加热板上固着液滴内部的浮力-Marangoni对流

为了揭示液滴内部的流动与传热机制,本文对加热板上固着液滴内部的浮力-Marangoni对流进行了数值模拟研究,首先对浮力对流和Marangoni对流进行分离研究,后研究了两者共同作用的情况,并讨论了Pr数、Re数、Bi数、Ra数、Ma数和接触角对内部流场和温度场的影响.结果表明,纯Marangoni对流在液滴右侧中截面上表现为逆时针涡旋,而纯浮力对流存在静态分岔现象,给定不同方向的涡旋作为初始条件,当Ra数大于临界Ra数后,最终稳定时液滴内部的涡旋方向与给定的初始方向一致,在本文的数值模拟中该分叉点约为2 500.液滴内部的浮力-Marangoni对流也存在静态分岔,在Ra数较小时,液滴内部的涡流方向纯Marangoni对流方向一致,在Ra数大于临界Ra数后,该方向与所给初始流场一致,但该分叉点受Ma数影响,增大液滴的Ma数可以使分岔受到抑制.     

浮力-热毛细对流表面位型研究

将栅线结构光源系统与图像处理技术相结合,形成一种实时诊断浮力-热毛细对流表面形貌的光学测量系统.研究两端带有温差的矩形池内薄层流体的表面变形.应用图像互相关方法对实验结果进行计算和分析,得到了流体表面变形定量的实验结果.实验结果表明了在浮力-热毛细对流的发展过程中,出现流体表面变形,温度梯度、表面张力、浮力、液固亲润问题、以及壁面温度决定了该表面变形大小和变形方向.     

对流热流量的热线测量法

测量流过某一截面上的对流热流量,可以归结为测量流过该截面上各点介质的温度和速度。本文介绍利用热线风速仪恒流电桥将热线作为电阻温度计测定截面上各点温度的方法,设计了一种简易的探头温度校正装置。在速度测量中,讨论了热线温度补偿方法,介绍了极低速时的热线校正方法并建立了一种相应的校正设备。     

环形液池内中等Pr数流体的浮力-热毛细对流

为了了解水平温度梯度作用时环形液池内的浮力-热毛细对流特性，利用 有限差分法进行了非稳态三维数值模拟，环形液池外壁被加热，半径为40 mm，内壁被冷却， 半径为20 mm，液池深度为3~17 mm，液池内流体为0.65cSt的硅油，其Pr数 为6.7. 模拟结果表明，当水平温度梯度较小时，流动为轴对称稳态流动，随着温度 梯度的增加，流动将会失去其稳定性，在浅的液池内(d=3mm)，转化成三维振荡 流动，在深的液池内(d≥6mm)，转化成三维稳定流动；模拟计算的 临界温差及表面温度分布图像与实验结果基本吻合.     

液层厚度对浮力-热毛细对流面型的影响

将Michelson光学干涉测量系统与图像处理技术相结合，发展形成一种实时诊断热毛细对流和浮力对流流体表面形貌的实验测量系统. 采用光学干涉测量方法研究了两端带有温差的矩形池内薄层流体的对流、表面变形、以及表面波的基本问题. 应用Fourier变换方法对实验结果进行计算和分析，得到了流体表面变形和表面波的定量的实验结果. 实验结果表明了在浮力-热毛细对流的发展过程中，首先出现流体的表面变形，之后在该变形的基础上，叠加了一个表面波的信息，该表面变形和表面波与流体的温度梯度、表面张力、以及浮力有直接的关系；表面波隐藏在表面变形内.     

凝固界面前沿自然对流的实验研究

本文在水平Bridgman定向凝固装置中,对超薄试样中固液界面前沿的自然对流进行了实验研究。发现在厚度仅为200μm—700μm的试样中,由于水平温度梯度的存在,凝固界面附近仍然存在着不可忽视的自然对流,并系统研究了各种不同因素对界面前沿热对流的影响规律,为界而过程的微观分析和凝固理论的研究提供了丰富的信息,为采用该装置进行透明有机物模拟研究提供了设计依据。     

环形液池内中等Pγ数流体的浮力-热毛细对流

为了了解水平温度梯度作用时环形液池内的浮力-热毛细对流特性,利用有限差分法进行了非稳态三维数值模拟,环形液池外壁被加热,半径为40 mm,内壁被冷却,半径为20 mm,液池深度为3～17mm,液池内流体为0.65cSt的硅油,其Pr数为6.7.模拟结果表明,当水平温度梯度较小时,流动为轴对称稳态流动,随着温度梯度的增加,流动将会失去其稳定性,在浅的液池内(d=3 mm),转化成三维振荡流动,在深的液池内(d≥6mm),转化成三维稳定流动;模拟计算的临界温差及表面温度分布图像与实验结果基本吻合.     

