FORCED CONVECTION HEAT TRANSFER OF INTERACTING SPHERES

A basic understanding of the highly nonlinear interaction effects of closely spaced spheres on fluid mechanics and heat transfer parameters is important for improving the design of a variety of thermal fluid-particle systems. Fluid flow patterns, isotherms, as well as Nusselt number distributions and extended correlations are presented for steady laminar axisym-metric flow past a linear array of three spheres. A finite-element method is used to solve the complete Navier-Stokes and heat transfer equations for the system parameter ranges 1 Re_d  200 (Reynolds number) 2 d_ij10 ( particle spacings)and 0. 1  prRe_d 2000 (Piclet number). The verified computer simulation results indicate that convection heat transfer parameters of interacting spherical particles deviate significantly from solitary spheres for all Reynolds and Piclet numbers.     

COMBINED FORCED AND BUOYANCY-INDUCED CONVECTIVE HEAT TRANSFER IN A PARTIALLY CLOSED VERTICAL CHANNEL

This paper reports a numerical study of mixed convection flow over a horizontal surface in a partially closed adiabatic symmetric vertical channel. For low Rayleigh numbers, the forced flow enhances the buoyant convective heat transfer rates. For higher Rayleigh numbers, complex interactions arise between the buoyancy-induced flow and forced convection. The shearing between the buoyancy-dominated wall layer and the forced flow over it induces a stagnation region and a pair of weak recirculating cells in the middle of the channel above the horizontal surface. This effectively reduces heat transfer rates. Nusselt number data are presented for a wide range of Rayleigh and Reynolds numbers. Finally, the effect of the inlet height of forced flow on the transport is discussed.     

MIXED CONVECTION HEAT AND MASS TRANSFER IN RADIALLY ROTATING RECTANGULAR DUCTS

The present work investigates mixed convection heat and mass transfer in the entrance region of radially rotating rectangular ducts with water film evaporation along the porous duct watts. Mechanisms of secondary vortex development in the ducts under various conditions are examined by a vorticity-velocity numerical method. Emphasis is placed on the rotation effects, including both Coriolis and centrifugal buoyancy forces, and the mass diffusion effect on the flow structure and heat transfer characteristics. Results are presented in particular for an air-water vapor system under various conditions. Predicted results show that the effects of liquid film evaporation along the porous duct walls on the mixed convection neat transfer are rather substantial. The magnitude of the evaporative latent heal transfer may be 10 times greater than that of sensible heat transfer. The predictions also demonstrate that the distributions of Nu, Sh?z?, and fRe are closely related to the emergence, disappearance, growth, and decay of the rotating-induced secondary vortices. Additionally, a higher Nu?z? is found for a rectangular duct with a larger aspect ratio ( γ = 2) due to the relatively stronger secondary flows.     

NUMERICAL SOLUTION OF FORCED CONVECTION HEAT TRANSFER IN He II

A theoretical investigation of one-dimensional forced convection heat transfer in He II is conducted. The problem of interest involves a flow tube containing He II, which is heated at its midpoint along its length. Two modes of heating are analyzed: step function and square pulse. The one-dimensional He II energy equation is used to find the temperature distribution along the tube for both steady-state and transient situations. For the steady-state case, a numerical integration routine is used to obtain a solution, whereas for the solution of the transient case, a finite-difference scheme is developed. The numerical temperature profiles are then shown to compare well with the results of an experiment.     

太阳辐照、大气对流下未饱和土壤中热质迁移过程的实时仿真研究

建立了太阳辐照、大气对流下土壤水、盐、热迁移过程的数学模型,在此基础上成功一套面向用户的土壤热质迁移动态过程实时仿真软件,该软件采用菜单化、图形化设计,具有界面直观优美、功能多、精度高、速度快、使用方便和易于扩展的优点,并能在配置较简单的微机上运行,具有较大的实用价值。     

采用床内强制对流进行传热传质的固体吸附式循环分析

采用一维两温度模型，以活性炭纤维－氨为工质对，模拟计算了对流热波循环的吸附床加热过程和冷却过程中床内的温度分布和变化趋势，并分析计算了对流热波循环的性能参数。系统的回热率达０．４，热泵效率达１．７８，热泵系统的能量密度为１６１６Ｗ／ｋｇ。对系统加以优化，可获得更高的回热率和ＣＯＰ。     

