On the identification and mapping of three distinct stages of single vapor bubble growth with the corresponding microlayer dynamics

Abstract

An experimental study to establish a one-on-one correspondence between microlayer dynamics and growth process of single vapor bubble under nucleate pool boiling regime has been presented. The transient evolution of microlayer and the corresponding vapor bubble growth have been simultaneously recorded using a thin film interferometer and one of the gradients-based imaging techniques, namely rainbow schlieren deflectometry. Based on interferometry measurements, the microlayer is observed to undergo three distinct stages. Each of these stages is identified by distinct microlayer structure namely, inverted conical, flattened peripheral region and truncated conical structure during the initial, transitional and diffusion-controlled growth phases of the vapor bubble. Contribution of microlayer towards bubble growth has been established by quantifying the time rate of change of net mass of the superheated fluid entrapped in the microlayer region and the corresponding rate of change of mass of the growing bubble. The analysis showed that the ratio of mass of the superheated fluid in the microlayer region to the mass of the vapor bubble does not exceed a maximum of ~ 15%. For any given heat flux level, the growing vapor bubble is seen to acquire its maximum size in the transitional regime of the growth process during which the microlayer exhibits a flattened peripheral region. Possible impact of varying heat flux level on microlayer dynamics and movement of the apparent contact line of the vapor bubble has also been discussed. A reasonably good agreement of the experimental data with some of the well-established bubble growth models for initial microlayer thickness has been observed. Based on the experimental results, critical value of empirical parameter ‘Cslope’ (parameter correlating the bubble base radius and the maximum microlayer thickness) for transition from inertia-driven to diffusion dominated growth has been identified to be ~ 0.0042.