纳米流体Rayleigh-Benard自然对流流动及换热的数值模拟

发布时间：2018-11-21 16:22

【摘要】：纳米流体具有良好的导热性能,在日常生活和工业中应用十分广泛。自从纳米流体的概念提出之后,国内外学者开始对其进行研究,研究内容基本包括:纳米流体稳定性、物性、传热特性、强化换热机理等各个方面。但是对于纳米流体自然对流的流动及换热研究还处在初级阶段,并且纳米流体自然对流与强制对流的研究结果具有很大差异,至今还没有一个确切理论能解释产生这种差异性结果的原因。本文应用FLUENT软件对矩形腔体内Rayleigh-Benard自然对流的流动及换热进行数值模拟研究。首先,在矩形腔体内充满流体,在不同瑞利数下形成稳定的对涡。用流函数等值线示意对流形成的过程,Rayleigh-Benard自然对流是重力、流体粘滞力、浮升力共同作用的结果;当矩形腔体中心点流体速度达到最大并且不随时间变化时,对流稳定,并且Rayleigh-Benard自然对流的旋涡流动是稳定的、自发的且旋涡流动方向按逆、顺时针交替出现。其次,采用多相流混合模型,对矩形腔体内的纳米流体进行模拟,模拟计算得到的Nuavg值与文献值、经验公式值进行对比,存在一定的误差,但其大小均在允许范围内。在一定范围内对模拟值进行拟合,拟合后的系数与经验公式的系数对比相差很小,证明了模型的正确性。最后,在验证模型的正确性之后,对矩形腔内纳米流体进行数值模拟。获得结果:纳米流体Rayleigh-Benard自然对流的形成过程Nuavg随时间的变化呈现出非线性性;Rayleigh-Benard自然对流中纳米流体与水传热的对比情况:在瑞利数小于6×106时,水的换热情况好于纳米流体,当瑞利数大于6×106时,纳米流体的换热情况好于水的;纳米粒子粒径越小换热效果越好;基液物性参数的综合比较是衡量换热效果的依据,与强制对流换热中只比较基液的导热系数有很大的区别;在自然对流和强制对流中添加金属粒子比添加金属氧化物粒子的换热效果要好。
[Abstract]:Nano-fluids have good thermal conductivity and are widely used in daily life and industry. Since the concept of nano-fluid was put forward, scholars at home and abroad began to study it, including the stability of nano-fluid, physical properties, heat transfer characteristics, enhancement of heat transfer mechanism and other aspects. However, the study of natural convection and heat transfer of nanoscale fluid is still in the primary stage, and the results of the study on natural convection and forced convection of nanoscale fluid are quite different. Up to now, there is no definite theory to explain the cause of the difference. In this paper, FLUENT software is used to simulate the natural convection flow and heat transfer of Rayleigh-Benard in rectangular cavity. Firstly, the rectangular cavity is filled with fluid, and a stable pair vortex is formed under different Rayleigh numbers. The Rayleigh-Benard natural convection is the result of the interaction of gravity, fluid viscosity and floating lift. When the velocity of the fluid at the center of the rectangular cavity reaches the maximum and does not change with time, the convection is stable, and the vortex flow of the natural convection in Rayleigh-Benard is stable, and the vortex flow direction is reversed and the vortex appears alternately clockwise. Secondly, the multi-phase flow mixing model is used to simulate the nanoscale fluid in rectangular cavity. The Nuavg value obtained by simulation is compared with the literature value and the empirical formula value. There is a certain error, but its size is within the allowable range. The simulated values are fitted within a certain range, and the comparison between the coefficients of the fitted model and the empirical formula is very small, which proves the correctness of the model. Finally, after verifying the correctness of the model, the numerical simulation of nanoscale fluid in rectangular cavity is carried out. The results are as follows: the formation process of Rayleigh-Benard natural convection in nanoscale fluid is nonlinear with the change of Nuavg with time. Comparison of heat transfer between nano-fluid and water in Rayleigh-Benard natural convection: when Rayleigh number is less than 6 脳 10 ~ 6, the heat transfer of water is better than that of nano-fluid, and when Rayleigh number is greater than 6 脳 10 ~ 6, the heat transfer of nano-fluid is better than that of water. The smaller the particle size is, the better the heat transfer effect is, and the comprehensive comparison of the physical properties of the base solution is the basis for measuring the heat transfer effect, which is different from the heat conductivity of the base solution only compared with the forced convection heat transfer. The heat transfer effect of adding metal particles in natural convection and forced convection is better than that of adding metal oxide particles.
【学位授予单位】：东北电力大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TK124

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