Laminar tube flow heat transfer in non-newtonian fluids under arbitrary wall heat flux

Analytical solutions are developed for the wall temperature profile of a power law fluid in laminar flow in a circular tube. This profile is first developed for the boundary condition involving uniformly constant heat flux at the wall. This is next extended for the boundary condition involving an arbitrarily varying heat flux at the wall. The computed results are finally compared with measured values obtained from a horizontal recirculating flow experimental unit.     

水平螺旋槽管降膜流动和传热特性研究

对水平螺旋槽管壁面降膜形成及传热特性进行了理论和实验研究,得到了液膜厚度及速度的解析及数值解.结果表明,降膜液膜特性主要受槽道结构和液膜表面张力控制.管壁温度沿周向向下逐渐升高,而且在定热流密度下保持不变,而液膜温度则沿周向逐渐上升.相比光管,螺旋槽管降膜具有更高的传热系数.     

横管降膜过程中液膜内流动和传热特性分析

横管降膜流动过程中,液膜速度和温度及其分布是影响传热传质的关键因素,由于实验研究方法的局限性,实验研究结果一般只是液膜内各参数的平均值,而液膜内部的速度和温度具体分布特性却很难得到。借助FLUENT软件,利用VOF模型研究了橫管外液膜速度和温度及其分布特性。通过建立三维数理模型,模拟研究了常温常压下,橫管外液膜无相变条件下横管液膜的传热过程,并从边界层的角度解释了液膜波动对传热过程的影响。     

Evaporation heat transfer of falling water film on a horizontal tube bundle

A numerical simulation and experimental study were carried out for evaporation heat transfer of a falling water film on a smooth horizontal tube bundle evaporator. A laminar model and a turbulence model were respectively adopted to calculate the heat transfer coefficients of falling water film on horizontal heated tubes. The calculation zone on the heated tube was divided into the top stagnation zone and the lateral free film zone. The initial boundary conditions for the free film zone were determined from the calculated results of the stagnation zone. The modified wall function method was used for the turbulent flow. Comparisons between the experimental data and the numerical solutions by use of two flow models show that the experimental data lie between the laminar model solutions and the latter turbulence model solutions and that they are closer to the latter solutions. Finally, a simple dimensionless correction based on the numerical simulations is proposed for predicting the evaporation heat transfer of falling water film for actual engineering applications. © 2001 Scripta Technica, Heat Trans Asian Res, 31(1): 42–55, 2002     

海水淡化系统水平管降膜蒸发器传热系数研究

针对海水淡化系统水平管降膜蒸发器,总结和分析管内冷凝侧与管外蒸发侧的换热系数关联式,比较管内径、入口蒸汽流速、蒸汽冷凝温度、出口蒸汽干度对管内蒸汽冷凝侧换热系数的影响;研究传热温差以及喷淋密度对管外蒸发侧换热系数的影响。结合不同的污垢系数,进行了总传热系数的影响因素分析,为海水淡化系统的工程设计提供依据。     

椭圆横管外降膜流动和换热特性分析

本研究基于VOF算法编写用户UDF(自定义函数),采用FLUENT软件建立了椭圆横管外降膜流动和换热的计算模型。根据CFD(计算流体力学)模型计算和分析了在不同长短轴比下管外降膜速度分布、压力分布、液膜内温度分布和管外换热分布的变化规律。研究结果表明:长短轴比的变化影响了管外液膜速度分布、压力分布和膜内温度分布;相比圆管,椭圆管的管外膜内液体流速更快。壁面压力沿周向逐渐减少并在X=0.9附近快速回升;随长短轴比e的增加,周向压力最小值位置逐渐向后推移。局部Nu数分布与压力分布在趋势上存在一致性。当e=1.65附近时,椭圆的换热性能最优;最后,通过对管形的研究分析,提出横管的传热分区模型。     

Characteristics of heat and mass transfer in vapor absorption of falling film flow on a horizontal tube

The absorber is an important component in absorption machines and its characteristics have significant effects on the overall efficiency of absorption machines. This article reports on the results of numerical studies on the characteristics of falling film LiBr–H₂O solution on a completely wetted horizontal tube and the associated vapor absorption in the Reynolds number range of 5 < Re < 100. The boundary layer assumptions are used for the transport of mass, momentum and energy equations and the finite difference method is employed to solve the governing equations in the film flow. The heat and mass transfer coefficients are expressed in the forms of Reynolds number, Prandtl number and Schmidt number, at the usual absorber pressure and inlet solution concentration. The results can be used as a reference in designing actual absorption chiller.     

