作阶段工质在各并联管路中互激振荡流动。在高加热功率下,管内工质的互激振荡流动更为剧烈,热量输送距离更远。研究结果为平行流热管换热器的优化设计提供了参考依据。     

Heat transportation by oscillatory flow with steady flow component

This paper deals with heat transportation by an oscillatory flow composed of a sinusoidal oscillatory flow superimposed with a steady flow. Velocity and temperature fields, heat transportation rate, work rate, and heat transportation efficiency were investigated through numerical analysis. Results obtained elucidated that (1) the phase difference between velocity and temperature variation remained the same as that of the sinusoidal reciprocal flow without the use of a steady flow component. (2) In the upstream direction heat was mainly transported by the steady flow component and in the downstream direction transportation was mainly performed by the oscillatory flow component. (3) The heat transportation rate of the present oscillatory flow composed of both steady and oscillatory flow components was less than the arithmetic sum of the rates produced by the steady flow and the sinusoidal oscillatory flow. (4) The heat transportation rate was increased immensely by superimposing the steady flow on the sinusoidal oscillatory flow. (5) Conversely, work done by the present oscillatory flow increased only slightly. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(7): 482–500, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20130     

Augmentation of heat transportation by an oscillatory flow in grooved ducts

This paper deals with heat transportation by oscillatory flow in grooved ducts. The heat transportation rate, work rate, heat transportation efficiency, and dispersions of fluid particles and heat were analyzed with the computer code FLUENT. The frequency and tidal amplitude of the oscillatory flow were 0.05 Hz and 45 mm, respectively. The internal diameters of the contraction and expansion sections of the grooved ducts were 6 and 12 mm, respectively. The groove lengths varied from 0 to 40 mm and the pitch of the grooves was fixed at 10 mm. We found that: (1) The grooved duct with S=15 mm had the highest heat transportation rate, which was about 4.5 times that of the smooth round pipe. (2) The grooved duct with S=20 mm had the greatest heat transportation efficiency, which was about 6.4 times that of the smooth round pipe. (3) Enthalpy transportation by the dispersion motion of fluid particles played a substantial role in the heat transportation of the grooved ducts with 5 mm≤S≤40 mm. (4) The grooved duct with S=10 to 15 mm had the greatest dispersion of fluid particles, which explained their having the greatest heat transportation rate. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(2): 68–85, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20192     

Oscillation-controlled heat-transport tube (heat transfer coefficient in tubes in heating and cooling regions)

In the present study, heat transfer coefficients of oscillatory flow at inner surfaces of the heating and cooling regions of oscillation-controlled heat-transport tubes (OCHTs) are investigated numerically. The numerical simulation is conducted under the following three conditions for the tube walls at the heating and cooling regions: isothermal, extremely thin, and actual wall systems. Based on the numerical results and Hausen's correlating equation for laminar flow heat transfer in tubes, a correlating equation of the heat transfer coefficient is developed which can be generally applied to these three conditions. Next, using this correlating equation and the authors' simplified model of overall thermal resistance in OCHTs, heat-transport rates are predicted, and it is found that the predicted results are in good agreement with the numerical results. Finally, numerical simulation is conducted also to compare the heat-transport rates of OCHTs with those of the conventional forced circulation type under the same pumping power. The results indicate that there exist oscillatory flow regions in which the heat-transport rate of the phase-shifted OCHTs is larger than that of the conventional circulation type. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(6): 415–430, 1998     

Convective heat transfer induced by fluid oscillations

Numerical simulation was conducted on the heat transfer process in a two-dimensional flow field imposed by a sinusoidal oscillation of the fluid. When gravitational force was ignored, the heat transfer at the wall was similar to forced-flow turbulent heat transfer. On the other hand, considering gravity, two modes of mixed convection heat transfer were observed depending on the thermal boundary conditions. For high-viscosity fluids, a bifurcation of the flow pattern appeared, which was mainly determined by the oscillation amplitude. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 26 (1): 39–53, 1997     

物性参数对环形空间流动和传热影响的模拟研究

利用有限容积法,建立了环形空间内单相流体竖直向上流动过程中流动和传热的稳态模型。模型将环形空间内管设置为具有固定生热速率的发热体;流体与内管壁之间设置流动和传热边界层,以更精确的描述壁面位置流体与固体之间动量和热量的耦合传递过程。通过与常物性模型的对比,流体密度、导热系数和黏度随温度变化的变物性模型,在传热能力上具有一定的减少,流体与固体传热面之间的界面剪切力稍有下降。通过比较常物性模型和变物性模型的Re和Ri,结果表明,随着流体强制循环速度的加大,流体物性变化对流动和传热过程的影响逐渐减小。     

滑移区气体-颗粒流动与传热特性研究

针对气体-颗粒微尺度流动与传热过程开展数值模拟研究,所构建模型中气体处理为可压缩、变物性流体,并在颗粒表面采用速度滑移和温度跳跃边界条件以考虑气体稀薄效应。在数值模拟基础上,研究分析稀薄效应对颗粒与其周围气体流动与换热的影响程度,并进一步提出新的阻力系数与传热努谢尔特数关联式。研究结果表明,气体稀薄效应将减小颗粒阻力系数,同时抑制颗粒与其周围气体的传热过程。     

