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

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 x 10 mm(2) is subject to an electrical heat source. The micro-pin-fins are etched as cubic columns of 100 mu m size and arranged in a staggered arrangement with 400 mu m 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 mu m and an opening with a size of either 15 mu m or 45 mu m 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/m(2)s and heat flux ranging from 0 to 10 W/cm(2), and at two saturation temperatures of 35 and 50 degrees C. For the case of single-phase heat transfer, the experimental results are compared with the pin-fins having 45 mu m 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 mu m. The microgap having nucleation cavity with cavity opening-width of 15 mu m results in a 3.44 degrees C smaller surface-superheat than that of 45 mu m 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. (C) 2019 Elsevier Ltd. All rights reserved.