Non-contact experiments to quantify the microlayer evaporation heat transfer coefficient during isolated nucleate boiling regime

Abstract

Experiments to understand the relationship between dynamics of superheat layer, microlayer evaporation and bubble growth process during single bubble formation under saturated pool boiling regime have been conducted. Thin film interferometer and rainbow schlieren deflectometry have been employed in tandem to simultaneously map the transient evolution of microlayer and superheat layer along with the bubble growth process. During initial time instances of growth process, heat flux dissipated in the microlayer region is determined to be as high as ~1 MW/m2 for the range of experimental conditions employed. As a result, a large temperature drop is observed in the microlayer region. As the bubble growth process enters its final stages, reduced temperature levels of the heated substrate result into a significant reduction in the depletion rate of the inner core region of microlayer, while the evaporation rate of its peripheral region is seen to increase. The consolidated evaporative heat transfer coefficient for the entire bubble cycle is determined to be ~140 kW/m2K for q″ = 50 kW/m2 and 170 kW/m2K for q″ = 80 kW/m2. The corresponding accommodation factor required for predicting the evaporative heat transfer coefficient values through kinetic theory-based model is determined to be ~0.015.