Thermo-Hydraulic Analysis of Semi-Transparent Flat Plate Pulsating Heat Pipes Tested in 1 g and Microgravity Conditions

Four Closed-Loop Flat Plate Pulsating Heat Pipes (FPPHP) with four different channel internal diameters have been tested under terrestrial (1 g), hyper (1.8-2 g) and micro-gravity conditions, during the ESA 64th Parabolic Flight campaign, at the same operating conditions (heat power range, cooling fluid temperature, vertical BHM orientation). During terrestrial gravity periods, the fluid stratifies with the liquid at the bottom of the system, and, in the lower part of the channels, where the heat is applied (Bottom Heated Mode), the thermo-hydraulic heat and mass transfer mode is purely linked to pool boiling, regardless the diameter. Instead, during microgravity periods, the fluid circulates naturally into a slug and plug flow pattern regime. Dry-out phenomena occur almost immediately after the change in gravity level followed by a fast motion of the liquid plugs, promoting a mass transfer through all PHP channels. A comparative analysis of the evaporator temperatures and the menisci velocities obtained through video post-processing, shows the influence of the channel diameter on the heat and mass transfers occurring inside the different PHPs. A quantitative comparison of the FPPHP thermal performance shows that, during microgravity transient phases, there is a limit of the channel diameter beyond which the thermal performances does not increase anymore, despite the decrease in viscous pressure losses.