多孔介质喷雾干燥过程的热质传递

多孔介质是大量干燥过程的主体，由于实际多孔介质干燥过程的复杂性，建立通用的干燥过程传热传质模型十分困难。本文通过分析喷雾干燥过程中高初始含湿多孔介质与干燥介质之间的传热传质机理以及各因素对传热传质的影响，根据马歇尔方程探讨了干燥介质与料雾之间的水蒸汽分压差在干燥过程中的变化情况，反映了多孔湿介质在喷雾干燥操作中的传热传质过程的几种特性，为确定实际生产中喷雾干燥器的操作奈件指明了新的出路。     

高初始含湿多孔介质喷雾干燥过程的热质耦合传递

多孔介质是大量干燥过程的主体,由于实际多孔介质干燥过程的复杂性,建立通用的干燥过程传热传质模型十分困难。通过分析喷雾干燥过程中高初始含湿多孔介质与干燥介质之间的传热传质机理以及各因素对传热传质的影响,根据马歇尔方程探讨了干燥介质与料雾之间的水蒸汽分压差在干燥过程中的变化情况,反映了多孔湿介质在喷雾干燥操作中的传热传质过程的几种特性,为确定实际生产中喷雾干燥器的操作条件指明了新的出路。     

附加空气层的多层墙体热湿耦合非稳态传递模型及验证

为准确预测附加空气层的多层墙体内的温湿度分布和动态变化,研究多孔介质墙体内的热湿耦合非稳态传递规律,基于Luikov、Fick定律等基础传递理论,推导出热湿空气在墙体内部的瞬态耦合传递控制方程。通过对控制方程驱动势、方程项系数的改进,以空气含湿量和温度为驱动势,建立了建筑多孔介质墙体热湿耦合传递非稳态模型。采用有限容积法隐式差分格式设计了MATLAB模拟计算程序,设置相应的初始条件和边界条件,计算附加空气层的多层墙体内温度、湿度、传热和传湿量随时间变化的分布规律。最后,通过对比新建模型模拟结果与WUFI软件的模拟计算结果,验证了模型的准确性和可行性。     

存在非均匀内热源的多孔介质内自然对流传热传质的数值研究

采用数值方法分析了具有非均匀分布内热源的竖直同心套管内多孔介质中的传热传质,浮力比N由1.5变为-1.5后,流体由顺时针流动变为逆时针流动且流体速度加快。｜N｜〈1时边界层的发展主要由热浮力控制。管隙中心处浓度分布在R=0.8处出现一个最小值。     

具有非均匀内热源的多孔介质中自然对流传热传质的数值研究

采用数值方法分析了具有非均匀分布内热源的竖直同心套管内多孔介质中的传热传质,内热源分布系数M较大时,造成流场中心的逆时针环流向中心挤压。浮力比N由1.5变为-1.5后,流体由顺时针流动变为逆时针流动且流体速度加快。Nusselt数在Z=0.7处出现转折。随M增大内壁面Nusselt数变化范围增大,并且转折点前移。上壁面Sherwood数也呈先增大后减小的趋势,并且在R=0.9处出现转折。     

寒冷地区湿传递对多层墙体热湿性能的影响

墙体的传热传湿对建筑围护结构的热工性能、建筑能耗和室内环境有十分重要的影响。以相对湿度和温度为驱动势建立多层墙体一维非稳态热、湿和空气耦合传递模型（HAM模型）,并利用有限元法进行数值求解,重点关注湿传递对传热的影响机理。结果表明：考虑传湿时墙体交界面处湿度梯度大,相变速率大;墙体内部会产生湿积累,缩短墙体的使用年限;墙体内部温度上升幅度和上升速率大,墙体交界面处局部Nusselt数变化受湿传递的影响大;相变潜热量占总传热量的26.1%,计算空调负荷时不可忽略。     

含湿率对多孔介质热导率测量准确性的影响

本文利用HotDisk热常数分析仪对不同含湿率时含湿沙的热导率进行测量研究,并对孔隙尺度水分形态、分布及演化过程进行了观察实验研究。研究发现,低含湿率（＜25%）情况下,介质的有效热导率难以准确测得。低含湿时孔隙内水分主要以孤立液桥形式存在,测试过程中,贴近探头附近的水分由于受探头加热发生蒸发扩散而逐渐减少,导致探头周围试样构成持续发生变化,因而测量得到的热导率准确性较差。含湿率较高时,孔隙内水分相互联通,测试时水分蒸发扩散速度相对较慢,且存在毛细回流现象,探头周围试样构成维持恒定,因此可以准确获得热导率。     