EFFECTS OF BINARY MIXTURE CONDENSATION ON TURBULENT MIXED CONVECTION HEAT AND MASS TRANSFER IN A VERTICAL CHANNEL

Abstract

A numerical analysis was carried out to study the detailed heat and mass transfer processes between a condensation liquid film and mixed turbulent moist airflow. Results show that the condensation latent heat transfer is more important for a system with higher inlet relative humidity or lower inlet Reynolds number of a moist airstream. The heat and mass transfer coefficients are higher for a system with higher inlet relative humidity and inlet Reynolds number of moist air. In addition, the aiding-buoyancy forces cause diminution in heat and mass transfer results compared with the corresponding results of forced convection.     

太阳能蒸馏器中一组传热传质关系式

建立了盘式太阳能蒸馏器中一组传热传质关系式,并用多级迭盘式蒸馏室进行稳态模拟实验。发现该关系式在较大的雷利数范围及较宽的温度段内能很好地预示蒸馏器的产水量。     

考虑建筑房间内导热辐射与自然对流耦合时外围护结构对对流换热的影响

对具有导热和表面辐射换热相互耦合的建筑房间内的自然对流进行了数值研究。计算采用层流模型,为SIMPLE算法,QUICK差分格式。计算结果表明,辐射参与换热对流动将产生显著影响,会使房间内形成二次涡流。在自然对流的房间内,辐射换热比对流换热更占主导地位。当具有外围护结构时,导热效应使总对流换热有所增长(曲线的初始部分),但当导热系数比超过一定值时(kr≥10),再增加固体的导热性能,对房间内的流动和换热的影响就不明显了。从数值上证实了在实际建筑环境中,只要外围护结构的厚度达到一定数值就可达到隔热保温的要求。再增加厚度并不会得到更好的效果。     

HEAT AND MASS TRANSFER FROM A PERMEABLE CYLINDER IN A POROUS MEDIUM WITH MAGNETIC FIELD AND HEAT GENERATION/ABSORPTION EFFECTS

We formulate the problem of coupled heat and mass transfer by natural convection from a horizontal cylinder embedded in a uniform porous medium in the presence of an external magnetic field and internal heat generation or absorption effects. The cylinder surface is maintained at a constant temperature and a constant concentration and is permeable to allow for possible fluid wall suction or blowing. The resulting governing equations are nondimensionalized and transformed into a nonsimilar form and then solved numerically by an implicit, iterative, finite-difference method. Comparisons with previously published work are performed and excellent agreement is obtained. A parametric study of the physical parameters is conducted and a representative set of numerical results for the stream function, temperature, concentration profiles, and the Nusselt and Sherwood numbers is illustrated graphically to show interesting features of the solutions.     

NATURAL CONVECTION HEAT TRANSFER FROM A CYLINDER COVERED WITH AN ORTHOTROPIC POROUS LAYER

The problem of laminar natural convection from a horizontal cylinder with an orthotropic porous layer on its outer surface was investigated numerically. The effect of several combinations of pressure loss coefficients and porous layer thickness on the normalized average Nusselt number was studied over a wide range of Rayleigh numbers. The results showed that the change in the normalized average Nusselt number was insensitive to the value of the radial pressure loss coefficient but decreased, nonlinearly, with the value of the tangential pressure loss coefficient. The optimal porous layer thickness for maximum reduction in the normalized average Nusselt number was inversely proportional to the Rayleigh number.     

相变微胶囊悬浮液层流强迫对流换热实验研究

对以溴代十六烷(C16H33Br)为相变材料的相变微胶囊悬浮液(MPCMS)作了管内层流恒定热流密度加热条件下的强化传热性能测试和分析.悬浮液质量浓度为5.0%、10.0%和15.8%(文中百分数若无特别指出均为质量分数),整体雷诺数和斯蒂芬数分别在400～1900和1.1～8.8范围内.实验结果表明:实验工况下与水相比,使用MPCMS可降低壁面温度3.9℃,Nux数可提高27%～42%,传热实验过程中MPCMS样品压降增加不大,压降可降低26.7%(um=1.3m/s),在其输传热综合效果较好.     

