Piston effect in a supercritical fluid sample cell: A phenomenological approach of the mechanisms

We report on the analysis of the response of a near-critical fluid sample c ell submitted to heating at different distances from the critical temperatu re T-c, in the Mir station microgravity environment. We recall the hydrodyn amic and thermodynamic bases of the mechanisms of fast adiabatic heating, a lso called "thermalization by Piston Effect" (PE). We give a phenomenologic al approach of these mechanisms in a highly compressible critical fluid sub mitted to heating until a steady state is reached, and present the main res ults of numerical experiments in a Van der Waals fluid. We then use this ph enomenological understanding to analyze an experimental case of heating in reduced gravity onboard Mir, using the Alice 2 facility between 1996 and 19 99 (Cassiopea to Perseus missions). We have measured the characteristic tim e of the PE as a function of the critical temperature distance, and investi gated the crossover behavior from the pure adiabatic regime for a surroundi ng wall of infinite conductivity, to the bottleneck conductivity regime ind uced by a real cell. Close to T-c, we have evidenced the key influence of t he geometry of the cell and of the thermal properties of the wall materials .