Experimental Investigation of Filling Ratio Effect in a 12-Turn High Temperature Oscillating Heat Pipe

A 12-turn high temperature oscillating heat pipe (HTOHP) charged with NaK-78 was tested experimentally to investigate the effect of filling ratio on thermal performance and orientation robustness. The HTOHP was fabricated from stainless steel 316 with a total length of 711.2 mm and tested in a closed chamber. Five filling ratios were tested (30%, 40%, 50%, 60%, and 70%) at heat inputs ranging from 500 W – 3250 W for inclination angles of 90°, 60°, 30°, 0° (horizontal), -30°, -60°, and -90°. For all cases, the effective thermal conductivity increased with heat input. The 40% filling ratio yielded the highest effective thermal conductivity (a maximum of 1836.8 W/m·K at 3250 W and 30°) of all tested filling ratios but began to fail at 0° when oscillations were no longer aided by gravity. The 70% filling ratio yielded the second highest effective thermal conductivity, outperforming 30%, 50%, and 60% in almost all cases and operated at all tested angles (including the -90° case of evaporator above the condenser).