Heater size and heater aspect ratio effects on subcooled pool boiling heat transfer in low-g

Pool boiling heat transfer measurements using heaters of varying aspect ratio were obtained in low-g (0.01 g ± 0.025 g) aboard the KC-135 aircraft. The heater aspect ratio was varied by selectively powering 2 × 2, 2 × 4, 2 × 6, 2 × 8, and 2 × 10 arrays of heaters in a 10 × 10 heater array containing individual heaters 0.7 × 0.7 mm2 in size. Electronic control circuitry was used to maintain an isothermal boundary condition on the heater surface while the power dissipated by the heater was measured. The working fluid was FC-72 at 101 kPa and three different bulk subcoolings. Low-g boiling behavior was governed by the dynamics of the primary bubble. For both square and rectangular heaters, CHF appeared to be a result of the competition between increasing heat transfer associated with the satellite bubbles and the decrease in heat transfer due to growth of the dry area under the primary bubble as the wall superheat increases. At low subcooling on rectangular heaters, surface tension acted to pull the bubble into a spherical shape, allowing liquid to rewet the surface. At high subcooling and high superheat, thermocapillary convection caused the large bubbles that formed on the surface to shrink by increasing the condensation on the bubble cap, resulting in more wetted area. The presence of thermocapillary convection at higher subcooolings may be due to FC-72 being a mixture of various components.