多孔介质传热传质中耦合扩散效应的研究

本文采用加罚函数有限元方法数值模拟在温度梯度和浓度梯度同时存在时双浮升力自然对流及其伴随的传热传质,着重探讨了多孔介质中传热传质的交叉耦合扩散Soret效应和Dufour效应的基本影响规律,展示了流场、温度场和浓度场随热附加扩散准则数和扩散附加热准则数的变化状况。     

考虑毛细滞后效应的未饱和含湿多孔介质传热和传质理论

毛细滞后是未饱和多孔介质中很重要的特性之一。在许多情况下，毛细滞后现象对介质中传热和传质过程有着显著的影响。本文引入最小梯度概念，用以描述多孔体中毛细水运动滞后的定量行为，进而建立了考虑毛细滞后效应的未饱和含温多孔介质传热和传质的系统理论，为进一步定量研究毛细滞后对传热传质的影响打下了理论基础。     

多孔介质非饱和渗流的阈梯度理论

一、前言 毛细滞后现象对非饱和多孔体中的传热与传质过程有重要的影响,但由于对毛细滞后现象尚缺少基本的认识,目前关于多孔介质传热传质的理论暂且都回避对这一问题的处理,这也正是当前有关多孔介质传热传质特性数据和实用计算方法缺乏和可靠性差的重要原因。     

分形介质的传热与传质分析(综述)

本文论述了分形介质的分形理论和数学基础,并简要综述了用分形理论和方法研究分形介质的传热与传质特性(如多孔介质的渗透率、热导率以及池核态沸腾换热)方面目前所取得的研究进展,最后扼要展望了用分形理论和方法进一步研究分形介质的传热与传质的可能的若干课题和方向。     

对流边界条件高强度快速传热传质研究

在考虑非傅立叶效应和非斐克效应的基础上,对所建立的高强度快速传热传质的耦合方程进行了对流边界条件的数值模拟．结果表明：由于超常传热传质的耦合作用会造成多孔介质内部“局部增湿”现象,且温度和湿度的变化会出现滞后现象．     

多孔介质快速干燥过程热质耦合方程的代数显式解析解

对多孔介质快速干燥过程的传热与传质耦合方程组导出了两套代数显式解析特解。这些解首先可以作为计算传热传质学的标准解,用以检验数值计算的准确性、收敛性与稳定性等,还可以启发数值工作者改进计算技巧例如差分格式与网格生成技术等。当然,解析解还会有其相应的理论价值。     

饱和含混多孔介质自然对流传热传质的电化学方法实验研究

本文建立了饱和含湿多孔介质自然对流传热传质实验台，利用电化学方法，在封闭腔体的两竖壁上建立浓度梯度，实验探讨了两竖壁保持不同的温度和浓度时，腔体内部的温度分布及边壁上的传热传质特性。     

多孔介质高强度传热传质的理论研究

提出了高强加热下含湿多孔介质传热传质新模型,模型包括的水分种类齐全、水分迁移机制全面,假设条件相对较少,考虑了非Fourier传热效应和非Fick传质效应,模型通过具体的含湿量分区分析得到简化。     

多孔介质快速干燥过程中热质耦合效应的研究

本文针对一维多孔介质的第一类边界条件,对高强度快速干燥条件下热质交换的普遍微分方程组进行求解。研究了高强度快速干燥过程中多孔介质内部传热传质之间的相互耦合现象以及初始边界条件的变化对热质传递的影响,为多孔介质内部温度场和湿度场的控制奠定理论基础。     

基于质量积耗散的膜法除湿组件结构设计

建立了错流中空纤维膜液体除湿组件的二维传热、传质数学模型,引入研究传质过程质量积耗散的湿阻来衡量溶液除湿过程中水分传递的不可逆性。通过分析膜法除湿组件中除湿与传热耦合的湿空气水分传递过程,得到了热湿传递过程中传质的阻力表达式。通过一个典型设计实例,得出了可以降低中空纤维膜除湿组件不可逆传质损失的管长、根数、管间距、排列方式等设计参数。     

封闭腔内纳米流体强化自然对流换热的数值模拟

采用F luen软件对封闭腔内Cu-H2O纳米流体强化自然对流换热进行了数值模拟,重点分析Cu纳米粒子添加量和Gr数对换热性能的影响,并解释其换热机理。研究结果表明:在水基液中加入Cu纳米粒子可以显著提高基液的自然对流换热特性。对于一给定的Gr数,随着纳米粒子质量分数的增加,纳米流体的速度组成部分增加,纳米流体质量分数越大,x方向和y方向的速度峰越大,因此加速了流体中能量传输。另一方面,随着Gr数的增加,流线图中旋涡逐渐变大,流线间强度增加,说明换热效果逐渐增强。当Gr数较小时,传热主要是由热壁和冷壁之间的热传导引起的,随着Gr数的增大,换热逐渐变为由对流换热占主导地位。     

THERMOSOLUTAL CONVECTION WITH COUPLED HEAT AND MASS TRANSFER PROCESS IN A SQUARE ENCLOSURE

Thermosolutal convection flow and its effect on the heat and the mass transfer in a square enclosure is studied experimentally. Both thermal and solute diffusion are induced from the sides, and natural convection is initiated by the combined thermal and solutal buoyancies, which either augment or oppose to each other. The solute diffusion is initiated in an electrochemical system that uses copper sulfate-sulfuric acid solution as an electrolyte. Depending on the magnitude of buoyancy ratio, three different kinds of flow regimes and structures can occur, which lead to different distributions of concentration in the enclosure. The formation and growth of layered flow structure is attributed to the solutal boundary-layer flow that can intrude and accumulate along the horizontal wall. The nearly stagnant layer that occurs can reduce the heat transfer rate. The Nusselt numbers at different flow regimes are measured and correlated in terms of relevant nondimensional parameters. This suggests the correlation of Sherwood number in different ranges of buoyancy ratio. The visualization of flow structures and measurements of both heat and mass transfer allow better understanding of the complicated system.     