水平管外降膜蒸发传热特性的数值模拟

为深入研究液膜内的微观传热机理,对水平管外降膜蒸发的传热特性进行了数值模拟,获得了液膜厚度、液膜流动速度和传热系数等热力参数在液膜内的分布特性。通过与实验数据的对比验证了数学模型的准确性。研究结果表明:在饱和蒸发温度62℃、传热温差2.8℃、管外径25.4mm和液膜入口速度0.071~0.15 m/s条件下,沿圆周方向,液膜厚度减小,传热系数增加,直至达到液膜热力发展区,膜厚和传热系数趋于稳定;受液膜内温度变化的影响,液膜内的粘度、表面张力和导热系数的变化对液膜传热特性产生显著影响。     

水平管降膜蒸发器传热传质实验研究

水平管降膜蒸发器因传质传热系数高而被广泛应用于淡化水处理中。搭建了水平管降膜蒸发传热实验台,通过对实验结果的归纳,得到了水平管降膜蒸发器的蒸发量随喷淋密度、蒸发温度、热通量的变化规律及热量利用率随蒸发温度的变化规律。结果表明,热通量范围不同时,蒸发量随喷淋密度的变化规律不同;蒸发量随热通量的增大而增大,随蒸发温度的增大而增大;热量利用率随蒸发温度的增大而增大。     

Heat transfer characteristics of falling film evaporation on horizontal tube arrays

A falling film heat transfer test facility has been built for the measurement of falling film evaporation in a vacuum of about 1000 Pa. At this condition, only convective evaporation occurred in the liquid film. The Reynolds numbers of falling film over a range from 21.6 to 108.1 were tested on six-tube arrays made of enhanced or smooth tubes. Results show that the tubes with both enhanced outer and inner surfaces give high heat flux. Besides, as the Reynolds number increases, the heat transfer enhancement ratio of falling film evaporation decreases. A semi-analytical correlation is established to predict the heat transfer coefficients of falling film evaporation on smooth tube arrays, considering the contributions of partially dryout and fully wet regimes, respectively. For enhanced tubes, the heat transfer enhancement ratios to the smooth tubes were also correlated.     

Characteristics of transient heat transfer during quenching of a vertical hot surface with a falling liquid film

An experimental study has been conducted to elucidate characteristics of transient heat transfer during quenching of a vertical hot surface with a falling liquid film. The experiment was done at atmospheric pressure for the following conditions: an initial surface temperature from 200 to 400^°C, a subcooling of 20– 80 K, average velocity of 0.52– 1.24 m/s, and the block material is copper and carbon steel. The surface temperature and heat flux are estimated from the measured temperatures in the block during the quench by a two‐dimensional inverse solution. It follows that as the position of wetting advances downward, the position at which the heat flux becomes a maximum also advances downward. The time at which the position of maximum heat flux begins to move is one of the most important parameters and can be predicted by a proposed correlation. In addition, it is revealed that the maximum heat flux for copper depends on the length to which it occurs from the leading edge. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(6): 345– 360, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20167     

黏性流体在光滑横管外壁成膜机理研究

采用低雷诺数、小流量模型对黏性流体在水平管外形成的薄液膜的流速以及膜厚分布进行分析和计算；给出了边界层坐标系的二维边界层方程，求得了其相似性解；根据液膜自由表面上的运动学边界条件，给出关于膜厚的一阶偏微分方程，利用特征线法求得了膜厚分布的数值解。结果表明：重力驱动在光滑横管外壁面白发形成均匀的液膜是不可能实现的，必须外加一个能产生特殊流场的气刀来改变其原有受力情况，才有可能实现均匀液膜。在某一确定的时刻，膜厚沿着x轴逐渐增厚；随着时间的推移，各处膜厚逐渐减小。这些对于确定气刀的结构和安装的位置是十分重要的。     

Numerical simulation of horizontal tube bundle falling film flow pattern transformation

It is important to study the falling-film pattern of a horizontal tube bundle in order to set up a heat and mass transfer model accurately. The falling-film pattern of a horizontal tube bundle is simulated in this paper. The technique is based on computational flow dynamics (CFD) for the two-phase flow of gas and water. The experimental results were used to validate the mathematical model. It indicates that the simulation results accord with experimental data well. The simulated results show that the flow pattern varies with different flow rates. Under the different flow rates, it observes the droplet, droplet-columnar, columnar, columnar-sheet and sheet flow patterns. The critical value is 0.0125 kg/s between droplet and columnar, and the critical value is 0.02 kg/s between columnar and sheet.     

Transition and flow boiling heat transfer inside a horizontal tube

Experiments on transition and flow boiling heat transfer with refrigerant R114 inside a horizontal tube were performed at bubble flow, critical heat flux and in the transition region between bubble flow and film boiling at mass fluxes between 1200 and 4000 kg/m² s and in the pressure range between 5 and 15 bar. In comparison with pool boiling bubble flow heat transfer depends essentially on the mass flow rates and on the vapor quality. The critical heat flux depends less on the temperature difference than in pool boiling heat transfer and exhibits a maximal and a minimal value as a function of the pressure. The critical heat flux increases with mass flow rate as already shown by Collier. In the region of transition boiling the heat flux over the difference between wall and saturation temperature approaches a horizontal curve. Therefore in this region an evaporator may always be operated under stable conditions and burn out does not occur.     

