高聚物对封闭方腔热对流的流动与传热特性影响数值模拟

发布时间：2018-01-17 13:41

本文关键词：高聚物对封闭方腔热对流的流动与传热特性影响数值模拟　出处：《哈尔滨工业大学》2015年硕士论文　论文类型：学位论文

【摘要】：在流体热对流流动中,尤其当流体处在湍流时,其流动情况比较复杂,此时对于热对流中的流动特性和传热特性而言,因流体自身的复杂性而使其较为复杂。在此基础上,在流体中添加一定量的高聚物后,高聚物可以改变该溶液的粘弹性特性,使得其流动特性和传热特性因此而发生变化。为了研究高聚物添加对热对流流动和传热特性的影响,在本文中,选取自然对流和Rayleigh-Bénard(RB)两种具有代表性的热对流,分别对高聚物添加造成的影响进行研究与分析。首先,为了验证高聚物添加后,Cross方程可以准确的表示高聚物溶液的物性参数,通过实验测量了浓度为200ppm、500ppm、1000ppm高聚物溶液的粘度随剪切速率变化的曲线;通过资料查得不同浓度高聚物的松弛时间?、零剪切粘度0?以及幂指数n,将其代入Cross方程中,取与实验数据相同的剪切速率,根据公式得出对应的粘度值。将相同浓度下实验测得的粘度随剪切速率变化的曲线与Cross方程得出的粘度与剪切速率变化曲线相比较。其次,本文在建立相应不同热对流的几何模型后,对不同Ra数下的数值模型分别进行网络无关性验证来确保数值模拟计算的可行性与精确性,在得到不同工况下对应的最佳的网格后,基于CFD模拟,分别模拟研究在不同Ra数、不同介质下的流体流动和传热特性变化,综合其速度变化以及温度方面的变化,探讨高聚物添加对热对流的影响。再次,在封闭方腔自然对流中,当Ra数逐渐增大时,对于任何介质而言,流体的流动速度和传热能力都将增加,且增加幅度随着Ra数的增加而变大。而高聚物的添加对方腔自然对流的流动和传热有着较大的影响,高聚物的添加降低了原流体流动的阻力而使得流体的整体速度增加,使得冷热壁面处的速度边界层变薄;同样数值模拟结果显示在高聚物添加后热壁面处的Nu数变小,壁面处的温度边界层增厚,流体的传热减弱,根据不同Ra数下的Nu数得出不同介质下这两者之间的关系。最后,对于RB热对流而言,高聚物添加对流体产生的影响与方腔自然对流结果一致,不同的是RB热对流较方腔自然对流复杂。对RB热对流整体而言,添加高聚物后,剪切特性占据主要地位,使得流体剪切变稀,溶液的流动阻力减弱,速度增加。同时,虽然速度整体增加,但是在流体传热方向上的速度值反而减小,在一定程度上抑制了温度的传递,流体热输运能力降低,传热降低。同样的,在同一Ra数下,高聚物添加使得RB热对流壁面处的速度边界层变薄、温度边界层增厚。
[Abstract]:In the case of thermal convection, especially when the fluid is in turbulent flow, the flow is complicated, and the flow characteristics and heat transfer characteristics in the thermal convection are discussed. On the basis of the complexity of the fluid, the viscoelastic properties of the solution can be changed by adding a certain amount of polymer to the fluid. In order to study the effect of polymer addition on the thermal convection flow and heat transfer characteristics, in this paper. Two kinds of representative thermal convection, natural convection and Rayleigh-B 茅 nardberg, were selected to study and analyze the effect of polymer addition. In order to verify that the cross equation can accurately represent the physical properties of polymer solution, the concentration of 200 ppm ~ 500ppm was measured experimentally. The curve of viscosity change with shear rate of 1000ppm polymer solution; The relaxation time of polymers with different concentrations was obtained by the data. , zero shear viscosity 0? The power exponent n is substituted into the Cross equation and the shear rate is the same as the experimental data. According to the formula, the corresponding viscosity value is obtained. The curve of viscosity changing with shear rate measured under the same concentration is compared with the curve of viscosity and shear rate obtained by Cross equation. Secondly. In order to ensure the feasibility and accuracy of the numerical simulation, the corresponding geometric models of different thermal convection are established in this paper, and the network independence of the numerical models under different Ra numbers is verified separately in order to ensure the feasibility and accuracy of the numerical simulation. After obtaining the best mesh under different working conditions, the fluid flow and heat transfer characteristics in different Ra numbers and different media were simulated based on CFD simulation. The influence of polymer addition on thermal convection is discussed by synthesizing the variation of velocity and temperature. Thirdly, in the closed square cavity, when the Ra number increases gradually, for any medium. The flow velocity and heat transfer capacity of the fluid will increase, and the amplitude of increase will increase with the increase of Ra number. The natural convection flow and heat transfer in the admixture of polymer have a great influence on the flow and heat transfer. The addition of polymer reduces the flow resistance of the original fluid and increases the overall velocity of the fluid, which makes the velocity boundary layer thinning at the cold and hot wall. The same numerical simulation results show that the Nu number at the hot wall becomes smaller, the temperature boundary layer at the wall becomes thicker and the heat transfer of the fluid decreases after the addition of polymer. According to the Nu number of different Ra numbers, the relationship between them is obtained in different media. Finally, for RB thermal convection, the effect of polymer addition on the fluid is consistent with the results of natural convection in square cavity. The RB thermal convection is more complex than the natural convection in the square cavity. For RB thermal convection as a whole, the shear characteristics play a major role after the addition of polymer, which makes the fluid shearing thinning and the flow resistance of the solution weakening. At the same time, although the velocity increases as a whole, the velocity value in the direction of heat transfer of the fluid decreases, to a certain extent, the transfer of temperature is restrained, the heat transfer capacity of the fluid decreases, and the heat transfer decreases. At the same Ra number, the addition of polymer makes the velocity boundary layer thinning and the temperature boundary layer thickening at the RB thermal convection wall.
【学位授予单位】：哈尔滨工业大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TK124

【引证文献】