Effect of bubble coalescence on the wall heat transfer during subcooled pool boiling

• Bubble coalescence is studied during nucleate pool boiling.
• Heat fluxes are measured on constant temperature microheaters.
• Very fast coalescence did not increase the heat fluxes between bubbles.
• A coalescence number is given to quantify conditions for very fast coalescence.
• Significant bubble motion after coalescence increased the surface heat fluxes.

This study experimentally investigates the effects of bubble coalescence on the heat fluxes during nucleate pool boiling. A microheater array was used to generate vapor bubbles in FC-72 liquid with constant surface temperature boundary conditions while the heat flux at selected locations was measured for various superheats using a high speed data acquisition system. The heat flux for boiling with coalescence was found to fluctuate much more than when only a single bubble formed on the surface due to the vaporization of the liquid layer trapped between the bubbles and oscillations of the bubbles after coalescence which resulted in relatively long periods of fairly high heat fluxes. The observations also showed that some coalescence events were not accompanied by an increase in the heat transfer rate as the liquid layer between the bubbles was physically pushed away by the rapid bubble growth during the inertial bubble growth stage instead of evaporating. A mechanistic model was developed to predict the conditions for which the heat fluxes do not increase during coalescence. In many other instances, the coalescence dynamics were such that the only bubble remaining on the surface was swept off by the liquid motion from the previous coalescence event with no nucleation of another bubble. This was followed by a long period without bubble nucleation followed by sudden nucleation and explosive coalescence of two bubbles giving rise to a large bubble which departed violently with a large increase in the heat transfer rate.