MIXED CONVECTION HEAT TRANSFER IN A TEE BRANCH

The heat transfer characteristics of a Newtonian fluid in a two-dimensional, planar, right-angled tee branch are studied over a range of inlet Reynolds numbers and Grashof numbers. The flow and heat transfer equations, subject to the Boussinesq approximation, are solved using a finite-element discretization. The effects of the branch length and the grid size on the interior flow field are examined to assess the accuracy of the solutions. Results are presented for two types of experimentally realizable boundary conditions—equal exit pressure at the outlet of each branch and specified flow split between the branches. The thermal boundary condition of uniform wall temperature is examined. The effect of increasing Reynolds number is to increase the size and strength of the recirculation zones in both the main and side branches, while that of increasing Grashof number is to decrease such an effect. For the case of equal exit pressures there is a significant flow reversal in the side branch and the exit flow rate from the main branch increases linearly with increasing Gr/Re². For the case of specified flow split, an increasing back pressure is required to be maintained at the exit of the main branch to regulate the flow split at the desired level.     

NON-DARCIAN FORCED CONVECTION ANALYSIS IN AN ANNULUS PARTIALLY FILLED WITH A POROUS MATERIAL

Numerical solutions are presented for fully developed forced convection in concentric annuli partially filled with a porous medium. The porous medium is attached at the inner cylinder, which is maintained at uniform heat flux or at uniform wall temperature while the outer cylinder is adiabatic. The Brinkman-Forchheimer-extended Darcy model was used to model the flow inside the porous medium. The dependence of the fluid flow and heat transfer on several parameters of the problem is thoroughly documented. The inertia coefficient at which the inertial effects reduce the flow rate by 5% is determined as a function of the Darcy number for various thicknesses of the porous substrate. It is also shown that a critical thickness at which the value of the Nusselt number reaches a minimum does not exist if the effective thermal conductivity of the fluid-saturated porous medium is much higher than the fluid conductivity.     

FORCED CONVECTION IN MICROPOLAR FLUID FLOW OVER A WAVY SURFACE

Forced convection in the boundary layer flow of a micropolar fluid over a wavy surface is studied by the coordinate transformation and the spline alternating direction implicit method. Effects of the vortex viscosity parameter and the wavy geometry on the velocity, the local skin friction coefficient, and the local Nusselt number (Nu) are studied. Results show that the harmonic curves for the local skin friction coefficient and the local Nu have the same frequency as the frequency of the wavy surface. Moreover, the vortex viscosity parameter tends to decrease the heat transfer rate and to increase the skin friction coefficient.     

低熔点熔融盐管内强迫对流换热的实验研究

利用课题组自行配制的二元混合熔融盐(KNO3-Ca(NO3)2)开展导热油与低熔点熔融盐的管内强制对流实验研究.通过实验得到导热油与低熔点熔融盐的总换热系数,并通过最小二乘法和Wilson分离法得到了管内低熔点熔融盐侧的对流换热系数及其准则关联式.与不同的经典传热关联式对比,最大偏差为+23％.考虑高温熔融盐的变物性特...     

BUOYANCY-AIDED MIXED CONVECTION WITH CONDUCTION AND SURFACE RADIATION FROM A VERTICAL ELECTRONIC BOARD WITH A TRAVERSABLE DISCRETE HEAT SOURCE

This article presents the results of a comprehensive fundamental numerical study of the problem of buoyancy-aided mixed convection with conduction and surface radiation from a vertical electronic board provided with a traversable, flush-mounted, discrete heat source. Air, a radiatively transparent medium, is considered to be the cooling agent. The governing equations in primitive variables for fluid flow and heat transfer are first converted into stream function–vorticity form, and are later converted into algebraic form using the finite-volume method. The resulting finite-difference equations are solved by Gauss-Seidel iterative technique. The governing equation for temperature distribution along the electronic board is obtained by appropriate energy balance. The effects of pertinent parameters, viz., location of the discrete heat source, surface emissivity of the board, and modifiedRichardson number, on various results, including local temperature distribution along the board, maximum board temperature, and contributions of convection and surface radiation to heat dissipation from the board, are studied in great detail. The fact that any design calculation that ignores surface radiation in problems of this kind would be error-prone is clearly highlighted.