CRITICAL HEAT FLUX ON MICROSTRUCTURED SURFACES IN MICROGRAVITY AND IN THE PRESENCE OF ELECTRIC FIELD: PRELIMINARY RESULTS OF A PARABOLIC FLIGHT CAMPAIGN
Boiling heat transfer is used in a wide range of industrial processes, including desalination, electronics cooling, power generation, and two-phase thermal management in space stations and satellites. Critical Heat Flux (CHF) is an important performance-limiting condition in many boiling heat transfer processes. In particular, in microgravity conditions, CHF encounters a drastic reduction. Thus, it is important not only to understand the mechanisms that lead to CHF but also to explore and identify ways to enhance it in micro-g. In this work, we explored the possibility to enhance CHF combining both engineered surfaces (passive technique) and electric field (active technique).
In normal gravity applications, it has been demonstrated that CHF is greatly enhanced on microstructured surfaces; this is often associated with imbibition, which, remarkably, is independent of gravity. On the other hand, the application of an electric field creates a driving force able to remove vapor away from the surface. Therefore, their combined application appears a suitable technique to improve CHF in micro-g.
Experiments were run during a parabolic flight campaign held by ESA on Airbus A310-ZeroG. A dedicated apparatus was built and operated with bare and microstructured boiling surfaces, using slightly subcooled FC72 at 1 bar. A DC electric field was applied via a metal grid laid 6 mm over the heated surface. The results showed that CHF on bare surfaces is reduced in microgravity. In the presence of microstructures, CHF improves, although it does not reach the same value as in normal gravity. The application of the electric field induces a further improvement of CHF in microgravity conditions. It is thus demonstrated that vapor removal mechanisms, like buoyancy or electric force, play a role which is additional to imbibition in determining the value of CHF on microstructured surfaces.