一种用于分析封闭腔内湍流自然对流换热的k-ε新模型

由于目前用于求解湍流自然对流流动与传热的k-ε模型在应用过程中存在不足,结合高雷诺数k-ε模型需要借助壁面函数法来确定壁面上相关参数值和低雷诺数k-ε模型在近壁区布置更多节点以便获得粘性底层详细信息的特点,重新定义了湍流普朗特数σt的计算式,提出了一种修正的k-ε新模型;利用该模型对封闭方腔内的湍流自然对流流动与传热进行了数值分析。结果表明:与文献中数值模拟结果相比,当108≤Ra≤1014时本文模型所得壁面平均努塞尔特数更接近文献中的实验值,与实验值之间的相对误差在8%以内;壁面的局部努塞尔特数与文献中的实验值吻合得较好。这说明本文模型用于求解封闭腔内湍流流动与传热问题是合适的,比其它湍流模型更能准确地描述封闭腔内湍流自然对流换热中边界层发展与壁面传热特性之间的内在联系。     

热毛细对流的液桥几何位形及浮力效应

本文讨论重力对不同高度、直径比液桥的热毛细对流的影响。当液桥高度、直径比增大时,液桥中的等流函数线呈双涡结构,这种流动图样并不必然与热毛细振荡流相联系。在地面热毛细对流实验中模拟空间微重力情况,液桥高度需小于1.5mm。在微重力环境中,液桥内的流场和温度分布介于地面相同参数液桥的上部加热和下部加热两种结果之间。因此,可以用地面实验结果估计空间液桥的对流和热输运情况。     

Mixed Convection Effect on Melting from a Vertical Plate in a Porous Medium

In the present work, the effect of mixed convection about vertical surfaces on the phenomenon of melting process in a fluid-saturated porous medium is analyzed on the basis of boundary layer approximations. Similarity solutions are obtained for aiding external flow. The final similarity equations are integrated numerically by use of the fourth-order Runge–Kutta method. Results are reported for the flow and thermal fields in the melt region. The melting phenomenon decreases the local Nusselt number at the solid–liquid interface.     

A Two-Equation Analysis of Convection Heat Transfer in Porous Media

This paper presents a two-equation analysis on the convection heat transfer in porous media based on the modeling developed by Carbonell and Whitaker (1984). The porous system under consideration is bounded by two parallel walls and heated uniformly from one side surface. The Darcy flow is imposed and the fully developed heat transfer is assumed. General solutions, which take into account the additional convective and conductive terms, are obtained for the temperature fields and the Nusselt number. The detailed studies are presented for the porous systems characterized by consolidated and unconsolidated circular unit cells. The results show that, for the consolidated unit cell case, a prediction without the additional convective term overestimates the heat transfer, while for the unconsolidated unit cell case, this effect is negligible. The additional conductive terms are also examined and found to act conventionally as part of the conductive terms.     

Mixed Convection on a Horizontal Surface Embedded in a Porous Medium: the Structure of a Singularity

The mixed convection boundary-layer flow on a horizontal impermeable surface embedded in a saturated porous medium and driven by a local heat source is considered. Similarity solutions are obtained for specific outer flow variations and these are shown to have a solution only for parameter values greater than some critical value. When this is not the case the solution develops a singularity at a finite distance from the leading edge. The nature of this singularity is also discussed.     

Intensification of Heat Transfer in a Downward Liquid-Film Flow

Heat transfer in a film flow of the FC-72 dielectric liquid down a vertical surface with an embedded 150×150 mm heater is experimentally examined in the range of Reynolds numbers Re = 5–375. A chart of liquid-film flow modes is constructed, and characteristic heat-transfer regions are identified. Data on the dependence of heater-wall temperature and local heat flux at the axis of symmetry of the heater on the longitudinal coordinate are obtained. Local and mean heat-transfer coefficients are calculated. It is shown that enhanced heat transfer is observed in the region where rivulets starts forming in the low-Reynolds-number liquid-film flow.     

