Numerical Study of Bubble Coalescence Heat Transfer During Nucleate Pool Boiling

Bubble growth during nucleate boiling in a large pool of liquid was modeled by numerically solving the unsteady Navier-Stokes laminar flow equations with the energy equation to predict the vapor and liquid flow fields. The analysis assumed two-phase, transient, three-dimensional, laminar flow with the Boussinesq approximation for the buoyancy. The volume of fluid method was used with the level set method to predict the bubble interface motion. The numerical investigations studied the dynamics and heat transfer rates associated with the coalescence of bubbles generated on two microheaters. The results for various wall superheats and liquid subcoolings illustrate the bubble growth and interaction dynamics throughout the coalescence process and the wall heat fluxes associated with the bubble nucleation and coalescence. In some cases, the bubble coalescence traps an evaporating liquid layer between the bubbles that then quickly evaporates, resulting in high heat fluxes. In other cases, the bubbles very quickly coalescence while the bubbles are still in the fast inertial controlled growth regime and the liquid layer between the bubbles is pushed out without evaporating, resulting in low heat fluxes as the surfaces are covered with vapor. These results show how similar conditions can lead to very different heat fluxes during coalescence as has been seen experimentally.