Flow and heat transfer characteristics in pulsating pipe flows (effects of pulsation on internal heat transfer in a circular pipe flow)

Effects of pulsation on flow and heat transfer characteristics are experimentally examined in the pulsating pipe flows having sinusoidal velocity fluctuations around a nonzero mean. By systematically varying three pulsation parameters (the amplitude, frequency, and mean velocity), time-averaged and fluctuating temperature profiles are measured under the heating condition of constant wall temperature using saturated vapor. The mean Nusselt number, Nu_p, is calculated, and compared with that in ordinary turbulent pipe flows without pulsation. The results show that Nu_p, decreases initially as the pulsation amplitude increases, then recovers gradually, and finally becomes much greater than the original value. In pulsating pipe flows with a nonzero mean velocity, therefore, pulsation cannot always promote heat transfer, but sometimes suppresses it, depending mainly on the pulsation amplitude and mean velocity. It is also found that these heat transfer characteristics of a pulsating pipe flow are controlled by the transition of flow patterns with pulsation amplitude from a fully turbulent flow to a conditionally turbulent flow via a transitional flow. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25(5): 323–341, 1996     

An Investigation of the Swirling Flow and Heat Transfer in a Duct

This paper presents the results of experiments and numerical simulation of the turbulent swirling flow and heattransfer in a duct.The calculated results are in good agreement with data obtained by measurements.It isfound that the swirling flow promotes heat transfer to the wall of the duct;the swirl numbers are dependentupon the vane exit angles of the swirler,distance from the swirler and the duct Reynolds number.But the decayof swirling flow in streamwise direction is related to local Reynolds numbers and is independent of the swirlerexit angle.The swirl flow characteristics presented in this paper may be used for engineering purposes.     

开缝型翅片流动与传热三维数值模拟

通过数值模拟,研究空调系统使用的开缝型翅片的传热与阻力特性。对三种型式的开缝型翅片进行模拟,得出了流场和温度场。通过对比分析发现,双边交替开缝的slit-2型翅片,换热性能最好,X型双向开缝片的性能次之,单边开缝的slit-1型翅片换热效果低于前两种。数值模拟还得出,空气流过slit-x型翅片的阻力最大,流过slit-1型翅片的阻力最小。     

微尺度通道内流动沸腾研究综述

阐述了微尺度通道内传热问题出现的工程背景——高密度微电子器件的冷却。对当前国内外微尺度通道内流动沸腾换热特性的研究现状进行了归纳。突出分析了工质种类、微尺度通道的几何参数和工质的工况参数等对微尺度通道内流动沸腾换热特性的影响。同时分析了微尺度通道内流动沸腾换热的强化机理、流动阻力特性、压降关联式和沸腾换热关联式的理论和实验研究。最后根据分析对今后的工作提出了一些建议。     

A spatially advancing turbulent flow and heat transfer in a curved channel

Direct numerical simulation was performed for a spatially advancing turbulent flow and heat transfer in a two‐dimensional curved channel, where one wall was heated to a constant temperature and the other wall was cooled to a different constant temperature. In the simulation, fully developed flow and temperature from the straight‐channel driver was passed through the inlet of the curved‐channel domain. The frictional Reynolds number was assigned 150, and the Prandtl number was given 0.71. Since the flow field was examined in the previous paper, the thermal features are mainly targeted in this paper. The turbulent heat flux showed trends consistent with a growing process of large‐scale vortices. In the curved part, the wall‐normal component of the turbulent heat flux was twice as large as the counterpart in the straight part, suggesting active heat transport of large‐scale vortices. In the inner side of the same section, temperature fluctuation was abnormally large compared with the modest fluctuation of the wall‐normal velocity. This was caused by the combined effect of the large‐scale motion of the vortices and the wide variation of the mean temperature; in such a temperature distribution, large‐scale ejection of the hot fluid near the outer wall, which is transported into the near inner‐wall region, should have a large impact on the thermal boundary layer near the inner wall. Wave number decomposition was conducted for various statistics, which showed that the contribution of the large‐scale vortex to the total turbulent heat flux normal to the wall reached roughly 80% inside the channel 135^° downstream from the curved‐channel inlet. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20275     

Research on a new heat transfer model for liquid film convective evaporation in annular flow

Based on the phenomenon of turbulence restraint in liquid‐vapor interface, an analytical model is proposed for annular flow with a velocity distribution. The liquid‐vapor interface affecting district mixing length model was amended, and a new liquid film convective evaporation heat transfer model at the annular flow was developed. Compared with the experimental data, the results show that the new model is better than the model based on full tube flow velocity distribution. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 524–530, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10051     

Enhanced heat transfer using oscillatory flows in solar collectors

In this work, we propose the use of oscillatory laminar flows to enhance the transfer of heat from solar collectors. The idea is to explore the possibility of transferring the heat collected from a solar device to a storage tank by means of a zero-mean oscillating fluid contained in a tube. This method takes advantage of the fact that the effective thermal diffusivity of a fluid in oscillatory motion is several orders of magnitude higher than the fluid molecular diffusivity. Therefore, the axial transport of heat along the tube is substantially higher when the fluid oscillates than when the fluid is static. Also, preliminary estimations show a dramatic heat transfer enhancement using oscillatory flows compared with the forced convection of heat by standard unidirectional flows. We explore the behavior of the effective thermal diffusivity using both Newtonian and viscoelastic fluids. For the Newtonian fluid a single maximum value of this quantity is exhibited for a given oscillation frequency. In contrast, several maxima for different resonant frequencies are observed for the viscoelastic fluid. Further, the absolute maximum of the enhanced thermal diffusivity for the viscoelastic fluid is several orders of magnitude larger than that of the Newtonian fluid.     

