Explosive nucleation in microgravity: The Constrained Vapor Bubble experiment

Nucleate boiling was observed in the microgravity environment on International Space Station during operation of the Constrained Vapor Bubble (CVB) heat pipe experiment. Surveillance images of the boiling dynamics within this constant volume system were correlated with transient liquid pressures and transient temperature profiles. Nucleation events occurred in a non-periodic but non-random way and individual events were recorded over a twenty (20) hour time period. Each nucleation event originated at the heater surface and new bubble growth was accompanied by a shock wave that passed through the heat pipe and partially collapsed the original vapor bubble. The critical size of the equilibrium homogeneous nucleating bubble radius was determined using the experimental data and standard thermodynamic descriptions of boiling to be on the order of 140 nm for a superheat of 42 K. Maximum heat input to the heat pipe closely followed the timing of the nucleation event. Maximum heat loss, via thermal radiation from the walls of the device, followed the timing of bubble motion and bubble coalescence in the device. The whole process resulted in about a 10% increase in the overall heat transfer rate. Using literature data for an evaporating droplet on a surface, a Kelvin–Clapeyron model for incipient boiling was used to compare the CVB and droplet experimental results. A simple model for the effect of location on the nucleation probability in the CVB was developed.