Transport-mechanisms in natural nucleate boiling in absence of external forces

In literature it is generally supposed that under terrestrial conditions the driving force in natural, nucleate boiling heat transfer is namely buoyancy caused by earth gravity, which is expressed in the empirical correlations for technical applications. However, experiments in microgravity performed during the past three decades demonstrate unanimously that up to a medium level heat flux the overall heat transfer in pool boiling is nearly independent from gravity. We refer and discuss in this paper on results of experiments performed with various liquids and liquid states and also using various heater geometries on mission platforms which provide low gravity for short and long periods. Beside the measurements of the experimental parameters to determine the heat transfer, we observed the macroscopic boiling process itself with movie films and videos in order to study the bubble dynamics. From these records we learned about the mechanisms of heat and vapour bubble transport, about the interaction between solid heater, superheated liquid, and vapour without gravity or other external force only generated by the bubbles themselves, and we observed significant details about the boiling process not recognized so far. These findings are essential for a better understanding of the complex physical process; and therefore they are important for the formulation of empirical correlations, and in future for numerical simulations to predict properly boiling heat transfer for technical applications.