Experimental investigation of bubble coalescence heat transfer during nucleate pool boiling

•High speed, local heat flux measurements during coalescence, departure and nucleation of FC72.•Heat flux measurements for various separation distances and superheats.•Fast coalescence resulted in no large heat flux increase between the bubbles.•Heat fluxes depend strongly on the superheat and the location under the bubble.•Heat fluxes are generally higher for nucleation and departure than for coalescence or rewetting. This study focuses on analyzing the bubble dynamics during coalescence for nucleate pool boiling and its effects on the heat transfer. The study uses experimental observations and heat flux measurements to characterize the bubble dynamics, fluid flows and thermal effects due to nucleation, coalescence and departure during nucleate pool boiling. The investigations used using various types of microheaters 50–200 µm in size etched on glass substrates to obtain time and space resolved heat transfer measurements. The purpose is to describe the dynamics of the fluid motion as well as the thermal interactions with and without coalescence between growing and departing bubbles. The investigations were performed for various spacings and positions on the heater array with various subcoolings and wall superheats. The experimental investigations illustrate the mechanisms promoting bubble coalescence and the effect of coalescence on the wall heat transfer. The heat flux for boiling with coalescence fluctuated much more than for single bubble boiling due to the vaporization of the liquid layer trapped between the bubbles. In some cases, the heat fluxes during coalescence were as large as during nucleation and departure with some very large heat flux peaks that could lead to damaging thermal stresses in the surface.