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 walls with finite thermal properties, was studied in space during the 1994 IML-2 mission. Experiments were performed in the range 2500 to 1 mK above the critical point in which simultaneous density and temperature measurements are conducted during a number of transient heating runs. The results of these measurements show clearly that a fast isentropic thermalization takes place uniformly throughout the sample, with essentially 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 the finite thermal impedance of the cell walls. It has been possible to do so in a manner that satisfactorily represents the observations. The success of this description enables the separation of isentropic thermalization from true heat transport effects, thereby opening the way to a determination of the thermal diffusivity of the fluid at temperatures as close as 1 mK to the critical temperature. In addition, the observed 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 thermal expansion coefficient.