多孔介质方腔内置芯片热流耦合的LBM数值模拟

基于格子Boltzmann方法的渗流多孔介质耦合双分布模型,对表征体元（REV）尺度下含电子芯片的多孔介质自然对流进行数值模拟研究,主要研究不同物性参数对多孔介质自然对流的影响以及单电子芯片尺寸、多芯片布局等因素对电子芯片表面散热性能的影响。得出了如下研究结果：对于恒温单芯片的多孔介质自然对流,在达西数Da=10-2时存在临界芯片尺寸。在临界芯片尺寸条件下,流场扰动较更小的芯片尺寸更强,传热性能却几乎不变。不同瑞利数Ra条件下临界芯片尺寸不同,Ra越大,临界芯片尺寸越大,在Ra=103时临界芯片尺寸为0.203125倍方腔边长,Ra=104时临界芯片尺寸为0.25倍方腔边长,Ra=105时临界芯片尺寸为0.390625倍方腔边长。当多孔介质渗透率降低时,即Da=10-4时,不存在临界芯片尺寸,且芯片表面和冷壁处的平均Nusselt数均随Ra的增大而增大。对于恒温多芯片的多孔介质自然对流,在多孔介质渗透率较大(Da=10-2)的情况下芯片横排布置可取得最佳换热效果,在渗透率较小(Da=10-4)时芯片布局宜采用对角分布。     

多孔介质内复合对流传热传质的数值分析

采用数值方法分析了具有非均匀内热源的竖直套管中复合对流传热传质,考查内热源分布系数M和热质二浮力比N对速度、温度、浓度分布以及Nusselt数和Sherwood数的影响。结果表明：当N〉1时速度V为正,其值随N的增加而增大;当N〈-1.5时,V则先负后正。随着M增大,等温线和等浓度线分布更加密集,传热和传质过程更加明显。     

耦合表面张力的封闭腔体内管外自然对流传热特性

采用格子-玻尔兹曼方法（LBM）构建了考虑表面张力影响下封闭腔体内加热圆柱外自然对流数学模型。在分别对自然对流与表面张力模型模拟验证的基础上,探究了液固表面张力与重力场下浮升力同时作用下加热圆柱外流体自然对流的传热特性。结果表明,在给定Ra数下（10³≤Ra≤10⁶）,随着表面张力的增大（Oh数减小）,封闭腔体内管外自然对流扰动会加剧,流型会变复杂,壁面换热效率有明显提高。在Ra数为10⁵,长度比（加热圆柱半径和方腔长度之比）为1/10情况下,加入表面张力σ=0.076302N/mm（Oh=0.122）,腔体左侧壁面平均努塞尔数和加热圆柱壁面与未加表面张力相比分别提高了93.5%和60.35%。当表面张力大小和自然对流的浮升力相当时,此时的流场波动更加明显剧烈,在一定范围内增大浮升力反而减弱换热。     

Natural convection heat and moisture transfer with thermal radiation in a cavity partially filled with hygroscopic porous medium

This article deals with natural convection heat and moisture transfer with thermal radiation in a cavity partially filled with hygroscopic porous medium. The governing equations for the momentum and heat transfer in both free fluid and hygroscopic porous medium and moisture content transfer in hygroscopic porous medium were solved by the finite element method. The radiative heat transfer is calculated by making use of the radiosity of the surfaces that are assumed to be grey. Comparisons with experimental and numerical results in the literature have been carried out. Effects of thermal radiation and Rayleigh number on natural convection and heat transfer in both free fluid and porous medium and moisture content transfer in porous medium were analyzed. It was found that surface thermal radiation can significantly change the temperature and moisture content fields in the regions of free flow and porous medium. The mean temperature at the interface decreases, the temperature and moisture content gradients are created on the upper two corners of the porous medium region, and the moisture content in the porous medium decreases in the porous medium as Ra increases.     

