Heat-transfer effects of coalescence of bubbles from various site distributions

A boiling-heat-transfer study with controlled coalescence of bubbles has been performed by producing dual or multiple individual bubbles from microheaters (0.27mm x 0.27mm). The effects of separation distances between the bubbles and the number of bubbles participating in the process were investigated with various heater configurations. Each heater is kept at a nominally constant temperature during the experiment so that the time- and space-resolved heat flux can be measured at a frequency of ca. 4500 times per second. In each case, the heaters were set at the same superheat and the superheat was varied from 35 to 85 K. Based on the heat fluxes measured by the data-acquisition system, we found that the heat-transfer enhancement rate becomes higher as the separation distances increase between the bubbles. The bubbling-cycle duration decreases with the separation distance. The heat-transfer enhancement is attributed to rewetting of the dry heater surface and turbulent mixing effects. Also, together with visualization results, we have found that the coalescence process is associated with a combination of bubble oscillation, stretching and sliding on the heaters. We have also found that multiple coalescences took place with a group of bubbles, and the heat fluxes are proportional to the number of bubbles involved and the separation distances between bubbles. The internal heaters tend to have lower heat-flux levels.