The microlayer and force balance of bubbles growing on solid in nucleate boiling

This study uses interface-resolved computational fluid dynamics simulations to investigate the dynamics of bubble growth on a horizontal solid surface, with a focus on the characteristics of the microlayer and the forces involved in the process. The simulations exclude phase change effects to concentrate on hydrodynamics and employ an external mass source for controlled bubble inflation. This mass source follows a time-dependent bubble radius growth law, $R\left(t\right)\propto t^{1/2}$, which is typical for heat-transfer-controlled growth of a vapour bubble during nucleate boiling of water at atmospheric pressure. The numerical framework is validated against recent experimental measurements of bubble shape and microlayer profile. The results of this work indicate that the rate of bubble growth significantly influences microlayer formation. Faster growth rates produce a near-hemispherical bubble shape with an extended radial microlayer on the solid, while slower rates yield a taller, more spherical bubble with a shorter microlayer. All microlayer profiles exhibit an outwardly-curved shape, with a maximum microlayer thickness increasing with the growth rate. The study also examines the force balance on the bubble, revealing that the net vertical force of the bubble does not equal zero even when the bubble remains attached to the solid surface. Our analysis of the bubble motion demonstrates that previous force balance models used for determining bubble detachment lack robustness. The microlayer profile obtained in this work is important for boiling heat transfer studies as the microlayer contributes significantly to local heat transfer. The force balance analysis shows the need for a new approach to determine bubble detachment behaviour, which is vital for predicting flow boiling rates.