Microlayer Formation, Evaporation and Bubble Growth in Nucleate Boiling

Abstract

<p>A numerical study of microlayer formation and evaporation is reported in which computer programs developed in order to predict the isothermal (initial) and the instantaneous microlayer thicknesses as well as the time varying mass evaporated from the microlayer for any specified bubble growth rate are presented. The results of these programs show that the isothermal microlayer thickness δ₀(r) is of the order of 1-5 μm and its profile agrees with the author's experimental data. The results of the microlayer evaporation show that the microlayer may contribute up to 100% of the bubble mass and that the instantaneous microlayer profile δ(r,t) agrees with some photographically measured data from the same reference for certain selected bubbles.</p> <p>Another computer program was developed to solve the Navier-Stokes equations, which account for all forces in the liquid acting on the bubble, in conjunction with the energy equation in order to study the entire hydrodynamic and thermodynamic fields around a growing bubble on a heating surface. The velocity, pressure and temperature distributions were analysed and the time varying mass evaporated from the bubble cap was computed. The results show that the mass evaporated from the bubble cap is less by an order of magnitude than the mass evaporated from the microlayer.</p> <p>Finally, a numerical model for bubble growth was constructed using the programs mentioned above, assuming the mass transfer across the liquid-vapour interface from both the microlayer and bubble cap to be the predominant mechanism for bubble growth. Some aspects of bubble growth are discussed including the time varying bubble mass, vapour pressure, bubble growth rate and bubble growth relationship. A comparison between this numerical model and the author's experimental data for the instantaneous microlayer thickness shows good agreement.</p>