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

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.