Physics of boiling and bubble dynamics with and without gravity: Conclusions of space experiments

Boiling heat transfer on a miniature heater has been studied in FREON R11 u nder microgravity conditions during the Spacelab Mission IML-2 in 1994 and compared with measurements on Earth. Several boiling modes have been observ ed depending on the liquid state, the subcooling of the liquid, and the hea t flux. The most important role in boiling heat transfer plays the surface tension; the wetting behavior between the liquid and the solid surface of t he heater; momentum of vapor formation; coalescence processes; and, in subc ooled liquids, the thermocapillary driven convection. In the nucleate boili ng region, we could not observe a remarkable influence of the gravity on th e heat transfer. In the film boiling and transition boiling regions at mode rate subcooling, a maximum reduction of 50% of the heat transfer coefficien t compared to the Ig data was found at this state. A single bubble with a 4 0 times larger diameter than the heater diameter of 0.26 mm was attached at the top of the heater developing a strong thermocapillary jet flow. The ob served heat flux densities and the values of the heat transfer coefficient are very high, even higher compared with wires. These experiments are basic studies to investigate the influence of the buoyancy force on the heat tra nsfer and on the bubble dynamics, but in addition, they are simulations for the direct cooling of small electronic devices by pool boiling heat transf er, which becomes very important due to high thermal loads of modern electr onic components even for the application in space vehicles.