NATURAL CONVECTION FROM DISCRETE HEAT SOURCES IN A VERTICALLY VENTED RECTANGULAR ENCLOSURE

Natural convection in a discretely heated, vertically vented enclosure has been investigated experimentally. A vertically vented enclosure is one whose top and bottom boundaries are partially open, allowing ambient fluid to be drawn in by buoyancy. Mach-Zehnder interferometry was used to visualize the temperature field within the enclosure and to determine the local and average heat transfer characteristics of the discrete heat sources. A smoke-generation technique was used to visualize the flow structure. The experimental parameters investigated include Grashof number, vent gap width, and heater locations for a dual heater configuration and a single-enclosure aspect ratio. The vent gap width was varied between a fully open condition (discretely healed parallel plates) and nearly closed. The Mach-Zehnder interferograms and local heat transfer results suggest a complex buoyancy-driven flow field that depends intimately on the rent gap width. For some geometric configurations, the heat transfer coefficient was found to be nearly uniform across the length of the heater, while for others traditional boundary-layer-type heat transfer characteristics were found. In general, the heat transfer coefficient increased with increased vent gap spacing. However, a maximum in average Nusselt number was observed for a dimensionless gap spacing of G/W – 0.67 for one of the two dual-heater placement configurations studied. The heat transfer characteristics were compared to those of a single isolated plate and unobstructed parallel-plate channels.     

The influence of cross effects on the characteristics of heat and mass transfer in the conditions of conjugate natural convection

Mathematical modeling of nonstationary conjugate heat and mass transfer is carried out with regard to Soret and Dufour effects in an enclosure with heat-conducting impermeable walls of finite thickness with local heat and mass sources under convective-radiative heat exchange with the environment. The study is based on the two-field method. Temperature, concentration, and stream function distributions in the object analyzed are obtained over a wide range of changes of governing parameters.     

Transient electrohydrodynamic convective flow and heat transfer of MWCNT - Dielectric nanofluid in a heated enclosure

A computational research was performed to analyze the electrohydrodynamic (EHD) convective heat transfer in a differentially heated dielectric-MWCNT nanofluid layer. The study was conducted on a square enclosure subjected to a temperature gradient between these two vertical walls as well as a potential difference between these horizontal walls. The enclosure was filled with MWCNT oil-based nanofluid; the MWCNT nanoparticles were dispersed in a perfectly insulating thermal oil with a volume fraction of hardly exceeded 0.4%. The governing equations were derived with the assumption of homogeneous nanofluid and were solved with employing finite volume method. Based on the obtained results, it was found that the increase of Rayleigh number, electric Rayleigh number and nanoparticle concentration enhanced the heat transfer. For high thermal and electric Rayleigh number values, the flow and heat transfer became time dependent and accordingly a frequency study was also performed. It was found that the inclusion of an electric field with the addition of nanoparticles led to a significant heat transfer enhancement of about 43%.     

三维方腔内竖直平板混合对流换热的数值研究

本文建立了三维方腔内竖直板混合对流换热的数学模型,探讨Re数,进风口位置、竖直板位置对混合对流换热的影响.结果表明:随Re数增加,竖直板下部的平均Nu2下降,而总体平均Nu1增加.随送风口高度增加,竖直板对流换热量增加.在竖直板位置L1/H=0.5～1.5的范围内,竖直板处于中间位置L1/H=1.0时,竖直板混合对流换...     

Numerical analysis of nonsteady-state conjugate natural convection between two concentric spheres

Mathematical modeling of the regimes of free convection heat transfer between two concentric spheres is performed. The temperature of the internal sphere is constant during the whole process; the external sphere with a heat-conducting enclosure of finite thickness is affected by the surrounding medium. The boundary-value problem is formulated based on the mass, momentum, and energy conservation laws in dimensionless stream function-velocity vorticity vector-temperature variables in spherical coordinates. The obtained distributions of temperature and streamlines represent the effect of both energy source and environment on the flow regimes and the heat transfer.     

封闭圆内开缝圆自然对流换热的振荡特性

本文通过数值计算探讨了封闭圆内开缝圆自然对流换热的振荡特性。数值计算以整个圆为计算区域,采用了非稳态的数学模型和具有QUICK差分格式的SIMPLE算法。在相同条件下计算结果和实验结果符合很好。数值结果显示, 当几何结构一定时,Rayleigh数Ra小于某个临界值时,流动和换热处于稳态,并且关于垂直中心线对称;Ra大于这个临界值时,流动和换热是振荡的,非对称的。数值实验还表明,流动和换热出现振荡时的临界Rayleigh数Rac与开缝圆的开缝度有关,且流动和换热的振荡会出现对称振荡和非对称振荡两种情形。