Heat Transfer Mechanisms for Flow Boiling in Microgravity using Fluorescing Materials as Temperature Sensors

The objective of this study was to gather local heat transfer data on flow boiling in microgravity over a variety of flow parameters. Results were used to identify dominant heat transfer mechanisms and refine conventions that will be used for space-based flow boiling heat exchangers. Obtaining a fundamental understanding of the nature of flow boiling fluid mechanics and heat transfer in space environments allows more compact and efficient heat exchangers to be implemented for future space-flight missions and/or satellites. The experiment was conducted using high-speed CMOS cameras to record HFE 7000 flowing through a transparent sapphire tube (6 mm inner diameter, 8 mm outer diameter). A novel technique using temperature sensitive paint (TSP) was used to measure wall temperature along the inside of the tube. TSP fluoresces when excited with blue or UV light, and the intensity of its emission decreases with increasing temperature. This experiment was designed to track simultaneous intensity changes of the temperature sensitive paint and flow visualization with CMOS cameras in order to obtain a temperature distribution along the inner wall of the test section tube. A transparent conducting polymer film was laminated onto the outside of the tube so it couldbe electrically heated to initiate flow boiling. The temperature data were used to obtain values for local and averaged heat transfer within the test section. Ground-based experiments have validated this method by reproducing single-phase heated flow correlations. Local data provided insight into the dominant heat transfer mechanisms in microgravity and aided understanding of flow conditions which are gravity independent. It was found that single-phase liquid convective heat transfer is the dominant mechanism in microgravity flow boiling. The effect of increasing void fraction within the flow can be accounted for by incorporating flow acceleration, but the effects of flow regime transition and annular flow film thickness have yet to be determined. The resulting data from this experiment will help develop more accurate correlations for space-based heat exchangers, allowing spacecraft to distribute heat and power more consistently.