Thermo-fluidic characterization of a deployable pulsating heat pipe tested at different gravity levels

The present work describes the experimental investigation of a deployable Pulsating Heat Pipe (PHP) (i.e. the adiabatic section is shaped as a torsional spring) on ground and onboard a zero gravity aircraft to test its technological readiness level in microgravity/hypergravity conditions and to infer on the thermohydraulic effect of the 3D shape with respect to an equivalent standard PHP. For sake of comparison indeed, a planar horizontal PHP having the same length of the adiabatic section is developed as a reference case. The deployable PHP at 180deg (i.e. unfolded configuration, evaporator and condenser on the same horizontal plane), is tested for two different coil orientations, up and down respectively, and compared to the planar one. The thermal resistances of the first (180-up) and the planar one in horizontal position are equivalent, while the second (180-down) reaches the highest thermal resistance. Weightlessness allows to suppress the gravitational force intrinsically present in a 3D shaped PHP when tested on ground, and thus enable operation even in the worst tested position (180-down). Therefore, the deployable PHP, filled with HFE7000 at 70 % filling ratio, is tested for three different opening configurations, namely, 0 degrees, 90 degrees and 180 degrees down deploying angles, three different heat loads (14, 24, 34 W), under different gravity conditions (parabolic flight campaign). In normal gravity condition, the maximum temperature reached at the evaporator section was slightly affected by the mutual position of the evaporator and condenser (0-90-180 deg). Instead, the gravity field continuous variation (normal-hyper-micro) during each parabola enhances fluid oscillations, ensuring slug-plug flow at low heat input (14 W), for all configurations.