Boiling Heat Transfer and Bubble Dynamics in Microgravity

This article presents results for pool boiling heat transfer under microgravity conditions that the author and his team have gained in a succession of experiments during the past two decades. The objective of the research work was to provide answers to the following questions: Is boiling an appropriate mechanism of heat transfer for space application? How do heat transfer and bubble dynamics behave without the influence of buoyancy, or more general, without the influence of external forces? Is microgravity a useful environment for investigating the complex mechanisms of boiling with the aim of gaining a better physical understanding? Various carrier systems that allow simulation of a microgravity environment could be used, such as drop towers, parabolic trajectories with NASA's aircraft KC-135, ballistic rockets such as TEXUS, and, more recently, three Space Shuttle missions. As far as the possibilities of the respective missions allowed, a systematic research program was followed that was continuously adjusted to actual new parameters. After a general survey concerning the importance of boiling for technical applications, an introduction is given especially for those individuals not closely familiar with the fields of microgravity and boiling. Surprising results have been obtained: not only that saturated pool boiling can be maintained in a microgravity environment, but also that at small heater surfaces and lower values of heat fluxes, even higher heat transfer coefficients have been attained than under terrestrial conditions. The bubble departure can be attributed to surface tension effects, to "bubble ripening" and coalescence processes. Under subcooled conditions only, thermocapillary flow was observed that transports the heat from the bubble interface into the bulk liquid, but does not enhance the heat transfer compared with boiling at saturated conditions. Direct electrical heated plane surfaces lead to a slow extension of dry spots to dry areas below bubbles, the increasing surface temperature suggesting transition to film boiling. The critical heat flux in microgravity is lower than under earth conditions, but considerably higher than the hitherto accepted correlations predict when extrapolated to microgravity. The nearly identical heat transfer coefficients received for nucleate boiling under microgravity as well as terrestrial conditions, and for both saturated and subcooled fluid states, also suggest identical heat transfer mechanisms. These results lead to the conclusion that the primary heat transfer mechanism must be strongly related to the development of the microlayer during bubble growth. Secondary mechanisms are responsible for the transport of enthalpy in form of latent energy of the bubbles and hot liquid carried with them. Under terrestrial conditions, that mechanism is caused by external forces such as buoyancy; under microgravity conditions, the self dynamics of the bubbles and/or thermocapillary flow under subcooled conditions are responsible. The results demonstrate clearly that boiling can be applied as a heat transfer mechanism in a microgravity environment and that microgravity is a useful means to study the physics of boiling.