Combined heat and mass transfer in natural convection between vertical parallel plates

Abstract

This study is to examine the effects of latent heat transfer associated with liquid film vaporization on heat transfer in the natural convection flows driven by the simultaneous presence of combined buoyancy forces of thermal and mass diffusion. Results are especially presented for an air‐water system under various conditions. The influences of channel length and system temperatures on the momentum, heat and mass transfer in the flow are investigated in great detail. The important role of transport of latent heat of vaporization under the situations of buoyancy‐aiding and opposing flows is clearly demonstrated.     

Effects of wetted wall on laminar natural convection in vertical annular ducts

Abstract

A detailed numerical analysis is performed to investigate the effects of latent heat exchange, in connection with evaporation of the liquid film on the wall, on the natural convection heat transfer in vertical concentric annuli. Major governing parameters identified are Gr_T, Gr_M, Pr, Sc, and N. Results are specifically presented for an air‐water system under various heating conditions to illustrate the latent heat transport during the evaporation process. The effects of the channel length, ratio of radii N and wetted wall temperature on the momentum, heat and mass transfer are examined in detail. Tremendous enhancement in heat transfer due to the exchange of latent heat was clearly demonstrated.     

Combined heat and mass transfer in MHD free convection from a vertical surface with Ohmic heating and viscous dissipation

The problem of combined heat and mass transfer of an electrically conducting fluid in MHD natural convection adjacent to a vertical surface is analyzed, taking into account the effects of Ohmic heating and viscous dissipation. The resulting governing equations are transformed using suitable transformations and then solved numerically by an implicit finite-difference technique. The solution is found to be dependent on the governing parameters including the magnetic field parameter, the buoyancy ratio between species and thermal diffusion, the Eckert number, the Prandtl number, and the Schmidt number. Effects of these major parameters on the transport behaviors are investigated methodically and typical results are illustrated to reveal the tendency of the solutions. Representative results are presented for the velocity, temperature, and concentration distributions, as well as the local skin-friction coefficient, local Nusselt number, and the local Sherwood number.     

Numerical study of Soret and Dufour effects on heat and mass transfer from natural convection flow over a vertical porous medium with variable wall heat fluxes

A study has been carried out to obtain the solutions for heat and mass transfer from natural convection flow along a vertical surface with variable heat fluxes embedded in a porous medium due to thermal-diffusion (Soret) and diffusion-thermo (Dufour) effects. The buoyancy induced boundary layer adjacent to a vertical surface is analyzed using a non-Darcy flow model. The parameters for inertia, buoyancy ratio, exponent of heat flux, position and diffusion have been examined. The governing differential equations of continuity, momentum, energy and concentration are transformed into a set of coupled equations and solved using similarity analysis with numerical technique. Results show the velocity, temperature and concentration profiles related to local Nusselt and Sherwood numbers at different magnitude of Soret and Dufour numbers.     

Radiation and natural convection heat transfer from wire-and-tube heat exchangers in refrigeration appliances

The air-side heat transfer from wire-and-tube heat exchangers of the kind widely used in small refrigeration appliances has been studied. Radiation and free-convection components have been separately investigated. The radiation component was theoretically computed using a diffuse, gray-body network with interactions between each part of the heat exchanger and the surroundings. For the free-convection heat transfer component, a semiempirical correlation was developed on the basis of experimental tests conducted on a set of 42 low-emittance exchangers with various geometrical characteristics. Comparisons between overall heat transfer predictions and a second, independent set of experiments on eight high-emittance exchangers showed satisfactory agreement. The proposed analysis is suitable either to determine the heat transfer performance of an existing (already sized) exchanger or to design a new one for prescribed heat duty and working temperatures.     

强制对流换热实验测试系统的研制

运用虚拟仪器技术,研制了一套强制对流换热实验台自动化测试系统.测试系统实现了温度和风速的精确测量,以及风机转速和加热功率的自动控制,测试软件可以直观地观察整个换热过程中各个测量点的状态变化情况,进而得到整个实验过程从启动到稳态过程的全部数据,并对测量结果进行了不确定度分析,系统平均换热系数测量不确定度为4.9W/(m2.℃).     

