Conjugated heat transfer to laminar flow with internal heat source in a parallel plate channel

A steady heat transfer problem was analyzed as a conjugated problem between a fully-established laminar flow and the duct wall, under two kinds of boundary conditions at the outer wall surface. The internal heat generation is described as a linear function of the fluid temperature. The influence of the heat conduction in the wall was found to be important for the case with small wall conductance under a boundary condition of the first kind, and also for the large wall-conductance case under the second condition. Such influence is more significant for the case with an internal heat source.     

Transport phenomena over partially active surfaces

Transport phenomena over partially active surfaces have been examined theoretically and experimentally. Boundary conditions of the first and third kind have been considered. The theoretical results were achieved by solving the Laplace differential equations with mixed boundary conditions at the surface. The solution methods of conformal mapping, Fourier series and finite differences for boundary conditions of the first kind are discussed and compared with the simple model of Schlünder. There is good agreement if the fraction of active surface is larger than 2%. For boundary conditions of the third kind (e.g. first-order reaction) the model had to be extended and is thereafter in qualitative agreement with the differential equation solution. Our calculations show only a weak dependence of the mass transfer rate on the fraction of active surface. The decrease with active surface fraction is slower if the boundary layer is thick, the active patches are small and the reaction is slow compared with diffusion.

As an example of a boundary condition of the first kind, evaporation through a perforated foil was examined. The catalytic combustion of hydrogen at partially active surfaces was studied as an example of a boundary condition of the third kind. The experimental results confirm the expected dependence of overall mass transfer on the active surface fraction, although the experimental data show reasonable scattering in the case of the catalytic reaction. These deviations are due to problems in achieving a uniform and constant catalytic activity during the measurements.     

浮顶罐内含蜡原油静态储存中的冷却胶凝规律

将失流点以下的含蜡原油看作是多孔介质体系,以附加比热容法描述蜡的结晶潜热,动量源项方法表征蜡晶网络结构对液态原油的流动阻力,基于有限体积法数值模拟含蜡原油的冷却胶凝过程。结果表明：传热机制和边界条件主导了凝油结构的演变进程。在自然对流作用下,凝油最先在罐底和罐壁所包围的区域内产生,且其始终是罐内胶凝最严重区域。罐顶最先形成完整的凝油层,其发展先后经历了慢速增长和快速增长两个阶段,且其凝油层厚度逐渐趋于均匀分布;其次是罐底,其发展过程与罐顶相反;最后是罐壁,其凝油层的演变具有从罐底沿罐壁向罐顶推进的特点。罐内对流越强,罐顶凝油层的增长速率越缓慢,罐底凝油层的增长速率越快。基于温度场及凝油结构的演变规律,可以将含蜡原油的冷却过程分为3个阶段,即自然对流占主导的第1阶段,导热逐步取代自然对流的第2阶段,及以导热为主导机制、边界条件调控下的第3阶段,同时给出了不同阶段原油温度分布和散热损失规律的细节。     

插入物强化管内单相流体传热的研究

为寻求强化低Re数下气体传热的较好插入物型式,在Re=500-10000范围内,对6种插入物用氢气泡示踪法进行流态显示,在恒壁温条件下进行竖直管内空气传热和阻力对比试验,得出各插入物的f-Re和Nu-Re关联式,并对各插入物进行传热强化性能评价.在此基础上进行参数范围为e/D=0.067—0.234、S/e=3.89—13.25的螺旋片试验,得出其阻力和传热关联式.综合研究流态显示结果和传热性能评价,认为在层流或过渡流区,利用螺旋流与利用边界层分离二者相比较,前者可以耗能较少而获得同样的传热强化效果.     

