Thermal convection in subcooled nucleate boiling and microbubble emission boiling: Insights from schlieren and chronophotography techniques

Abstract

In this study, the bubble-induced thermal convection in nucleate boiling and microbubble emission boiling (MEB) under highly subcooled conditions is investigated through using high-speed schlieren and chronophotography techniques. The spatio-temporal evolution of thermal plumes in nucleate boiling and MEB is revealed based on schlieren technique and power spectral density (PSD) analysis. The results show that bubble condensation in nucleate boiling generates the most intense thermal plumes, particularly at high heat fluxes. Using chronophotography technique, it is found that the thermal convection in MEB, driven by bubble oscillations, exhibits a wave-like pattern over time, while bubble collapses generate densely striated schlieren patterns. PSD analysis highlights significant differences in the decay profiles, cutoff frequencies, and spectral indices of fluctuations of schlieren grayscale between nucleate boiling and MEB. Comparative analysis with previous particle image velocimetry studies suggests that MEB may facilitate efficient heat transport from the heating surfaces to the cold bulk, thereby preserving the subcooling of the liquid near the heating surface even at very high heat flux. These insights deepen our understanding of the fundamental differences in heat transfer mechanisms between nucleate boiling and MEB.