Investigation of heat, mass transfer and fluid flow characteristics in evaporative condensers

Heat and mass transfer and fluid flow characteristics in evaporative condensers is discussed. A complex pattern of water temperature and air enthalpy change was detected which depended upon elevation above the sump level. The results have indicated that the spray-filled space beneath the coil has a substantial effect of heat rejection in this type of apparatus. On the other hand, the effect of the upper spray nozzle zone is insignificant. It is possible to optimize the combination of various heat and mass transfer spaces, as well as the effect of extended surfaces and spray-filled spaces. Specifically, it will help to design better evaporative condensers with closely spaced staggered tubes, and to optimize the heat transfer and energy efficiency characteristics of such units.     

Numerical analysis of conjugate natural and mixed convection heat transfer of nanofluids in a square cavity using the two-phase method

In the present study, the problem of conjugate natural and mixed convection of nanofluid in a square cavity containing several pairs of hot and cold cylinders is visualized using non-homogenous two-phase Buongiorno's model. Such configuration is considered as a model of heat exchangers in order to prevent the fluids contained in the pipelines from freezing or condensing. Water-based nanofluids with Cu, Al₂O₃, and TiO₂ nanoparticles at different diameters ($25\text{nm}?d_p?145\text{nm}$) are chosen for investigation. The governing equations together with the specified boundary conditions are solved numerically using the finite volume method based on the SIMPLE algorithm over a wide range of Rayleigh number (${10}^4?\mathit{Ra}?{10}^7$), Richardson number (${10}^{-2}?\mathit{Ri}?{10}^2$) and nanoparticle volume fractions ($0?\phi ?5\%$). Furthermore, the effects of three types of influential factors such as: orientation of conductive wall, thermal conductivity ratio ($0.2?K_r?25$) and conductive obstacles on the fluid flow and heat transfer rate are also investigated. It is found that the heat transfer rate is significantly enhanced by incrementing Rayleigh number and thermal conductivity ratio. It is also observed that at all Rayleigh numbers, the total Nusselt number rises and then reduces with increasing the nanoparticle volume fractions so that there is an optimal volume fraction of the nanoparticles where the heat transfer rate within the enclosure has a maximum value. Finally, the results reveal that by increasing the thermal conductivity of the nanoparticles and Rayleigh number, distribution of solid particles becomes uniform.     

角膜真空冷冻干燥实验的传热传质分析

为保持角膜在真空冷冻干燥（以下简称冻干）过程中的活性,减少其内皮细胞的损伤,本文分析了冻干实验中前处理、预冻、干燥和复水检测等过程中,传热传质规律对角膜内皮细胞造成损伤的原因,在改进工艺参数的基础上,成功的冻干了合格的角膜。     

封闭腔内MWCNT纳米流体自然对流传热的数值模拟

采用Fluent软件对封闭腔内纳米流体层流自然对流换热进行了数值模拟研究.重点分析了Ra数和纳米颗粒的体积分数对自然对流换热特性的影响.数值模拟结果表明:在机油中添加多壁碳纳米管(MWCNT)粒子并没有提高基液的自然对流传热特性;对于给定的Ra数下,随着纳米颗粒体积分数的增大,纳米流体的传热特性也随之减弱;对于给定的体积分数,随着Ra数的增大,纳米流体的传热特性显著增强,但纳米流体的传热性能比机油的要弱,且在同一体积分数下随着Ra数的增大,传热性能减弱的程度要减小.     

