Nucleate boiling is widely used as a means of heat transfer in thermal management systems because of its high heat transfer rates. This study explored the effects of heat flux and surface geometry on heat transfer behavior and bubble dynamics of nucleate pool boiling in microgravity. A single platinum wire, a twist of three platinum wires, and a twist of four platinum wires were used as boiling surfaces for two separate experiments performed in microgravity on board NASA’s parabolic flight aircraft. Wire temperature, thermocouple, and video measurements were taken during a total of 44 microgravity parabolas. Results show that the crevices formed by wire twisting provide regions of localized superheating and are able to reduce the heat flux necessary for boiling onset to occur. This localized heating results in a lower average heater temperature and shortened superheating periods, but this effect decreases when more wires are present in the twist. This behavior was investigated and confirmed with a finite volume, transient conduction model. This model also showed that the water temperature profile at the bubble onset indicates that water at a certain distance from the wire surface, in this experiment 50 μm, needs to be heated to above saturation temperature in order to initiate and generate a burst of bubbles. A relative bubble area analysis method was able to quantify vapor production and bubble behavior across multiple frames of video. Application of this method revealed a transition of bubble behavior from large isolated bubbles to jet flows of small bubbles, and this method allowed the heat flux contribution of jet flows to be approximated. Additionally, a new mode of jet flows was observed. Particle image velocimetry was used to provide approximate velocities of small bubble jet flows and their influence on heat transfer to the bulk fluid.

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