基于非结构化VOSET方法的沸腾传热

Boiling heat transfer by using the VOSET method based on unstructured grids

将非结构化VOSET方法推广至求解气液界面存在相变的流动传热问题.为准确计算界面两侧因相变导致的能量跳跃,给出了含界面非结构网格控制单元温度的计算方法,并对非界面网格温度场采用隐式求解以提高计算精度.为验证所构建相变模型的正确性,编写程序分别模拟了恒壁面过热度和恒热流密度两种边界条件下的水平表面膜态沸腾,计算结果与Klimenko经验关联式吻合良好.通过圆弧表面膜态沸腾问题,验证了本文基于带相变非结构VOSET方法对不规则区域沸腾相变问题的适用性.通过模拟近临界压力水的膜态沸腾问题,并与Berenson和Klimenko经验关联式对比,验证了本文方法对实际沸腾问题的适用性.

It is well known that the fluid mass is not conserved by the level set methods while the accuracy of volume-of-fluid(VOF)methods is highly dependent on the calculation of interface normal and curvature from volume fractions. Advanced interfacial tracking methods appear by combining the two techniques to compensate each other. The coupled VOF and level set method named VOSET is one of such methods. This paper presents the extension work to include heat and mass transfer due to phase change for the VOSET method on an unstructured triangular grid. In this method, the liquid/gas interface is tracked by the VOSET method, which combines the advantages of both VOF and level set methods. The proposed method can not only preserve mass conservation but also facilitate the interface normal and curvature calculation with a high accuracy. The interfacial tracking methods are integrated to Navier-Stokes solvers accordingly. A critical issue for the mass transfer across the boiling interface is the treatment of energy jump. We propose a numerical algorithm to model energy jump based on VOSET for unstructured grids. The heat flux on each side of the interface within an interface cell needs to be computed separately. The interface is tackled as an internal boundary for temperature field. For cells with a pure phase, unlike previous studies, we solve the energy equation in an implicit way. For cells with a liquid/gas phase interface, an interpolation algorithm is developed to calculate the temperature. To validate the present numerical method,four classical boiling tests are implemented. To be specific, the first case is the constant heat flux film boiling on the horizontal plate. In this case, dimensionless wall heat flux 20.0 is set up for the bottom solid wall. The top boundary is open and the fluid is allowed to exit, no slip boundary is set to the bottom, periodic condition is set to the left boundary and symmetrical condition is imposed on the right boundary to save computational cost. The comparison of the Nusselt number with the analytical Klimenko correlation indicates that the difference between them is only 6.18%. The second case is the constant wall temperature film boiling on the horizontal plate. In this case, the boundary conditions are the same with the first case except for the bottom boundary, where the wall temperature keeps at 5 K. The computed space and time averaged Nusselt number agrees well with Klimenko correlation. Moreover, the results match well with those reported by Guo et al.qualitatively. The third case is the film boiling process on a circular surface, which is designed and simulated to verify the robustness of the present method dealing with boiling in complex domain. The numerical results demonstrate that the proposed numerical method can calculate the two phase boiling flow in irregular regions accurately. Finally, the research of film boiling of water at near-critical pressure is conducted. In this case, the wall superheat is set to be 10 K. The numerical results are consistent with those obtained by other methods in literatures, indicating the flexibility of the present numerical method to deal with real boiling problems.