Design of pulsating heat pipes. A novel non-equilibrium lumped parameter model for transient gravity levels

As relatively new and promising members of the wickless heat pipe family, Pulsating Heat Pipes (PHPs), with high effective thermal conductivity and construction simplicity, may answer to the present industrial demand of efficient thermal control, flexibility and low costs. In the last twenty years, many experimental and numerical works focused on PHPs, but despite the great efforts, their non linear, two-phase, internal flow remains essentially an unknown and, thus, none of the existing models is actually able to simulate it. One of the most important unsolved questions regards the influence that gravity may have on the device thermal-hydraulic behavior. Literature, indeed, reports very poor and contradictory data, even if modified gravity conditions arise in various applications from automotive to aerospace, from chemistry to material synthesis. This work aims to fill this serious lack of knowledge. Thus, a capillary, closed loop PHP made of a copper tube bended into 32 parallel channels and filled with FC-72 has been investigated both on the ESA ESTEC Large Diameter Centrifuge (ESA Educational project Spin Your Thesis! 2013), and on board of the ESA-NoveSpace Airbus A300 Zero-G flying parabolic trajectories (58th and 59th ESA Parabolic Flight Campaigns). For the first time, a planar PHP with circular cross section channels, equipped with 14 thermocouples and a pressure transducer has been fully, thermally characterized in several operative conditions from 0.01g to 20g. In addition, in order to provide a numerical tool able to help and support the experimental research in enlarging the present knowhow and spreading PHPs industrial application, the results of these experimental campaigns have been used to develop and validate a novel lumped parameters model. It uses an advanced numerical technique to allow fast simulations, extending sensitivity analysis and device designs. Lumped parameter models are not a novelty for PHPs per se: however, for the first time this kind of numerical tools has been applied to simulate transient operative conditions removing physical simplified assumptions and embedding directly phase changes processes. The resulting code showed very good prediction capability, being able to reproduce with high accuracy the experimental recorded data both in steady and transient conditions.