Investigation of the impact of near-wall mesh size on the transition from microscopic wall boiling mechanism to macroscopic multiphase-CFD models

CFD method enables high-fidelity investigation of complex systems. Numerous sensitivity studies on boiling submodels improve the applicability and reliability of the multiphase-CFD approach. These studies recommend that the mesh size near the wall should typically be large. A multiphase-CFD model for subcooled flow boiling in a tube was developed to investigate the impact of near -wall mesh size on calculation results. Five distinguished scale meshes were employed to evaluate the effect of mesh scale on the transition from microscopic wall boiling mechanism to macroscopic multiphase-CFD models. A more reliable multiphase-CFD thermal phase change model that accounts for superheated liquid layer evaporation was also developed based on the Lee model. The calculation results indicate that the near -wall mesh size is numerically robust in predicting wall temperature but significantly influences the heat flux partitioning. Smaller mesh sizes produce higher evaporation heat flux and a more significant increase in the deviation of liquid temperature from the saturation temperature, which leads to an incorrect reliance on the condensation model to calculate superheated liquid evaporation. This is why a larger mesh size is typically recommended in previous sensitivity studies. Applying the developed model presented in this paper can ensure that the deviation between the liquid temperature and the saturation temperature of the cell adjacent to the wall remains within acceptable limits. The void fraction displays a substantial mesh sensitivity, and as the mesh size decreases, the OSV (Onset of Significant Void) moves forward. However, the maximum mean void fraction remains unaffected by mesh size. The results of this research can provide valuable insights into the development of more accurate and reliable CFD models for subcooled flow boiling.