Simultaneous audio-visual-thermal characterization of transition boiling regime

Abstract

Safety concerns due to the highly unstable nature of boiling during the transition from the nucleate to the film boiling regime has limited investigations on this topic. Here we perform simultaneous characterization of the thermal, acoustic, and the bubble visualization data during the transition boiling regime. We show that the key features missed due to the high latency in the thermal data are captured in the audio (0-2kHz). We correlate the acoustic emissions with the high-speed visualization to elucidate the presence of an explosive boiling sub-regime, also referred to as the microbubble emission boiling (MEB). This regime is typically observed after the critical heat flux (CHF) and is characterized by numerous small sized bubbles (≈300±50μm) emanating from an unstable vapor film on the heater surface. We correlate this change in bubble behavior with the corresponding increase in the intensity and peak frequency of acoustic emissions up until the onset of a completely silent and stable film boiling regime. Conversely, MEB features are also observed during the transition from film boiling to the nucleate boiling regime in quenching experiments. We modify the heater surface and the boiling fluid, and vary the test conditions, to confirm the existence of MEB in all experiments over a wide range of subcoolings ranging from 2°C to 25°C. Our experiments suggest that the extent of the MEB increases with, (i) the increase in subcooling, (ii) the decrease in the surface tension of the boiling fluid, and (iii) the increase in surface roughness/wettability of the heater surface. We believe these unique acoustic features of the transition boiling regime can be used as a feedback to design measures to avoid catastrophic failures in boiling based energy and thermal management systems.