Vertical coalescence during nucleate boiling from a single artificial cavity

•An study of bubble growth from artificial cavity micro-fabricated on silicon wafer.•Vertical coalescence is observed at the boundary between the isolated and interference regimes.•Outside the isolated bubble regime, vertical coalescence decreases with pressure.•The effect of pressure on vertical coalescence is evidenced through experiments. In this experimental study bubble growth from an isolated artificial cavity micro-fabricated on a 380μm thick silicon wafer was investigated. The horizontally oriented boiling surface was heated by a thin resistance heater integrated on the rear of the silicon test section. The temperature was measured using an integrated micro-sensor situated on the boiling surface with the artificial cavity located in its geometrical centre. To conduct saturated pool boiling experiments the test section was immersed in degassed fluorinert FC-72. Bubble nucleation, growth, detachment and the evaporative heat flux at different pressures were analysed using high-speed imaging and temperature micro-sensors. Vertical coalescence was initially observed at the boundary between the isolated bubble and interference regimes. For wall superheats outside the isolated bubble regime, the occurrence of vertical coalescence is decreasing with increasing pressure. Although vertical coalescence seems initially more frequent with increasing wall superheat, the explicit dependency on temperature is covered by the scattering nature of the data. The applied heat flux was compared to the evaporative heat flux, nominally based on an arbitrary chosen area of influence on the boiling substrate.