Experimental study of channel roughness effect in diffusion bonded pulsating heat pipes

Abstract

This study investigates the influence of surface finishing on thermal performance of copper flat pulsating heat pipes (PHP). Two copper flat PHPs with overall dimensions of 208 × 150 × 5 mm3 were fabricated using the diffusion bonding technique. In both devices, the internal surfaces were modified. In the first one, the channel inner surface was sanded with standard sandpaper Grit N1200. In the second PHP (called hybrid), the evaporator zone was sanded with standard sandpaper Grit N100 and the condenser section with Grit N1200. Both devices counted ten parallel channels with hydraulic diameter of 2.5 mm. Distillated water was used as working fluid. The transient thermal performance and thermal resistance were analyzed, for filling ratios of 10, 15, 25, 37.5, 50 and 75% and for tilt angles of 0° and 90° bottom heat mode. Start-up, thermal stability and hysteresis were experimentally tested. Three regimes were observed: conduction, unstable, stable. In general, the hybrid PHP showed lower thermal resistances, except for the filling ratio of 75% at vertical position. The hybrid device shows stop-over instabilities at power inputs between 140 and 180 W, at vertical position and low filling ratios. Higher surface roughness increases the number of vapor nucleation sites, which, in turn, improves the phase change at evaporator zone and facilitates the start-up conditions at low filling ratios. However, increasing roughness also rises pressure drops, mainly at high filling ratios.