Incipient boiling in microgravity.

Abstract

An experimental study of incipient boiling in short-term microgravity and with a/g = $\pm$ 1 for both pool boiling and forced convection boiling was performed. Calibrated thin gold films sputtered on a smoothly polished quartz surface were used simultaneously for thermal-resistance measurements and heating of the boiling surface. The gold films were used for both transient and quasi-steady heating surface temperature measurements. Three test vessels were constructed for precise measurement and control of fluid temperature and pressure: a laboratory pool boiling vessel for the a/g = $\pm$1 experiments, a pool boiling vessel designed for the 131 m free-fall in the NASA Lewis Research Center Microgravity Research Facility for the microgravity tests, and a forced convection boiling loop for low velocity experiments with a/g = $\pm$1. Measurements included the heater surface temperature, the pressure near the heating surface, the bulk liquid temperatures, and the mass flow rate for the tests in the forced convection boiling loop. High speed photography (up to 1,000 frames per second) was used in the experiments. With high quality microgravity and the measured initial temperature of the quiescent test fluid, R113, the temperature distribution in the liquid at the moment of boiling inception resulting from an imposed step in heat flux is known with certainty not possible previously. The types of boiling propagation across the large flat heating surface, some observed here for the first time, are categorized; the conditions necessary for their occurrence are described. Explosive boiling propagation with a striking pattern of small scale protuberances over the entire vapor mass periphery not observed previously at low heat flux levels (on the order of 5 W/cm$\sp2$) is described. For the heater surface with a/g = $-$1, a step in the heater surface temperature of short duration was imposed. The resulting liquid temperature distribution at the moment of boiling inception was different from that obtained with a step in heat flux. This is the first study of incipient boiling to impose a step in heater surface temperature with the speed and certainty obtained here.