考虑界面蒸发时的降液膜流动传热

The interfacial evaporative heat transfer was included in the semi-empirical study of the heat transfer for the falling liquid film flow. The investigations showed that, the inclusion of the interfacial evaporative heat transfer in the turbulent model would lower the predicted convective heat transfer coefficient. Predictions of the new model resulted in a prominent deviation from that predictions of the normal model in the case of large mass flow rate and low wall heat flux. This deviation will be decreased with increasing wall heat flux, such that it will be asymptotic zero at very high wall heat flux. Predictions of the new model agreed well with the current experimental measurements. This study has verified that the Reynolds number is not the sole crucial parameter for heat transferof falling liquid film flow, and wall heat flux will be another important independent parameter. This result is consistent with our previous studies.     

FULLY DEVELOPED HEAT TRANSFER IN A ROTATING NON-ALIGNED STRAIGHT TUBE

Fully developed heat transfer in fluids flowing steadily through a rotating non-aligned straight tube is studied theoretically. A regular perturbation solution of the governing energy equation is developed for a constant wall flux boundary condition; this solution is valid only for “sufficiently mild” Coriolis disturbances. The asymptotic Nusselt number, complete to third order, is obtained from these results. Within the estimated region of validity of this solution the convective heat transfer coefficient depends on three independent dimensionless parameters. The first parameter serves as a Peclet number for energy transfer in the plane of the tube cross-section whereas the second and third parameters are proportional to the axial and transverse components of the Coriolis acceleration. Increasing the Coriolis acceleration will lead to transport enhancement relative to an analogous stationary straight tube exchange device.     

镁-氢化镁热化学蓄热系统数值分析

蓄热系统是解决热量供需不匹配的有效方式之一。根据热量储存原理的不同,可以将系统分为显热、潜热和热化学蓄热三种类型,其中热化学蓄热有其独特的优点。基于镁-氢化镁热化学蓄热系统蓄放热时的物理化学过程,建立了系统的二维非稳态数学模型,考虑了不同边界条件对系统的影响,通过数值计算,获得了系统的温度、反应速率、反应进度分布及系统的对外放热功率。研究结果表明:系统的蓄热密度为0.85 kW·h·(kg Mg)^-1,热量的传递是影响系统蓄放热效率的关键因素之一,并且当边界对流传热系数保持一定时,存在一个最佳的外界流体温度,使系统的平均放热功率达到最大。在系统以定壁温为边界条件时,系统最大的平均放热功率/质量值为0.79 kW·(kg Mg)^-1。     

Effect of interfacial velocity and interfacial tension gradient on momentum,heat and mass transfer

The effect of interfacial velocity and interfacial tension gradient on momentum, heat and mass transfer was studied theoretically. After reviewing current knowledge regarding interfacial velocity effects, the interfacial tension gradient effect is discussed as a numerical solution of the laminar boundary layer equations. The enhancement and suppression of interfacial mobility, and the rates of interphase momentum, heat and mass transfer were related to the Marangoni number. A diagram showing the effect of interfacial tension gradient on momentum, heat and mass transfer is presented.     

Heat and mass transfer when drying a spherical particle in an oscillating electromagnetic field

The problem of describing heat and mass transfer upon drying the spherical particle in oscillating electromagnetic field under boundary conditions of heat and mass transfer of the third kind has been formulated and analytically solved. The numerical analysis of the process has been performed.     

管内旋流场综合性能研究进展

针对目前换热设备的强化传热和防除垢问题,介绍了管内插入物、表面凹槽以及复合强化传热等几种管内旋流技术。管内插入物可破坏流体边界层,实现在线自动防除垢和强化传热,但插入物本身易因磨损导致换热效率下降;表面凹槽技术可对管内外流体实现双向传热,流体阻力与其它技术相比较小,但制造工艺相对复杂;复合强化传热技术传热能力较高,可综合各种传热技术的优点,且综合性能要比单一技术要高。对以上几种主要管内旋流技术进行分析比较,并提出未来管内旋流技术的主要发展趋势将以复合强化传热技术为主。     

