High-efficiency spray cooling of rough surfaces with gas-assist atomization

Abstract

Spray impingement heat transfer experiments were conducted to assess the role of surface roughness on heat transfer performance. Surfaces covered with arrays of micro-pillars with diameters in the range 0.8–50 μm were fabricated using standard lithography technique and etching processes. A recently developed air-assist nozzle was used to produce fine droplets of de-ionized (DI) water. Experimental results obtained indicate that the heat transfer performance is better for length scales in the 5–10 μm range than for sub-micron scales. Heat transfer coefficients in the two-phase evaporative regime were calculated by subtracting the sensible heat. Accounting for the enhancement in surface area allows for assessment of the effects of capillary wicking and thin film flow. Surfaces with bigger pillar size and larger spacing (∼20μm) exhibits greater heat transfer coefficient values due to higher permeability for wicking flow. The effect of nozzle/target spacing, porosity and liquid/air flow rates were also studied. At the highest liquid flow rate of 120 ml/min, a CHF value of 969 W/cm2 was observed, which is among the highest values documented in the literature, and suggests the potential for further enhancement at higher flow rates.