封闭腔内纳米流体自然对流换热的数值模拟

采用数值模拟方式研究充满了纳米流体的封闭腔内的稳态自然对流。重点分析了纳米颗粒的体积分数,Ra数以及不同类型纳米颗粒对自然对流换热特性的影响。数值模拟结果表明:在纯水中加入纳米颗粒可以显著提高基液的自然对流换热特性;对于给定的Ra数下,随着纳米流体的体积分数增大,纳米流体换热效果显著增强;对于给定的体积分数下,随着Ra数增大,纳米流体的换热强度也随之增大,并且换热机理由热传导为主变为热对流为主;通过Ag,Cu,CuO和Al2O3四种纳米颗粒的对流换热效果比较分析得出,金属Ag和Cu纳米颗粒比金属氧化物CuO和Al2O3的纳米颗粒制备的纳米流体的对流换热效果更好。     

倾斜方腔内纳米流体自然对流的数值研究

对充满Cu-水纳米流体的二维倾斜方腔内的自然对流换热进行了数值研究,研究了纳米颗粒体积份额φ、Ra数和腔体倾角对自然对流换热特性的影响。数值模拟结果表明:在纯水中加入纳米颗粒可以显著提高基液的自然对流换热特性;在一定的腔体倾角φ下,增大Ra均可以强化腔体内的自然对流换热,而且随Ra数的增大,强化效果越强;当Ra=104时,对不同的腔体倾角,纳米流体对换热强度提高的比例大致相当。     

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.     

周期性边界变化条件下封闭腔内纳米流体的流动模拟

文献采用数值模拟方式研究充满了纳米流体封闭腔内的瞬态自然对流。热边界条件为温度周期性变化的正弦函数。重点分析了在同一Ra下,正弦函数的振幅和频率(或周期)对纳米流体强化对流换热的影响;结果显示:随着热边界的正弦函数的振幅和频率变大,热边界的瞬态Nu数和平均Nu数也越大;同时还观察封闭腔内的漩涡中心,随时间变化周期性发生转移;这些结果对后期研究纳米流体的瞬态自然对流有很大的帮助。     

壁温周期振荡条件下方腔内纳米流体自然对流换热研究

对充满Cu-水纳米流体方腔内的非稳态自然对流换热进行了数值研究。综合研究了不同振荡波形下,纳米颗粒的体积份额和振荡波幅A对自然对流换热的影响。结果表明:在水中添加Cu纳米颗粒可以强化水的自然对流换热;体积份额和振荡波幅A的增大都加强纳米流体自然对流换热,但是不同振荡波形,加强程度不同并提出振荡面积S的概念,即振荡波形与其平均值围成的面积,曲线拟合显示时均努塞尔特数Nu随S增大呈指数曲线上升。     

层流状态下纳米流体的对流传热特性

通过数值模拟的方法研究了层流状态下雷诺数、体积分数、颗粒和基液种类以及颗粒粒径对纳米流体对流传热特性的影响。研究结果表明,纳米流体的对流传热系数明显高于基液,并且与基液和颗粒的性质、颗粒的体积分数及颗粒粒径密切相关。纳米流体的对流传热系数随着颗粒和基液热导率的增加、颗粒体积分数的增加以及颗粒粒径的减小而增大。研究发现,对于一定体积分数的Cu-水纳米流体,在层流状态下对流传热系数的提高程度基本保持一致,与雷诺数大小无关。     

纳米流体强化相变蓄冷的数值模拟

采用Fluen软件对封闭腔内Cu-H2O纳米流体强化相变蓄冷进行了数值模拟,重点分析Cu纳米粒子添加量和Gr数对蓄冷性能的影响,并解释其换热机理.研究结果表明:Gr数对其结冰时间影响不大,但纳米流体的质量分数是影响纳米流结冰时间的一个主要因素.对于一给定的Gr数,随着纳米粒子质量分数的增加,结冰时间减少,这是由于纳米流体相比基液具有较高的导热系数.另一方面,由于纳米流体融解潜热降低,则纳米流体结冰时,每单位质量的纳米流体需要的能量较少,所以纳米流体具有较高的热释放率,在相变储能应用中具有巨大优势.     

