Some physical and chemical proceddes in fluids

The ability to reproduce reduced gravity conditions for long periods is one of the reasons why the orbiting laboratory is so attractive. In this paper several fluid dynamics problem areas are reviewed in which zero-gravity conditions are of great importance. Although emphasis is placed on space processing, there are some older problems also in which gravity masks the phenomcna, impeding a reasonably simple approach to the solution. Three problems are considered: Thermal convection under reduced gravity. The dumping effect ofsurface gravity waves at the outset of convection induced by surface tractions is discussed in particular. The existence of convection is of concern for some satellite thermal control techniques presently used, and for most of the proposed manufacturing processes. Whereas convection should be normally avoided, problems related to the containerless stirring ofa melt constitute an exception. Secondly, gravity and chemical reactions. Although chemical reactions are independent of gravity because of the small mass of the molecules and atoms involved, in many cases the reaction rate dcpends on the arrival of the species to the reaction zone. When the arrival process is buoyancy-controlled, the net specd of the reaction will be affected by the gravity. Thirdly, two-phase flows under reduced gravity provkle interesting problems from boiling heat transfer to degasslng of melts. This part of the paper deals only with the measurement of sound veiocity in a liquid containing bubbles. It is suggested that such measurements should be mude under reduced gravity to provide reliable residís.