Effect of the gap height and channel aspect ratio on flow boiling characteristics in open microchannels

Open microchannels are capable of significantly lowering pressure drop and improving the stability of two-phase flows. That said, how gap height ( H g ) and channel aspect ratio ( AR ) affect flow boiling behaviors in such open microchannels is yet to be investigated. For this research, four copper-made open microchannel heat sinks were constructed, with H g values of 0.4 mm and 0.7 mm, and AR values of 1:1 and 2:1. Flow boiling experiments were carried out under mass fluxes of 305, 507, and 708 kg/m 2 s. The inlet temperature of deionized water was kept constant at 80 °C. The visualization results revealed the presence of bubbly flow, slug flow, stratified flow, and completely stratified flow within the open microchannels. These shifts in flow patterns were accompanied by corresponding changes in the heat transfer coefficient (HTC). At high mass fluxes ( G > 305 kg/m 2 s), the subcooled liquid in open microchannels delayed the generation of a stratified flow, which resulted in a decrease in the HTC. Low H g ( H g = 0.4 mm) reduced the influence of subcooled liquid on stratified flow and significantly improved HTC at a large heat flux. High AR ( AR = 2:1) considerably enhanced HTC and reduced the pressure drop.