Speaker
Description
Pulsating heat pipes (PHPs), also known as oscillating heat pipes, are passive heat transport devices characterized by ease of construction, low mass, ability to function as efficient heat switch, multi-fold times higher effective thermal conductivity than metallic counterparts and gravity-independent thermal performance. The on-orbit long-term reliability and operation of room temperature (or non-cryogenic) PHPs has already been established and demonstrated by JAXA and U.S. AFRL. Numerous experiments on ground and on parabolic flights have been commissioned by ESA for understanding the peculiarities of PHP behaviour under microgravity conditions. This work intends to spark investigations on PHPs with cryogens as working fluids particularly for space applications. Cryogenic PHPs retain the advantages of room-temperature PHPs while additionally addressing low temperatures (2.5-100 K).
CEA Paris-Saclay hosts state-of-the-art research facility for experimentally characterizing cryogenic PHPs. The development nucleated in 2015 as part of a novel proposal within the framework of SR2S European project for the future Mars mission which was to utilize nitrogen PHP to cool the thermal shield between superconducting magnets and human habitat. Since then, the research at CEA Paris-Saclay has been broadened to PHPs of different lengths ranging from 3.7 m to just 200 mm. Successful heat transport tests have been conducted with different cryogenic fluids such as argon, nitrogen, neon and helium (as low as 2.5 K) as well. The best thermal performance ever measured for neon PHPs is reported at CEA Paris-Saclay with thermal conductance as high as ~5.9 W/K in gravity-assisted vertical orientation and ~3.7 W/K in horizontal orientation (the closest condition to space environment for PHPs). The same PHP when tested with helium is able to transport heat with thermal conductance of ~1.0 W/K. In fact, it is interestingly seen that the helium PHP operates in spite of breaching the classical Bond number criteria for room-temperature PHPs. Another remarkable challenge accomplished at CEA Paris-Saclay is development of the first demonstrator of neon PHP employed as passive thermal link to cool a HTS magnet using a single cryocooler. This highlights the excellent capability of cryogenic PHPs to be dynamic and serve as heat switch.
With the aim of taking cryogenic PHPs in space, a collaborative project with CNES is under way to experimentally gauge the potential of miniature cryogenic PHPs in replacing metallic braids used for thermal coupling of cryocooler with an IR instrument focal plane in 45 K – 80 K temperature range. Additionally, in absence of cryofluid within the capillaries, the PHP system can inhibit the heat in-leak arriving from the redundant cryocooler. This miniature PHP is being designed to achieve thermal conductance >1.5 W/K with heat loads up to 6 W at a spatial distance of 150 mm. Some glimpses of the design phases for on-ground testing are showcased in this work.