低温推进剂贮箱增压过程的传热传质数学模拟

针对火箭发动机地面试验中低温液氧贮箱的预增压和增压过程建立了气相空间的传热、传质数学模型.运用实际气体的状态方程、连续性方程、能量守恒方程以及推进剂与气相空间的传热、传质方程等组成了关于气相空间参数的微分方程组,并运用四阶Runge-Kutta算法对其进行求解.获得了气相空间的压力、温度、增压气体流量、液氧挥发速率以及贮箱壁温等参数的变化规律.结果表明,在发动机启动前的预增压过程中,气相空间的温度和压力急剧增加,液氧的挥发速率也增加很快;发动机启动后的保持增压阶段,由于气相空间的体积不断发生变化,气相空间参数的变化趋于平缓,液氧表面向气相空间的传质速率也趋于稳定.     

Effect of thermal conductivity ratio on flow features and convective heat transfer

In this paper, we numerically investigate the steady laminar natural convection in a water-filled 2D enclosure containing a rectangular conducting body. Computations are performed for a wide range of dimensionless parameters including the Rayleigh number Ra, the thermal conductivity ratio k, and the location of the inner body δ, while the value of the Prandtl number is maintained constant at 6.8. This simulation spans four decades of Rayleigh number, Ra, from 103 to 106. The influence of these various dimensionless parameters on the flow behavior is investigated. Correlations of the averaged Nusselt numbers are obtained as a function of two parameters (Ra and δ) for each working thermal conductivity ratio. The results indicate that the heat transfer rate through the enclosure can be controlled by the position of the rectangular body. A comparative study was also carried out between a horizontal and a vertical conducting shape inside the enclosure. It is proved that a vertical position leads to a better heat transfer compared to the horizontal case.     

Combined momentum, heat and mass transfer in vertical slug flow absorbers

Numerical solutions have been obtained for the system of equations of momentum, heat and mass transfer describing the absorption of a refrigerant vapour from a Taylor bubble into the refrigerant-absorbent solution film around the bubble. The numerical results are compared with Nusselt's solution of the energy equation and with the penetration theory solution of the mass diffusion variation. Experimental data have been collected in vertical tubular absorbers in the slug flow region with the systems ammonia-lithium nitrate and ammonia-sodium thiocyanate. Four different absorber tubes have been tested with internal diameters of 10, 15, 20, and 25 mm. These data are compared with the numerical and theoretical results. The effect of the bubble nose on mass transfer is studied. Typical temperature profiles during the absorption process in absorption cooling/heating systems are shown.     

圆环腔内纳米流体自然对流的数值研究

采用Fluent软件对圆环封闭腔内的Ag-水纳米流体自然对流传热进行数值模拟,着重分析在不同瑞利数下Ag纳米颗粒的添加量和圆环内外壁半径比对圆环传热性能的影响.研究结果表明,随着瑞利数的增加,圆环间的换热强度不断加剧,换热由热传导逐渐向对流转变.添加纳米颗粒降低了换热性能,且随着颗粒浓度的增加换热效果不断恶化;同时,圆环半径比对换热有很大的影响,对一定的瑞利数而言随着半径比的减小,换热性能逐渐增强,且增大的趋势越来越显著.     

Experimental study of laminar natural convection heat transfer from a vertical cylinder to slush nitrogen under constant heat flux conditions

Natural convection heat transfer from a vertical cylinder immersed in slush and subcooled liquid nitrogen and subjected to constant heat fluxes was investigated in order to determine the relative merits of slush nitrogen (SlN₂) for immersion cooling. A glass dewar was used as a test vessel in which a cylindrical heater was mounted vertically, and heat transfer measurements were carried out for SlN₂ and subcooled liquid nitrogen (LN₂) in the laminar flow range. The results revealed advantages of SlN₂ over subcooled LN₂ in natural convection cooling. The local temperatures of the heated surface surrounded by solid nitrogen particles are measured to increase at much slower rates than in subcooled LN₂, which is due to the latent heat of fusion of solid nitrogen. Even after the solid nitrogen particles surrounding the heater are apparently depleted, the average heat transfer coefficients for SlN₂ are still found to be greater than those for LN₂ with the improvement in heat transfer being larger for lower Grashof number regime. Our analysis also indicates that solid nitrogen particles in close proximity to heated surface do not discourage local convection due to the porous nature of SlN₂, making the heat transfer in SlN₂ more effective than in the case of solid–liquid phase change of nitrogen involving melting and conduction processes.     