Successive linearisation analysis of unsteady heat and mass transfer from a stretching surface embedded in a porous medium with suction/injection and thermal radiation effects

In this study, we investigate the application of the new successive linearisation method (SLM) to the problem of unsteady heat and mass transfer from a stretching surface embedded in a porous medium with suction/injection and thermal radiation effects. The governing nonlinear momentum, energy and mass transfer equations are successfully solved numerically using the SLM approach coupled with the spectral collocation method for iteratively solving the governing linearised equations. Comparison of the SLM results for various flow parameters against numerical results and other published results, obtained using the homotopy analysis method (HAM) and Runge–Kutta methods, for related problems indicates that the SLM is a very powerful tool which is much more accurate and efficient than other methods. The SLM converges much faster than the traditional methods like the HAM and is very easy to implement. © 2011 Canadian Society for Chemical Engineering     

Quasi-steady state natural convection in a tilted porous layer

Quasi-steady state laminar natural convection in an inclined porous layer is studied analytically and numerically. On the basis of the Darcy-Oberbeck-Boussinesq equations, the problem is solved analytically in the limit of a thin porous layer heated on one side by a constant heat flux while the other boundaries are maintained adiabatic. For quasi-steady state, the flow and temperature fields and overall heat transfer rates are obtained in terms of the controlling parameters and the onset of convection in a bottom-heated horizontal system is predicted. It is also demonstrated for the case of a bottom-heated layer that for sufficiently small inclinations, multiple unicellular quasi-steady states exist, some of which are unstable. A numerical study of the same phenomenon, obtained by solving the complete system of governing equations, is conducted. Good agreement is found between the analytical predictions and the numerical simulation.     

有内热源的多孔层中自然对流的稳定性分析

采用局部非热平衡模型, 通过数值法和Garlerkin近似法, 分析存在均匀内热源和边界浓度梯度时, 有效热导率比、流体和固相间的传热系数、浓度梯度的大小以及内热源在流体与固相内的分布情况对水平多孔层中临界内热源Rayleigh数的影响, 来研究相关参数对自然对流的稳定性的影响, 并得到临界内热源Rayleigh数的表达式。结果表明, 浓度Rayleigh数的增加可以促进自然对流的形成;内热源为正时, 自然对流的形成区域主要位于上半区域;内热源为负时, 自然对流的形成区域位于下半区域, 内热源总是促进自然对流的发生;有效热导率比、流体和固相间的内部传热系数、内热源在流体与固相内的分布情况相互耦合, 影响自然对流的稳定性, 这种影响取决于各参数的范围。     

Mixed convection heat transfer in porous media in the non-darcy regime

In this study mixed convection heat transfer in a homogeneous porous duct of square cross section in a horizontal orientation is examined. Results from a generalized Forchheimer model are compared with that from the Darcy model. The heat transfer rate and the flow behavior depend on the following parameters: Grashof number, Gr = Q'gβKa/kv², an axial flow pressure drop parameter, ζ = (aK/vμ)dp'/dz', an inertial parameter ξ = mK/a, appearing in the Forccheimer model and the Prandtl number, Pr = C_pμ/k. In the Darcy limit, ξ → 0, the role of the axial flow parameter, λ is reduced to a mere scale factor and the flow behavior is determined by a single parameter, λ = Gr · Pr. Both the Darcy and the Forchheimer models exhibit dual solutions and a hysteresis behavior over a certain range of Gr. Such parametric dependence can be used as an additional tool along with carefully designed experiments to determine the importance of inertial and Prandtl number effects on convective heat transfer in porous media.     

LTNE条件下界面对流传热系数对部分填充多孔介质通道传热特性的影响

在多孔介质区考虑局部非热平衡,采用Brinkman-extended Darcy模型结合应力跳跃条件对部分填充多孔介质通道内流体传热特性进行分析。获得了各区域温度分布及Nusselt数解析解,并分析了各参数对温度及Nusselt数的影响。结果表明：界面对流传热系数H_s较小时,界面应力跳跃系数β和Darcy数Da的增加会减小流固两相间温差。而在高H_s下,Da减小也会减小两相温差。在Da、H_s和固流两相热导率之比K较大且空心率S（自由流体区高度与通道高度之比）和Biot数Bi较小时,流固两相间会在接近多孔介质区中部出现最大温差,而该最大温差会随着S增加和Da及H_s的减小向界面区移动。对于不同K与Bi,Nusselt数Nu与S的关系曲线存在不同的类型,与模型A（界面处多孔介质固相和流相根据各自温度梯度和热导率划分总热流）不同的是,采用模型C（界面处固相热流分配与自由流体区流相的热交换相关）所获得的Nu曲线类型与H_s有关。在K较小时,β对Nu的影响大于H_s对Nu的影响;而在K较大时,H_s对Nu的影响要远大于β对Nu的影响,且H_s增加会明显提高通道内的Nu。     

