倾斜矩形腔体内相变材料接触熔化研究

发布时间：2019-03-22 12:01

【摘要】：相变材料尤其是有机相变材料在作为蓄能材料时由于其导热系数小,往往需要通过增加翅片、金属粒子或是增加金属网格的结构来实现强化传热。而在相变熔化过程一般以接触熔化方式为主,装有相变材料的封闭容器中受重力的作用使得加热面和相变材料的固态部分紧密接触,熔化的液体受到排挤沿着加热面被排出从而形成了一层薄的液膜从而获得较高的加热率。从而在此基础上发现了在矩形腔体内通过倾斜可以获得比原来非倾斜状态更高的传热量,从而更快的熔化速率。实验采用有机玻璃腔体作为容器来构建可视化的环境将相变熔化过程通过相机记录下来并后期用图像处理可以得到液相份数以及固液界面线。通过可视化实验并通过图像数字化软件处理的方法可以确定倾斜角与完全熔化时间的关系,找到“转换角”,最终确定使得蓄能元件熔化效率最高的最佳倾斜角。由于相变熔化过程导致数学模型的非线性使得少数理想情况才存在分析解,大部分有限容积问题通过近似求解和数值模拟的方法获得。通过对非倾斜状态下的矩形腔体近似分析解模型与实验结果对比结果吻合程度良好,证明了图像处理的可行性。并在非倾斜的状态下提出了倾斜状态下的矩形腔体内接触熔化模型并得到了对应的近似分析解与倾斜情况实验对比,吻合良好,证明了在倾斜矩形腔体内接触熔化为其主要传热方式。采用数值模拟构建熔化模型并发现相变熔化过程存在5个阶段,第一阶段发生在熔化的开始阶段,固态向液态转变的阶段;第二阶段底部固态相变材料和加热面的液膜稳定形成的阶段;第三阶段随着液态相变材料的增多,使得右接触面和固态相变材料之间液体层逐渐趋于稳定;第四阶段稳定后的固态相变材料在重力的作用下下沉;第五阶段此阶段主要是针对倾斜角为45°和60°之间,在此角度之间受自然对流的影响明显。通过与近似分析解的对比可以看出,熔化初期主要以接触熔化为主,而对流换热在后期变得强烈变得不可忽略。
[Abstract]:Phase change materials, especially organic phase change materials, when they are used as energy storage materials, because of their small thermal conductivity, it is often necessary to enhance heat transfer by adding fins, metal particles or increasing the structure of metal mesh. In the process of phase change melting, contact melting is the main way, and the effect of gravity on the closed vessel containing phase change material makes the heating surface contact closely with the solid part of the phase change material. The molten liquid is extruded along the heating surface to form a thin liquid film to obtain a higher heating rate. On this basis, it is found that higher heat transfer and faster melting rate can be obtained by tilting in the rectangular cavity than in the original non-tilted state. An organic glass cavity is used as a container to construct a visual environment in which the melting process of phase transition can be recorded by the camera and then processed by image processing to obtain the number of liquid phases and the solid-liquid interface line. The relationship between the tilt angle and the complete melting time can be determined by visualization experiment and image digitization software processing, and the "conversion angle" can be found. Finally, the optimal tilt angle with the highest melting efficiency of the energy storage element can be determined. Due to the nonlinearity of the mathematical model caused by the melting process of phase transition, the analytical solutions exist in a few ideal cases, and most of the finite volume problems are obtained by means of approximate solution and numerical simulation. The approximate analytical solution model of rectangular cavity in non-tilted state is in good agreement with the experimental results, and the feasibility of image processing is proved. At the same time, the contact melting model of rectangular cavity in inclined state is put forward and the corresponding approximate analytical solution is compared with the experimental results in the inclined state, and the results are in good agreement with the experimental results. It is proved that contact melting in inclined rectangular cavity is the main heat transfer mode. The melting model is constructed by numerical simulation, and it is found that there are five stages in the melting process. The first stage occurs in the initial stage of melting, and the phase of solid-state to liquid-state transformation. The second stage is the stable formation of the solid phase change material and the liquid film on the heating surface, and the third stage makes the liquid layer between the right contact surface and the solid phase change material become stable gradually with the increase of the liquid phase change material. In the fourth stage, the solid phase change materials sink under the action of gravity, and in the fifth stage, the inclination angle is between 45 掳and 60 掳, which is obviously influenced by the natural convection. Through the comparison with the approximate analytical solution, it can be seen that the initial melting is dominated by contact melting, while the convective heat transfer becomes intense at the later stage and cannot be ignored.
【学位授予单位】：东华大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TB34

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