Boiling Experiments Under Microgravity Conditions

Abstract

In the past 50 years, many experiments on pool boiling and flow boiling in microgravity were conducted by using various microgravity facilities. However, the effect of gravity on heat transfer is not conclusive because there are contradictory trends of experimental results, for example, on the bubble detachment and the heat transfer characteristics in nucleate boiling, and because of the unknown effect of gravity on heat transfer due to two-phase forced convection and on the critical heat fluxes in different vapor quality regions. This might be caused by the existence of gravity fluctuation and the restriction of experimental conditions inherent in all microgravity experiments in addition to the limited opportunity of repeated experiments. In this article, the existing experimental research is reviewed, highlighting recent studies on both pool boiling and flow boiling, and the experiments conducted to date by the present authors since the mid-1990s are reviewed to clarify the unclear knowledge on the phenomena. Two experimental tools, i.e., a sensor for the measurement of liquid film thickness and a transparent heated tube, developed by the present authors for microgravity experiments, are introduced, and the results are summarized, especially on the measurement of the microlayer thickness underneath the bubbles and the gravity effect observed in flow boiling including the recent results. In addition, the space experiment planned for on board the international space station to obtain coherent data for gravity effects on flow boiling is introduced, and its objectives and outline of the prepared set-up are explained.

Keywords:

• boiling
• two-phase flow
• microgravity
• heat transfer