SUBCOOLED VOID GROWTH MECHANISMS AND PREDICTION AT LOW-PRESSURE AND LOW-VELOCITY

Measurements were made of void growth and wall temperatures for two circular annular geometries and forced-convective subcooled nucleate boiling conditions, with flow rates between 0.02 and 0.20 kg/s, pressures of 1.05, 2 and 3 bar, and inlet temperatures between 30 and 90-degrees-C. Void growth results at low pressure show that the highly subcooled void region is important, and void fraction at the onset of significant void may be as high as 10%. Observations of bubble ebullition and detachment mechanisms obtained from a high speed photographic study were compared to assumptions generally used in void growth modeling. The photographic results show that bubbles do not travel far downstream after nucleating, that there is no region of attached void and that bubbles slide along the wall before being ejected into the flow after the onset of nucleate boiling. Unlike findings for high pressure systems reported in the literature, the onset of significant void was found to be independent of (i) bubble detachment, (ii) the location where the bubble is first ejected from a bubble layer and (iii) the transition from partial nucleate boiling to fully developed boiling. A phenomenological void growth model is presented which accounts for the vapor volume inside a heated channel at atmospheric pressure, and includes the bubble ebullition cycle, formulated on the basis of information obtained from the high speed photographic study.