Numerical Simulation and Experimental Validation of the Dynamics of a Single Bubble During Pool Boiling Under Constant and Time-Varying Reduced Gravity Conditions

The numerical simulation and experimental validations of the growth and departure of a single bubble on a horizontal heated surface during pool boiling under reduced gravity conditions have been performed here. A finite difference scheme is used to solve the equations governing mass, momentum and energy in the vapor liquid phases. The vaporliquid interface is captured by level set method, which is modified to include the influence of phase change at the liquidvapor interface. The effects of reduced gravity conditions, wall superheat and liquid subcooling and system pressure on the bubble diameter and growth period have been studied. The simulations are also carried out under both constant and timevarying gravity conditions to benchmark the solution with the actual experimental conditions that existed during the parabolic flights of KC-135 aircraft. In the experiments, a single vapor bubble was produced on an artificial cavity, 10 μm in diameter microfabricated on the polished silicon wafer, the wafer was heated electrically from the back with miniature strain gage type heating elements in order to control the nucleation superheat. The bubble growth period and the bubble diameter predicted from the numerical simulations have been found to compare well with the data from experiments. INTRODUCTION Boiling, being the most efficient mode of heat transfer is employed in various energy conversion systems and component cooling devices. Applications of boiling heat transfer in space applications include thermal management, fluid handling, control and power systems. The key factors that are to be * This work received support from NASA under the Microgravity Fluid Ph ! author for correspondance addressed for the space systems based on Rankine cycle are the boiling heat transfer coefficients and the critical heat flux under depleted gravity conditions. Keshock and Siegel (1964) showed that the bubbles grow larger and show higher growth periods before getting detached from the heater surface under reduced gravity conditions owing to the reduced buoyancy force acting on the bubbles. Merte (1994) and Lee and Merte (1997) have reported the results of pool boiling experiments conducted in the space shuttle for a surface similar to that used in drop tower tests. The subcooled boiling was found to be unstable during long periods of micro gravity conditions. It was concluded that the subcooling has negligible influence on the steady state heat transfer coefficient. Ma and Chung (2001) experimentally studied single bubble dynamics in flow boiling of FC72 at terrestrial gravity and reduced gravity conditions in a 1 second drop tower. It was observed that the bubble departure diameter at reduced gravity conditions is larger than that of terrestrial gravity case. However no liftoff of the bubble from the heater surface was observed during the short micro gravity conditions. Straub, Zell and Vogel (1992, 1994) conducted series of nucleate boiling experiments using thin platinum wires and gold coated flat plate as heaters at low gravity conditions in the flights of ballistic rockets and in KC-135 aircraft. For a flat plate heater with R12 as the test liquid, boiling curves similar to those of normal gravity cases were obtained. Using R113 as the test liquid, rapid bubble growth and large bubbles were observed. However, neither the bubble growth rate nor the bubble diameter at the departure was given. For subcooled micro gravity cases, they observed a reduction of up to 50% in