Effects of heat flux, mass flux and two-phase inlet quality on flow boiling in a vertical superhydrophilic microchannel

Saturated flow boiling experiments were conducted to investigate the influence of surface wettability on the hydraulic and thermal transport performance in a large width-to-height aspect ratio, one-sided heated rectangular microchannel with deionized water as the working fluid. The contact angles of the bare silicon wafer surface and superhydrophilic surface after deposited by a thin film of 100-nm-thickness silicon dioxide were 65° ± 3° and less than 5° respectively, both of which were utilized as heated surfaces of the microchannel. Parametric experimental studies were carried out with the inlet vapor quality varied from 0.03 to 0.1 and the wall heat fluxes spanned from 4 W/cm2 to 20 W/cm2, at various mass fluxes ranging from 120 to 360 kg/m2 s. High speed flow visualizations were conducted coupled with instrumental measurements to illustrate the effects of heat flux, mass flux and two phase inlet quality on the local heat transfer coefficient, averaged heat transfer performance, two phase flow structure and pressure drop characteristics for surfaces with distinct surface wettability characteristics in the microchannel. Experimental results showed that the local heat transfer coefficient decreased first until approaching a minimum value and then increased towards the exit along the flow direction. Severe heat transfer deterioration was obtained for the bared silicon wafer surface with increased inlet vapor quality and heat flux, resulting from the local dryout phenomenon as can be verified by the flow visualization. While the heat transfer performance of the superhydrophilic surface was relatively constant due to continuous and uniform distribution of the thin liquid film on the heated surface during annular flow dominance and subsequent delay to partial dryout occurrence, which outperformed the untreated hydrophilic surface without additional pressure drop penalty.