Bubble coalescence at constant wall temperatures during subcooled nucleate pool boiling

Abstract

Bubble coalescence during subcooled nucleate pool boiling was investigated experimentally using constant wall temperature boundary conditions while the wall heat flux was measured at various locations to understand the effects of coalescence on the heat transfer. The effects of the subcooling on the coalesced bubble shape, size and departure time were also investigated. The observations showed that the coalesced bubble moved and oscillated on the heater surface with significant heat transfer variations prior to departure. The coalesced (or departure) bubble size and departure frequency decreased as the subcooling increased. The heat flux for boiling with coalescence fluctuated much more than for single bubble boiling with the heat flux fluctuations due to oscillations of the coalesced bubble on the heater surface resulting in frequent rewetting of the heated surfaces. In some cases, many additional small bubbles nucleated very near a larger bubble and then rapidly coalesced with the larger bubble which enhanced the overall heat transfer. The observations also showed coalescence with no increase in the heat transfer rate for very fast bubble coalescence such as would occur for large surface superheats as the boiling conditions approach CHF. A mechanistic model comparing the inertia and heat flux controlled bubble growth phases for inhomogeneous bubble growth shows that this coalescence without increased heat transfer corresponds to the inertial growth phase.