Heat and Mass Transfer Around a Bubble on a Horizontal Surface in a Subcooled Flow

Early state heat and mass transfer processes around a nucleated bubble in a subcooled flow boiling is studied numerically. The model uses both boiling and condensation processes, in which microlayer evaporation, thermal boundary layer conduction and kinetic theory evaporation and condensation heat and mass transfer mechanisms, are included. In addition, the model includes a microlayer thickness prediction. The model is applied on a two-dimensional and a three-dimensional computational domain at different subcooling temperatures and flow velocities in presence of gravity. The heated surface superheat and the system pressure are kept constant in all simulations. The two phase model, Volume of Fluid (VOF) in ANSYS/FLUENT is used. The results of the study show the importance of each of the heat transfer mechanisms in various stages of the bubble growth. At the early stages of the bubble growth, the microlayer heat transfer is the dominant transfer mechanism, however, as the bubble grows, the evaporation through bubble upper surface becomes significant. Furthermore, results indicate that the isothermal bubble assumption, which is used in prior models, is not valid for the whole life of bubble growth. The new bubble diameter correlation is concluded from theoretical analysis, which is the function of both √t and t with the coefficient of Ja and Pr numbers. The numerical model predicted bubble lift off diameter at p=2.5atm,〖∆T〗_sup=9℃,〖∆T〗_sub=15℃ is 0.36mm that is predicted experimentally 0.39mm at the same operating conditions. By 7% of underestimation of lift off diameter, the numerical model is validated.Ph.D