On turbulent pipe flow with heat transfer and chemical reaction

A theoretical model has been developed to describe heat transfer to a fluid which is flowing turbulently in a pipe and within which a first-order endothermic irreversible chemical reaction is taking place. It is based on the penetration model for the fluid boundary layer close to the pipe wall and enables the calculation of the effect of chemical reaction on the heat transfer coefficient of turbulent pipe flow.The model has been checked against experimental results for flow distribution and heat transfer in turbulent pipe flow without chemical reaction. Good agreement between the calculated and experimental results was found. The measured heat transfer coefficients for turbulent pipe flow with a uniformly distributed heat source within the fluid were also well predicted. Unfortunately no accurate experimental data on the effect of a first-order chemical reaction on the heat transfer coefficient of turbulent pipe flow are available. However, calculations made with the model show that this effect can be considerable and thus may not always be neglected in practice.     

HEAT TRANSFER DUE TO MHD SLIP FLOW OF A SECOND-GRADE LIQUID OVER A STRETCHING SHEET THROUGH A POROUS MEDIUM WITH NONUNIFORM HEAT SOURCE/SINK

An analysis is carried out to study the heat transfer characteristics of a second-grade non-Newtonian liquid due to a stretching sheet through a porous medium under the influence of external magnetic field. The stretching sheet is assumed to be impermeable. Partial slip condition is used to study the flow behavior of the liquid. The effects of viscous dissipation, nonuniform heat source/sink on the heat transfer are addressed. The nonlinear partial differential equations governing momentum and heat transfer in the boundary layer are converted into nonlinear ordinary differential equations using similarity transformation. Analytical solutions are obtained for the resulting boundary value problems in the case of two types of boundary heating, namely, constant surface temperature (CST) and prescribed surface temperature (PST). The effects of slip parameter, second-grade liquid parameter, combined (magnetic and porous) parameter, Prandtl number, Eckert number, and nonuniform heat source/sink parameters on the heat transfer are shown in several plots. Analytical expressions for the wall frictional drag coefficient and wall temperature gradient are obtained.     

Re-evaluation of the Nusselt number for determining the interfacial heat and mass transfer coefficients in a flow-through monolithic catalytic converter

The two-equation porous medium model has been widely employed for modeling the flow-through monolithic catalytic converter. In this model, the interfacial heat and mass transfer coefficients have been usually obtained using the asymptotic Nusselt and Sherwood numbers with some suitable assumptions. However, previously it seemed that there existed some misunderstanding in adopting these Nusselt and Sherwood numbers. Up to now, the Nusselt number based on the fluid bulk mean temperature has been used for determining the interfacial heat and mass transfer coefficients. However, the mass and energy balance formulations in the two-equation model indicate that the Nusselt number should be evaluated based on the fluid mean temperature instead of the fluid bulk mean temperature. Therefore, in this study, to correctly model the heat and mass transfer coefficients, the Nusselt number based on the fluid mean temperature was newly obtained for the square and circular cross-sections under two different thermal boundary conditions (i.e., constant heat flux and constant temperature at the wall). In order to do that, the present study employed the numerical as well as analytical method.     

SLIP FLOW AND HEAT TRANSFER OF A SECOND-GRADE FLUID IN A POROUS MEDIUM OVER A STRETCHING SHEET WITH POWER-LAW SURFACE TEMPERATURE OR HEAT FLUX

This article deals with the study of boundary layer flow of a second-grade fluid in a porous medium past a stretching sheet and heat transfer characteristics with power-law surface temperature or heat flux. The flow in the boundary layer is considered to be generated solely by the linear stretching of the boundary sheet adjacent to a porous medium, and boundary wall slip condition is assumed. In the energy equation effects of viscous dissipation, work done due to deformation and internal heat generation/absorption is taken into account. Closed form solutions are obtained for this problem.     