Flow transition for natural convective heat transfer in a porous medium saturated with water near 4°C

In this paper, the problem of buoyancy-induced convection flow in water-saturated porous media near 4°C is examined using a numerical model. Darcy's law is used to model flow behavior and a single equation convective heat transfer model is used for the energy equation. As the Boussinesq approximation is not valid for this case, a parabolic dependence of density on the temperature is used. Natural convection is generated and sustained by a uniform heat source. Flow behavior is governed by three natural parameters appearing in the model. They are: (i) dynamical parameter, (ii) geometric parameter, γ = b/a; and (iii) wall temperature, in relation to the reference temperature at the density extremum. For certain ranges of θ_w (<0) and Gr, interesting density inversion effects are possible. Transient solutions are obtained for various aspect ratios and modified Grashof number values. For a wide range of Grashof number, steady state solutions could not be obtained. Flow mutations into periodic and chaotic solutions are investigated for a range of Grashof number (100 to 40,000) and aspect ratio values (1 to 10).     

Natural convection and flow simulation in differentially heated isosceles triangular enclosures filled with porous medium

A finite element analysis is performed to investigate the effects of uniform and non-uniform heating of bottom wall on natural convection flows within isosceles triangular enclosures filled with porous medium. The detailed analysis is carried out in two cases depending on various thermal boundary conditions:

(I)

two inclined walls are maintained at constant cold temperature while the bottom wall is uniformly heated;

(II)

two inclined walls are maintained at constant cold temperature while the bottom wall is non-uniformly heated.

The present numerical procedure adopted in this investigation yields consistent performance over a wide range of parameters of Darcy number, Da (10^-5?Da?10^-3), Rayleigh number, Ra (10³?Ra?10⁶) and Prandtl number, Pr (0.026?Pr?1000) in all the cases mentioned above. Numerical results are presented in terms of stream functions, temperature profiles and Nusselt numbers. It is observed that at small Darcy numbers, the heat transfer is primarily due to conduction irrespective of Pr. As the Darcy number increases, there is a change from conduction dominant regime to convection dominant regime. Flow circulations are also found to be strong functions of Pr at large Da (Da=10^-3) and multiple circulation cells occur at small Pr with Ra=10⁶. Non-uniform heating of the bottom wall produces greater heat transfer rate at the center of the bottom wall than uniform heating case, but average Nusselt number shows overall lower heat transfer rate for non-uniform heating case. As average Nusselt number is same on both the inclined walls, the average Nusselt number for bottom wall is times that of the inclined wall which is well matched in two cases considered for verifying the thermal equilibrium of the system. The correlations are proposed for average Nusselt number as functions of Ra for various Darcy and Prandtl numbers.     

The effect of liquid film vaporization on natural convection heat and mass transfer in a vertical tube

A numerical analysis was carried out to investigate the effects of film vaporization on natural convection heat and mass transfer in a vertical tube. Results for interfacial Nusselt and Sherwood numbers are presented for air-ethanol and air-water systems for various conditions. Predicted results show that heat transfer along the gas-liquid interface is dominated by the transport of latent heat in association with the vaporization of the liquid film. Additionally, the predicted results obtained by including transport in the liquid film are contrasted with those where liquid film transport is neglected, showing that the assumption of an extremely thin film made by Chang et al. (1986) and Yan and Lin (1990) is only valid for systems with small liquid mass flow rates. For systems with a high liquid film Reynolds number, Re₁₀, the assumption of an extremely thin film is seriously in error.     

Radiation and conduction heat transfer coupled with liquid water transfer,moisture sorption,and condensation in porous polymer materials

A new one‐dimensional mathematical model to simulate the complex coupled heat and moisture transfer in porous polymer materials is presented. The new model takes into account effects of multiple involved processes such as radiation and conduction heat transfer, liquid capillary action, moisture sorption, and condensation. The technique of volume of fraction (VOF) is used to model the dynamic distribution of moisture in different phases, that is, vapor and liquid. The finite volume method (FVM) is used to develop the numerical computational scheme to solve the model. The temperature change on the fabric surface derived from the computational results of the model is compared with experimental measurements with reasonable agreement between the two. Further numerical simulations were carried out to investigate the complex interactions and coupling effects among the various heat and moisture‐transfer processes involved. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2780–2790, 2003