Laminar film condensation heat transfer of hydrogen and nitrogen inside a vertical tube

There have been a number of experimental investigations on condensing heat transfer to cryogenic fluids. The investigations with nitrogen and oxygen have shown reasonable agreement between experimental data and those predicted by Nusselt's theory. On the other hand, in the previous investigations with much colder fluids, such as hydrogen, deuterium, and helium, the experimental condensing heat transfer coefficients are smaller than those predicted by Nusselt's theory, and these differences become much larger when the film Reynolds number or decreasing temperature difference across the condensate film is decreased. In the present investigation, hydrogen and nitrogen were condensed inside a vertical tube (d = 15 mm, L = 30 mm) under steady‐state conditions, respectively, and condensing heat transfer coefficients were precisely measured. From the experimental results, the condensing heat transfer coefficients for saturated hydrogen and nitrogen vapors agree with those predicted by Nusselt's theory within ±20%. The results of the present study suggest that deuterium and helium might also behave as predicted by Nusselt's theory. © 2001 Scripta Technica, Heat Trans Asian Res 30(7): 542–560, 2001     

The effect of liquid film evaporation on flow boiling heat transfer in a micro tube

Flow boiling in micro channels is attracting large attention since it leads to large heat transfer area per unit volume. Generated vapor bubbles in micro channels are elongated due to the restriction of channel wall, and thus slug flow becomes one of the main flow regimes. In slug flow, sequential bubbles are confined by the liquid slugs, and thin liquid film is formed between tube wall and bubble. Liquid film evaporation is one of the main heat transfer mechanisms in micro channels and liquid film thickness is a very important parameter which determines heat transfer coefficient. In the present study, liquid film thickness is measured by laser focus displacement meter under flow boiling condition and compared with the correlation proposed for an adiabatic flow. The relationship between liquid film thickness and heat transfer coefficient is also investigated. Initial liquid film thickness under flow boiling condition can be predicted well by the correlation proposed under adiabatic condition. Under flow boiling condition, liquid film surface fluctuates due to high vapor velocity and shows periodic pattern against time. Frequency of periodic pattern increases with heat flux. At low quality, heat transfer coefficients calculated from measured liquid film thickness show good accordance with heat transfer coefficients obtained directly from wall temperature measurements.     

Investigations on heat and mass transfer characteristics of falling film horizontal tubular absorber

An experimental set-up is built incorporating only two principle components, viz, absorber and generator of vapor absorption refrigeration system (VARS) to investigate heat and mass transfer characteristics of absorber. The refrigerant, R134a (1,1,1,2-tetrafluroethane) is absorbed by R134a-DMAC (N,N-dimethylacetamide) solution flowing over the horizontal tubes arranged as tube bank. The effect of solution flow rate, coolant flow rate and temperature, heater load and concentration of R134a is studied. The performance parameters like solution exit temperature from tubes, state point temperatures, heat flux, mass flux, and overall heat and mass transfer coefficients are presented for different operating condition of absorber. For lower flow rate of the solution and higher flow rate of the coolant, the bulk solution temperature is found to decrease. The heat and mass transfer coefficients increase with mass flow rate of the solution. An increase in inlet temperature of coolant results into an increase in overall heat transfer coefficient and decrease in overall mass transfer coefficient.     

Boiling heat transfer and critical heat flux in liquid films falling on vertically-mounted heat sources

Boiling heat transfer measurements were obtained for an inert fluorocarbon (FC-72) liquid film injected over a vertical heated wall. Flow visualization revealed that vigorous boiling prior to burnout ruptured the liquid continuum, causing most of the film to separate from the heated wall leaving a thin liquid subfilm which maintained contact with the wall. The critical heat flux (CHF) was accompanied by dryout of the subfilm after total separation of the liquid near the upstream edge of the heater. A higher CHF was achieved by increasing the film velocity or by utilizing a shorter heater. Experimental data correlated favorably with predictions of a CHF model based on the Helmholtz instability and subfilm dryout.     

Laminar flow heat transfer from an argon plasma in a circular tube

The laminar flow of a plasma in the entrance region of a circular tube has been analyzed using an implicit finite-difference scheme. The solution is based upon the boundary-layer equations with the plasma radiation term retained in the energy equation, and the transverse convection term retained in both the momentum and energy equations. Numerical results have been obtained for an argon plasma having a linear enthalpy and cubic velocity profile at the tube entrance. At the low temperature limit of the analysis, the friction factor is in agreement with previously published results; and in all but a very small region near the tube entrance the local Nusselt number agrees with the Nusselt number for constant property, fully-developed flow to within approximately 17 per cent.