Coalescence Regimes During Single Bubble Nucleate Boiling Under Periodic Exponential Heating

Abstract Researchers have extensively studied the heat transfer ability of boiling under steady heating conditions. However, critical thermal failures in industries often involve a sudden and rapid increase in heat flux over a very short duration. This paper attempts to study the effect of transient (periodic) heating on nucleate boiling by comparing steady heat-flux-based pool boiling with that of exponential heat flux. This paper looks at vertical coalescence patterns formed during periodic heating, as single-bubble coalescence is viewed as a precursor to critical heat flux (CHF). An in-house CFD code based on the sharp interface level set method was used to study coalescence patterns under a periodic exponential heat flux of varying excursion rates (ethanol as working fluid). Simulations reveal that while no coalescence is observed in steady heating, various vertical coalescence regimes are identified for transient heating (upon reduction of excursion time): uncoalesced, periodic, chaotic, and chain. An increase in bubble frequency and vertical bubble coalescence is observed with reduced excursion time, while the departure diameter remains nearly constant. The simulations also reveal that for periodic coalescence, the sooner the first heat flux cycle is completed after the departure of the preceding bubble (at the nucleating cavity), the more likely it is for coalescence to occur. For chaotic and chain coalescence, the formation and dampening of oscillations at the interface of a coalescing bubble governs the frequency of coalescence. It is also observed that an increase in surface contact angle results in coalescence occurring at higher excursion rates.