HEAT-TRANSFER IN PURE CRITICAL FLUIDS SURROUNDED BY FINITELY CONDUCTING BOUNDARIES IN MICROGRAVITY

The behaviour of a near-critical sample of SF6, bounded by container w alls with finite thermal properties, was studied in space during the 1 994 IML-2 mission. Experiments were performed in the range 2500 to 1 m K above the critical point in which simultaneous density and temperatu re measurements are conducted during a number of transient heating run s. The results of these measurements show clearly that a fast isentrop ic thermalization takes place uniformly throughout the sample, with es sentially no effect on existing temperature and density gradients. The temperature rise caused by the isentropic thermalization is described quantitatively by a theoretical expression which takes into account t he finite thermal impedance of the cell walls. It has been possible to do so in a manner that satisfactorily represents the observations. Th e success of this description enables the separation of isentropic the rmalization from true heat transport effects, thereby opening the way to a determination of the thermal diffusivity of the fluid at temperat ures as close as 1 mK to the critical temperature. In addition, the ob served isentropic compressive heating mechanism suggests a new way for assessing specific important thermodynamic properties in the critical region, based on the experimental determination of the isentropic the rmal expansion coefficient.