Experimental investigation on the effect of transverse grooves in a channel on the thermal performance of a flat-plate pulsating heat pipe

Abstract

This study experimentally investigates the effect of transverse grooves in a channel on the thermal performance of a flat-plate pulsating heat pipe (PHP). Rectangular grooves are engraved on the sidewalls of the channel in the evaporator section in a direction transverse to the oscillating flow, and these grooves extend from the top surface to the bottom surface of the sidewalls. Two PHPs are fabricated for experimental evaluation using MEMS techniques: one with a grooved channel and another with a smooth channel. Both PHPs have overall dimensions of 29 x 29 x 1.5 mm3. A rectangular meandering channel is etched on a silicon wafer, and this channel possesses dual hydraulic diameters with alternating widths of 1 mm and 0.4 mm and a height of 0.5 mm. In the PHP with the grooved channel, the dimensions of the grooves are analytically determined to maximize the film evaporation heat transfer rate: the depth, the width, and the spacing are determined to be 50 mu m, 20 mu m, and 20 mu m, respectively. R-236fa is used as the working fluid, and the filling ratio is approximately 55 % by volume. Thermal performances of the PHP with the grooved channel and the PHP with the smooth channel are evaluated and compared through thermometry. Experimental results reveal that engraving transverse grooves on sidewalls of the channel effectively enhances the thermal performance of the PHP: the thermal resistance is decreased by an average of 34.6 % across the entire range of input heat fluxes, from the heat flux required for startup to the maximum allowable heat flux. High-speed photography is used, and it is observed that the liquid film in the grooved channel exhibits an 69.2 % thinner average film thickness than that in the smooth channel. This decrease in the average film thickness contributes to the increased evaporation heat transfer rate.