倾斜角度对封闭方腔内液体自然对流的影响

在多种倾斜角度下,对封闭腔内液体进行了数值模拟。重点分析了Ra数变化和倾斜角度对对流传热的影响。研究表明：随着Ra数的增大,换热由热传导为主转化为对流换热为主;倾斜角度对Nu数的变化影响很大,并且当倾斜角度处于90o～135o时,Nu数可以达到最大值。     

封闭三角形通道内热管的自然对流换热研究

论述和分析了封闭腔内自然对流换热的研究进展,运用Fluent软件对封闭三角形通道内的热管与壁面的二维散热问题进行了数值模拟,模拟了封闭腔内空气自然对流换热的温度场和速度场。     

涡量-流函数法模拟不同高宽比和角度的腔内自然对流

采用涡量-流函数方程,对FLUENT软件进行二次开发,对重力作用下封闭空腔内的二维自然对流换热流场、温度场进行了数值模拟.对非正方形的矩形空腔内的自然对流换热进行了数值计算,结果表明不同的高宽比例对自然对流换热有很大的影响;该文还对Ra=104时放置不同角度的封闭腔进行了数值计算,揭示了重力作用下不同角度时的自然对流换热变化,发现在某一倾斜角度时(θ=45°左右),平均换热系数Numean存在极大值.     

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.     

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.     

模拟界面换热系数和密度对热溶质对流影响

为了能够更好地控制凝固过程热和溶质的传输,以二元合金为例,研究了在单边散热条件下,不同界面换热系数和密度对热溶质对流的影响.研究表明:当材料密度一定时,随着界面换热系数的提高,沿散热方向上的温度梯度增大,热溶质对流的效果增强;当界面换热系数为1 000 W/(m2·K)时,在凝固初期铸件底部出现涡流现象;在界面换热系数不变的情况下,随着材料密度的增加,铸件内液体金属的热溶质对流效应减弱.     

Evaporation heat and mass transfer in natural convection pipe flow

Abstract

A numerical analysis has been performed to examine film evaporation on natural convection heat and mass transfer in a vertical pipe. Coupled governing equations for liquid film and induced gas flow were simultaneously solved by the implicit finite difference method. Results for interfacial heat and mass transfer coefficients are specifically presented for ethanol film and water film vaporization. The predicted results indicate that the heat transfer from gas‐liquid interface to the gas flow is predominated by the transport of latent heat in association with film evaporation. The results are also contrasted with those of zero film thickness and show that the assumption of extremely thin film thickness made by Chang et al. [5] and Yan and Lin [19] is only valid for a system with a low liquid Reynolds number Re _l1. But as the liquid Reynolds number is high, the assumption becomes inappropriate.     

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.     

微重力条件下分离结晶过程中熔体热毛细对流的数值模拟

用有限差分法对微重力条件下分离结晶生长中的熔体热毛细对流进行了数值模拟,熔体的深径比A取1和2,自由界面无因次宽度B取0.05,0.075和0.1;得到了分离结晶Bridgman生长过程中熔体热毛细对流的流函数分布和温度分布图,研究了流型的演变过程和流动的失稳机制.结果表明:当Marangoni(Ma)数比较小时,流动为稳态流动并只存在于自由界面附近,随着Ma数的增加,流动增强并逐步向熔体内部扩展,熔体内部温度分布的非线性性增加,自由界面速度增大;Ma数超过某一临界值后,流动转化为非稳态流动;流动失稳的物理机制是流速的变化和阻力的变化之间存在滞后.     

一种硅基集成PCR微芯片的传热数值分析

设计和制作了一种基于MEMS技术的硅基集成PCR（聚合酶链式反应）微芯片,采用有限容积法,边界条件考虑自然对流换热和辐射换热,对PCR微芯片的传热过程进行了数值模拟。主要分析了样品的升、降温速度和样品内部的温度均一性。分析结果表明芯片具有较高的升、降温速度,而且样品内部的温度均一性也满足PCR反应的要求。芯片在温度控制系统下进行了热循环反应,实现了GUS基因的扩增,获得了良好的实验结果。     

Numerical simulation of free convection of MHD non-Newtonian nanofluid within a square wavy enclosure using Meshfree method

Abstract

In this article, flow and heat transfer of nanofluids inside a wavy square enclosure filled with non-Newtonian (shear thinning) nanofluid under magnetic field has been simulated numerically. Single-phase model is used. The governing equations have been solved numerically using element free Galerkin method. The results are obtained for isotherms, streamlines, and average Nusselt number for various values of Hartmann number, Rayleigh number, nanoparticles volume fraction, and power-law index. Here, the main objective is to explore the effect of power-law index, Rayleigh number, Hartmann number, and volume fraction on average Nusselt number. It is found that in the absence of magnetic field, Nusselt number drops on increasing the value of power-law index whereas in the presence of magnetic field, heat transfer rate increases with increase in power-law index. With the increase in Rayleigh number and volume fraction of nanoparticles, the heat transfer rate increase in all cases. This type of problem has a direct application in the coolant systems, solar collector where the structure used is wavy in order to increase the rate of heat flow. Here, EFGM is efficiently applied for simulation due to irregular domain, which creates a novelty in the work.