• bubble coalescence;
  • heat flux;
  • transient conduction;
  • boiling

Bubble coalescence events and single bubble nucleation, growth, and departure were observed with a charge-coupled device (CCD) camera synchronized with a high-speed data acquisition system measuring the heat fluxes at different positions underneath the bubble. Four different kinds of coalescence events took place with characteristic heat flux signatures for each type of event. The heat transfer measurements under four typical heaters representing different positions under the bubble showed very different heat flux characteristics during each ebullition cycle. The results show that transient conduction due to the movement of the contact line during bubble interactions, sliding, stretching, and oscillating is the main mechanism resulting in the high heat transfer rates during coalescence. Fast evaporation of the liquid layer trapped between the two bubbles and higher heat fluxes near the contact line as the microlayer evaporated also generate high heat flux spikes but have less influence due to their short duration and small influence area. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1735–1745, 2013