Pool Boiling in Reduced Gravity

The main outcomes of the worldwide experimental activity dealing with pool boiling in reduced gravity are summarized. The currently available experimental facilities and experimental opportunities are examined, the main results obtained by the various experimental teams are reviewed, and highlights of current and future applications of boiling in space systems are given. The work initiated by several groups around the world seems to indicate that pool boiling (especially the subcooled one) may be safely sustained in micro-g conditions with appropriate measures and that improvements in performances (e.g. by application of other force fields) are possible. However, due to the high cost and low availability of flight opportunities, and to their limitations in space and time, a final assessment has yet to be completed, and some results are still controversial. In the second part of the paper, a review of the main pool boiling features and of the related models is carried out. The main effects of gravity and other force fields are stressed and compared with the above mentioned experimental results on earth and under reduced gravity conditions. The most commonly accepted viewpoints are reported for each aspect. The empirical correlations developed for boiling heat transfer in terrestrial conditions do not trivially extend their validity outside their range of application. Thorough experimentation in microgravity is thus needed to assess the performance of boiling heat transfer in such conditions. It is believed that, after further experimental activity, it will be possible to design efficient boiling systems for future spacecrafts. The research in microgravity, by eliminating the dominant effect of the buoyancy forces, may also help clarify the role played by the various mechanisms in the boiling phenomenon.