Influence of Variable Permeability on Combined Convection Along a Nonisothermal Wedge in a Saturated Porous Medium

Mixed convection along a vertical nonisothermal wedge embedded in a fluid-saturated porous media incorporating the variation of permeability and thermal conductivity is studied. The surface temperature is assumed to vary as a power of the axial coordinate measured from the leading edge of the plate. A nonsimilar mixed convection parameter and a pseudo-similarity variable are introduced to cast the governing boundary layer equations into a system of dimensionless equations which are solved numerically using finite difference method. The entire mixed convection regime is covered by the single nonsimilarity parameter =[1+(Ra _x /Pe _x )^1/2]^–1 from pure forced convection (=1) to pure free convection (=0). The problem is solved using nonsimilarity solution for the case of variable wall temperature. Velocity and temperature profiles as well as local Nusselt number are presented. The wedge angle geometry parameter is ranged from 0 to 1.     

Mixed Convection in a Horizontal Channel with Local Heating from Below

Mixed convection induced in the entrance region of a horizontal plane channel by a bottom heat source of finite dimensions is considered. The calculations were performed for the Prandtl number Pr = 1, Grashof numbers ranging from 4 · 10³ to 3.2 · 10⁴, and Reynolds numbers varying from 0 to 10. The dimensions of the heat source and its location were also varied. The results were obtained from a numerical solution of the 2D unsteady Navier-Stokes equations in the Boussinesq approximation, written in vorticity – stream function – temperature variables. The solution was found by the Galerkin finite element method.     

Mixed Convection Heat Transfer in the Annulus Between Two Concentric Vertical Cylinders Using Porous Layers

A numerical investigation of the steady-state, laminar, axi-symmetric, mixed convection heat transfer in the annulus between two concentric vertical cylinders using porous inserts is carried out. The inner cylinder is subjected to constant heat flux and the outer cylinder is insulated. A finite volume code is used to numerically solve the sets of governing equations. The Darcy–Brinkman–Forchheimer model along with Boussinesq approximation is used to solve the flow in the porous region. The Navier–Stokes equation is used to describe the flow in the clear flow region. The dependence of the average Nusselt number on several flow and geometric parameters is investigated. These include: convective parameter, λ, Darcy number, Da, thermal conductivity ratio, K _r, and porous-insert thickness to gap ratio (H/D). It is found that, in general, the heat transfer enhances by the presence of porous layers of high thermal conductivity ratios. It is also found that there is a critical thermal conductivity ratio on which if the values of Kr are higher than the critical value the average Nusselt number starts to decrease. Also, it found that at low thermal conductivity ratio (K _r ≈ 1) and for all values of λ the porous material acts as thermal insulation.     

Marangoni Mixed Convection Boundary Layer Flow

The paper deals with a steady coupled dissipative layer, called Marangoni mixed convection boundary layer, which can be formed along the interface of two immiscible fluids, in surface driven flows. The mixed convection boundary layer is generated when besides the Marangoni effects there are also buoyancy effects due to gravity and external pressure gradient effects. We shall use a model proposed by Golia and Viviani (L’ Aerotecnica missili e Spazio 64 (1985) 29–35, Meccanica 21 (1986) 200–204) wherein the Marangoni coupling condition has been included into the boundary conditions at the interface. The similarity equations are first determined, and the pertinent equations are solved numerically for some values of the governing parameters and the features of the flow and temperature fields as well as the interface velocity and heat transfer at the interface are analysed and discussed.     

Buoyancy Convection in Low Prandtl Number Liquids with Large Temperature Variation

Two-dimensional laminar convection in low Prandtl number liquids driven by the buoyancy force is studied. The liquid is contained in a closed square cavity with isothermal vertical walls kept at different temperatures. The top and bottom walls are assumed to be insulated. The thermal conductivity of the liquid is assumed to depend on temperature. ADI and SOR schemes are employed. The heat transfer is found to decrease appreciably across the cavity with a decrease in thermal conductivity.     

Mixed Convection in a Vertical Porous Channel

A numerical study of mixed convection in a vertical channel filled with a porous medium including the effect of inertial forces is studied by taking into account the effect of viscous and Darcy dissipations. The flow is modeled using the Brinkman–Forchheimer-extended Darcy equations. The two boundaries are considered as isothermal–isothermal, isoflux–isothermal and isothermal–isoflux for the left and right walls of the channel and kept either at equal or at different temperatures. The governing equations are solved numerically by finite difference method with Southwell–Over–Relaxation technique for extended Darcy model and analytically using perturbation series method for Darcian model. The velocity and temperature fields are obtained for various porous parameter, inertia effect, product of Brinkman number and Grashof number and the ratio of Grashof number and Reynolds number for equal and different wall temperatures. Nusselt number at the walls is also determined for three types of thermal boundary conditions. The viscous dissipation enhances the flow reversal in the case of downward flow while it counters the flow in the case of upward flow. The Darcy and inertial drag terms suppress the flow. It is found that analytical and numerical solutions agree very well for the Darcian model. An erratum to this article is available at .