Thermo-hydraulic analysis of the flat plate pulsating heat pipe tested under μ–gravity conditions

The high power and performance electronic devices in modern spacecraft systems with heat generation of hundreds of Watts per square centimeter crucially increases needs in high performance, low weight, energy efficient and reliable thermal management systems. Pulsating Heat Pipe (PHP) as a passive two-phase heat transfer device based on phase change induced motions of working fluid from evaporator to condenser could become a novel thermal management system due to high heat transfer capability, simple structure and ability of operating under different gravity levels and different positions. However, this relatively new technology with very complex thermo-hydrodynamic mechanisms inside is not fully studied and needs further investigations.
This work presents synthesis of the results obtained during 64th and 69th ESA Parabolic Flight Campaigns for the flat plate pulsating heat pipe tested under microgravity conditions in order to thermal performance investigations and liquid-vapor interfaces oscillation influence on the flow pattern inside the device.
Tested flat plate pulsating heat pipes represent a copper (64th PFC) / molybdenum (69th PFC) plate with milled squire shape channels (1.5 and 3 mm2) covered by the sapphire cover plate to allow visual analysis of the flow inside. Ethanol and dielectric fluid FC-72 were chosen as working fluids with volumetric filling ratio of 50 %.
Microgravity periods are mostly characterized by the significant temperature augmentation in the evaporator zone. This temperature rise is a result of fluid accumulation in the condenser and dry-out of evaporator ? heat transfers only by the FPPHP wall conduction.
Sometimes evaporator temperature drops because of flow pattern change from slug-plug to semi-annular and fluid reflows to the evaporator. This phenomenon was called “flow re-activation” and observed only for few cases during microgravity phases. Video post-processing was performed to define flow parameters influencing flow transition for microgravity conditions, as well as to determine liquid plugs velocities and accelerations. Classical criteria (We, Bo, Ga, Fr) used for slug-plug flow pattern definition were validated. Finally, the acting forces during stopover and re-activation were calculated.