A study of the effects of heater size, subcooling, and gravity level on pool boiling heat transfer

Pool boiling heat transfer measurements from different heater sizes were obtained in low-g (0.01 ± 0.025 g) and high-g (1.7 ± 0.5 g) aboard the KC-135 aircraft. Boiling on three heaters of different size (0.65, 2.62, 7.29 mm2) was studied. Control circuitry was used to maintain an isothermal boundary condition on the heater surface while the power dissipated by the heater was measured. Steady-state boiling data in low-g and high-g were obtained for various bulk fluid subcoolings and wall superheats. For the two larger heaters in low-g, critical heat flux (CHF) increased with increasing bulk subcooling and is significantly smaller than the CHF obtained in high-g. Satellite bubble coalescence with the primary bubble was the mechanism by which CHF occurred in low-g for larger heaters. At high subcoolings and wall superheats, above that of which CHF occurred, the onset of strong thermocapillary convection caused an increase in heat transfer above CHF. For the small heater in low-g, bulk subcooling had a negligible impact on boiling performance and a characteristically low heat flux was observed for the CHF. In high-g, boiling on heaters roughly the size of the bubble departure diameter was dominated by surface tension and characterized by a very low bubble departure frequency. For larger heaters in high-g, bubble departure frequency decreased with increasing subcooling but time and surface averaged heat transfer remained unaffected.