Experimental and numerical study on convective boiling in a staggered array of micro pin-fin microgap

Abstract

The flow patterns, heat transfer and pressure drop of convective boiling of dielectric fluid, FC-72, in a micro-gap are investigated experimentally. The surface of the microgap is enhanced with a staggered array of micro pin-fins. The enhanced surface of the microgap with the size of 10 × 10 mm2 is subject to an electrical heat source. The micro-pin-fins are etched as cubic columns of 100 μm size and arranged in a staggered arrangement with 400 μm pitch in both transverse and longitudinal directions. The inside of the micro pin-fins is nucleated with a cavity of cylindrical shape with diameter of 60 μm and an opening with a size of either 15 μm or 45 μm width. The opening of the cavities of the micro-pin-fins is aligned toward the down-stream. For the case of single-phase flow, a numerical analysis is performed, and the pressure drop and velocity fields are investigated in the micro-gap. The experiments were performed for various mass fluxes ranging from 94 to 275 kg/m2s and heat flux ranging from 0 to 10 W/cm2, and at two saturation temperatures of 35 and 50 °C. For the case of single-phase heat transfer, the experimental results are compared with the pin-fins having 45 μm cavity opening and found that the effect of cavity opening is significant when the mass flux is high. The surface-superheat at the onset of boiling is reduced by reducing the cavity opening-width from 45 to 15 μm. The microgap having nucleation cavity with cavity opening-width of 15 μm results in a 3.44 °C smaller surface-superheat than that of 45 μm opening-width. The convective heat transfer coefficient increases with the increase of mass flux, regardless of the flow type. Moreover, the variation of the pin-fin opening-widths does not influence the convection heat transfer rate. For single-phase flow, the heat flux shows a negligible effect on pressure drop, and the pressure drop increases with increasing mass flux. For two-phase flow, the pressure drop increases drastically with increasing heat flux. A smaller cavity opening-width results in a smaller pressure drop when the mass flux is high. The flow observation shows two distinct flow patterns in the microgap at the beginning of the bubble nucleation.