Analysis of coalescence phenomena on microheaters at two surface superheats

The bubble dynamics and detailed heat flux distributions under bubbles were investigated during coalescence on a microheater array at two surface temperatures to detail the effect of the superheat. Vertical coalescence events were observed at the lower surface temperature during single bubble boiling which increased the heat flux by pulling the smaller second bubble from the surface. The departure frequency at the lower superheat was faster than at the higher superheat during single bubble events due to more necking at higher superheats. The coalescence dynamics also differed for the two surface temperatures. Detailed analyses of the heat flux characteristics under the bubbles showed how the heat fluxes in the dryout area and around the contact line differed. The average heat fluxes were higher for the heaters outside of the contact line at higher superheats but were lower for the inner heaters. Transient conduction and vapor generation caused by rewetting were the main reasons for the heat flux enhancement. Bubble interactions including liquid motion around the bubble during the deformation process and jet flows from the smaller bubble towards the larger bubble were observed at both temperatures. The jet flows pushed the larger bubble so that the base area shrank which further increased the heat transfer.