MASS TRANSFER COEFFICIENTS FOR REACTIVE FLUIDS FLOWING IN CONDUITS WITH SEMI-PERMEABLE WALLS

The advancing front theory is an approximate solution for mass transfer into a reactive fluid when the reaction can be assumed to be very fast. The theory has had considerable use in predicting mass transfer characteristics for reactive fluids flowing in conduits. In this paper, the mass transfer coefficient, in the form of the local, fluid-side Sherwood number, is derived for reactive flow in conduits with semi-permeable walls. The local, fluid-side Sherwood number is given as a function of the Graetz number, the wall Sherwood number, and a dimensionless reaction strength parameter. The limiting cases of both the constant wall concentration boundary condition (Sh_w?∞) and the constant wall flux boundary condition (Sh_w?∞)are investigated. Comparisons of the results with the classical Graetz and Leveque theories give conclusions about the accuracy of the advancing front theory for the worst possible case.     

密闭二维腔内水中热声波的数值模拟

以边长为1 mm密闭二维腔中的液态水为研究对象,用数值计算的方法,通过在左侧边界施加阶跃温度或阶跃热流来产生热声效应。分析水中热声波的产生与传播特性,并研究右侧边界中点产生的热通量的强度。计算结果表明,在相同的热边界和几何边界条件下,液态水中热声效应引起的压力波幅值和热通量远大于空气;当加热阶跃温度不同时,水中产生的压力波幅值和热通量随温度升高而急剧增大;当阶跃热流加热时,压力波和热通量与阶跃温度加热时的有较大差异。     

CFD approaches for the simulation of hydrodynamics and heat transfer in Taylor flow

Previous studies on heat and mass transfer in the Taylor flow regime in microchannels have shown the transport (heat/mass) rates to be dependent on the length of the liquid slug. In order to understand the effect of slug length on transport rates and to have a one-to-one comparison with experimental data, a computational approach is required to simulate flows with liquid slugs and bubbles of controlled lengths.Here we describe and benchmark two approaches. The first, and conceptually simplest, is to generate bubbles and slugs in a long tube using a time-dependent boundary condition. In the second method, the flow and heat transfer in a single unit cell, consisting of a bubble surrounded by liquid slugs, is solved in a frame of reference moving with the bubble velocity. Both methods were implemented in ANSYS-Fluent.Simulations for a two-phase (liquid-only) Reynolds number of 713, Capillary number of 0.004 and void fraction of 0.366 for nitrogen-water flow were performed to compare the two techniques. There was a very large difference between the required computational mesh sizes and times for the two methods, with a wall clock time of 38 h on a single processor for the moving domain compared with 1460 h using four processors for the stationary domain approach. In addition, for a constant wall heat flux boundary condition, even with 14 bubbles present in a long tube thermal development was not achieved. The hydrodynamic and heat transfer results obtained from the two approaches were found to be very similar to each other and with results from our earlier verification and validation studies, giving a high degree of confidence in the implementation of both methods.     

基于热辐射成像建模求解的管式炉炉管温度检测

准确在线检测管壁表面分布式温度是优化管式炉加热工艺的关键所在。以一台工业管式裂解炉为试验对象,结合辐射图像处理方法,开展了管式炉炉管表面温度可视化检测研究。采用基于Monte Carlo的DRESOR法求解具有复杂边界条件的管式炉辐射成像模型,实现了炉管辐射与火焰辐射、炉壁辐射的解耦计算,对炉管表面温度与热通量分布进行了在线监测,并研究了二者随工质流动方向的变化趋势。经过验证,温度测量误差小于2%,测量误差主要出现在最高和最低温区域。该项研究将有助于指导管式炉燃烧调整,改进加热工艺,提高炉管表面受热均匀性,延长炉管工作寿命。     

Heat transfer to molten polymers flowing through tubes

A theoretical and experimental study of heat transfer to polymer melts flowing through circular tubes is presented. The mathematical model provides for shear heating and expansion cooling effects, and also heat of reaction during flow for various wall boundary conditions. Experimental results, obtained using low density polyethylene, show reproducible temperature and velocity profiles. The measured inlet melt temperature profile and the axial wall temperature profile provide the boundary conditions for the calculations. The experimental data confirm the predictions of the magnitude of the shear heating and expansion cooling effects during tube flow.