The study of buoyancy influence on mean flow and heat transfer under conditions of mixed convection in annular channels

Laser Doppler anemometry (LDA) measurements are reported on mean flow and turbulence in water as it flows downwards through a long vertical passage of annular cross‐section having an inner surface which can be uniformly heated and an outer one which is adiabatic. Under buoyancy‐opposed conditions, which can be achieved by heating the core and operating at a reduced mass flow rate, the flow near the inner surface is retarded, turbulent velocity fluctuations and turbulent shear stress are increased and the effectiveness of heat transfer is enhanced. When the influence of buoyancy is very strong, flow reversal occurs near the inner surface. Under such conditions, turbulence is produced very readily and the heat transfer process remains very effective, even when the Reynolds number is reduced to values at which the flow is laminar in the absence of heating. The measurements of turbulence in buoyancy‐opposed flow made in this study provide direct confirmation of the validity of the ideas currently used to explain the influences of buoyancy on mixed convection in vertical passages. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(1): 9–17, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20041     

Investigation on the characteristics of boiling heat transfer through narrow annular channels

Factors concerning the characteristics of boiling heat transfer are analyzed theoretically. Based on the experimental data of boiling heat transfer through annular channels with the gaps of 1–2 mm, three correlations which will be used to calculate the heat transfer in the similar conditions are given. The results obtained from these correlations are compared with experimental data. The main factors having influence on boiling heat transfer through narrow channels, and the desirable correlation are determined. This correlation can be used to predict the flow boiling heat transfer within the range of this experiment. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(2): 78–84, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20049     

Effect of Prandtl number on the turbulent thermal field in annular pipe flow

The effect of Prandtl number on the turbulent thermal statistics in fully developed annular pipe flow, with isoflux boundary conditions, is investigated by use of direct numerical simulation, for two values of Reynolds number. The Prandtl number has marked influence on the thermal field. With decreasing Pr, the conductive sublayer at both walls spreads from the walls to the core region, while the root mean square of temperature fluctuations and the turbulent heat fluxes are reduced near both walls. Asymptotic behaviours of these quantities are analyzed.     

Numerical simulation of laminar flow development with heat and mass transfer in PEM fuel cell flow channels having oxygen and hydrogen suction at one channel wall

Numerical simulation has been carried out of the fluid flow, heat and mass transfer for the developing laminar flow in polymer electrolyte membrane (PEM) fuel cell cathode and anode flow channels, respectively. Each flow channel is considered to be composed of two parallel walls, one porous (simulating electrode surface) and one non‐porous, or impermeable, wall (simulating bipolar plate surface). Various flow situations have been analyzed, and the local and the averaged friction coefficient, Nusselt number for heat transfer and Sherwood number for mass transfer are determined for various flow conditions corresponding to different stoichiometries, operating current densities and operating pressures of the cell. The effect of suction or injection (blowing) wall boundary condition has also been investigated, corresponding to the oxygen consumption in the cathode and hydrogen consumption in the anode. Correlations for the averaged friction coefficient, Nusselt and Sherwood numbers are developed, which can be useful for PEM fuel cell modeling and design calculations. Copyright © 2010 John Wiley & Sons, Ltd.     

同轴径向热管换热器壳程流场数值模拟

基于多孔介质模型和分布阻力方法,引入Al-sanea和Taborek两种阻力关系式模拟同轴径向热管换热器壳程的流场。结果表明:换热器壳程静压沿烟气流动方向呈线性分布;随入口烟气速度的增加,换热器阻力损失增大、压降增大;且随入口烟气速度的增加,压降增加的速率增大。     

Numerical simulation of heat transfer to separation air flow in an annular pipe

Separation and reattachment of air flow through a sudden expansion in an annular passage are considered in this study. Backward facing steps play a vital role in the design of many heat related applications where heat transfer is concerned. In the present work, numerical simulation is performed using computer fluid dynamics package (Fluent) to study the effect of step flow in an annular passage. The results are compared with the preliminary experimental findings. In the study, the flowing fluid was considered heated uniformly from the beginning of the expansion. Constant heat flux approach was also considered for the heat transfer investigation. Annular pipe flow system having a step ratio of D/d = 1.8 was considered where d and D are representing the diameter of the pipe before and after expansion. Numerical simulation review shows that the reattachment point extends further with the increase of velocity for different occasions. Finally, the local Nusselt number (Nu) in separation flow increases with the increase of Reynolds number (Re).