HYSTERESIS PHENOMENA IN SUBCOOLED FLOW BOILING OF WELL-WETTING FLUIDS

Detailed knowledge of the physical phenomena involved in subcooled boiling is of great interest for the design of liquid-cooled heat-generating systems with high heat fluxes. An electrically heated stainless steel tube, 2.3 m long, with a 7.57-mm inner diameter, was used to study the incipience of flow boiling in subcooled, well-wetting fluids (R-12 and R-114) over a wide range of pressure, mass flux, and heat flux. Hysteresis in boiling observed by increasing and decreasing heat flux seems to be ascribed to the small contact angle of refrigerants on surfaces such as stainless steel, allowing the larger cavities in the wall to be flooded. A higher temperature excess may be reached in the wall before boiling sets in than the temperature achieved when boiling has developed. Some existing correlations were tested, revealing the general inadequacy in predicting the higher superheating required in well-wetting fluids for the onset of nucleate boiling. Classic theory on bubble growth, heat transfer relationships, and the force balance equation at the bubble-liquid interface, coupled with an empirical correlation for the determination of the boiling incipience conditions, accomplish a very successful prediction of the experimental data.