Bubble — bubbles — boiling

A short overview of boiling research in microgravity performed during the past two decades is subject of this presentation. The research was concentrated on pool boiling without applying any external forces. The objective of this research was to answer the questions: Is boiling an appropriate mechanism of heat transfer in space applications, and how do heat transfer and bubble dynamics behave without buoyancy, shear or electrical field forces? Is bubble dynamics itself being able to maintain heat transfer during boiling? The correlations used today to calculate heat transfer coefficients for practical applications in pool boiling are more or less based on the assumption that buoyancy detaches the bubbles from the heating surface and carry vapor with hot liquid away. With this model heat transfer would break down in microgravity. That’s why microgravity itself is an outstanding environment to study boiling in order to gain a better understanding of the complex interrelated physical mechanisms. Various carrier systems that allow simulation of microgravity could be used, such as drop tower ZARM, drop shaft JAMIC, parabolic trajectories with NASA’s aircraft KC-135, ballistic rockets TEXUS, and finally three Space Shuttle missions. As far as the possibilities of the respective mission allowed, a systematic research program [1] was followed, which was continuously adjusted and updated to new results and parameters.We discuss the hydrodynamic and thermal behavior of single bubbles, the dynamics during coalescence processes and the interaction of bubbles at the hot wall during boiling with the processes: boundary layer superheat, nucleation, bubble growth, detachment and departure. Surprising results have been obtained, that not only saturated and subcooled boiling can be maintained in microgravity, but also that at lower heat fluxes an enhancement of heat transfer compared to terrestrial was observed, while most today used empirical correlations show a strong decrease extrapolated to lower gravity values. However, it must be pointed out that also the maximum accessible heat flux, the so called “critical heat flux”, is higher than predicted by present used relations, but as far as reliable values are available, reduced by about 50 % compared to terrestrials. With the simultaneously observed bubble dynamics the heat transfer results can be interpreted and both give rise to a better physical understanding of the boiling process.