Experiments were conducted in the microgravity of space in which a pool of liquid (R-113), initially at a defined pressure and temperature, was subjected to a step imposed heat flux from a semitransparent thin-film heater forming part of one wall of the container such that boiling is initiated and maintained for a defined period of time at a constant pressure level. Measurements of the transient heater surface and fluid temperatures near the surface were made, noting in particular the conditions at the onset of boiling, along with motion photography of the boiling process in two simultaneous views, from beneath the heating surface and from the side. A total of nine tests were conducted at three levels of heat flux and three levels of subcooling. They were repeated under essentially identical circumstances in each of three space experiments. The absence of buoyancy resulted in the onset of boiling at low heat flux levels, with what is defined as quasi-homogeneous nucleation taking place. The influence of these low levels of heat flux and the pressure effect used to produce the bulk liquid subcooling are accounted for by a modification of classical homogeneous nucleation theory.