蒸发式冷凝器传热传质性能研究

建立单级压缩制冷循环蒸发式冷凝器实验台,研究逆流状态下,迎面风速和喷淋密度对传热传质的性能影响。通过调节水泵和风机的频率改变风水量的配比关系,得出蒸发式冷凝器的传热传质性能变化情况。分析数据得出,当迎面风速为2.96 m/s,喷淋密度为0.057 kg/(m·s)时,总传热系数达最优值628 W/(m2·K),传质系数随风速的增大而增大,随喷淋密度的变化不明显。此外,通过实验数据回归得到传质系数计算关联式。     

Combined heat and mass transfer in natural convection flow from a vertical wavy surface

Summary In the present paper, effects of combined buoyancy forces from mass and thermal diffusion by natural convection flow from a vertical wavy surface have been investigated using the implicit finite difference method. Here we have focused our attention on the evolution of the surface shear stress,f(0), rate of heat transfer,g(0), and surface concentration gradient,h(0) with effect of different values of the governing parameters, such as the Schmidt number Sc ranging from 7 to 1500 which are appropriate for different species concentration in water (Pr=7.0), the amplitude of the waviness of the surface ranging from 0.0 to 0.4 and the buoyancy parameter,w, ranging from 0.0 to 1.Notation C species concentration in the boundary layer - C species concentration of the ambient fluid - C _w species concentration at the surface - D chemical molecular diffusivity - f dimensionless stream function - g acceleration due to gravity - Gr _x local modified Grashof number - N ratio of the buoyancy forces due to the temperature difference and the concentration difference - p pressure of the fluid - T temperature of the fluid in the boundary layer - T temperature of the ambient fluid - T _w temperature at the surface - u, v thex- andy-components of the velocity field - x, y axis in the direction along and normal to the plate Greek symbols thermal diffusivity - _T volumetric coefficient of thermal expansion - _C volumetric coefficient of expansion with concentration - stream function - nondimensional similarity variable - x/L - density of the ambient fluid - v kinematic coefficient of viscosity - stream function - dimensionless skin friction - fluid viscosity     

A variance propagation algorithm for stochastic heat and mass transfer problems in food processes

A variance propagation algorithm for stochastic coupled heat and mass transfer problems subjected to first order autoregressive random process boundary conditions was developed. The algorithm is based on the finite element formulation of Luikov's coupled heat and mass transfer equations and involves the numerical solution of coupled Lyapunov and Sylvester matrix differential equations. It offers a cheap alternative to the Monte Carlo method for the computation of the mean value and variance of the temperature and moisture content field. The algorithm is generally applicable and can easily be inserted in any existing finite element code. Also, it can be extended to other types of random processes. The algorithm was applied to analyse the drying of a soybean kernel. Simulation results show that random fluctuations of the process conditions may cause considerable variability of the temperature and the moisture content within the drying soybean kernel. This is an important feature to take into account for the design of a drying process, and for thermal food processes in general. Copyright © 2001 John Wiley & Sons, Ltd.     

微型热管技术的研究现状与发展

微型热管（micro heat pipe，MHP）被广泛用于冷却航天、航空、军用武器、车辆、计算机等众多领域的电子设备，是有效冷却高热流密度电子器件的主要途径之一，已成为现代热管技术重要的发展方向和研究热点。目前对MHP的研究主要集中在管内流动及传热、传质的机制研究，新型高效结构形式的设计，制造工艺技术的改进等方面。较系统地总结近来MHP在理论、设计及制造工艺等方面的技术研究进展，综述其应用现状并分析其发展趋势。     

Continuous and dispersed phase coefficients for heat and mass transfer involving single-file drops

An attempt has been made in this paper at modelling analogous situations of heat or mass transfer involving single-file drops in liquid-liquid systems. Experimental data on continuous and dispersed phase coefficients have been correlated.