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 cell submitted to heating at different distances from the critical temperature Tc, in the Mir station microgravity environment. We recall the hydrodynamic and thermodynamic bases of the mechanisms of fast adiabatic heating, also called "thermalization by Piston Effect" (PE). We give a phenomenological approach of these mechanisms in a highly compressible critical fluid submitted to heating until a steady state is reached, and present the main results of numerical experiments in a Van der Waals fluid. We then use this phenomenological understanding to analyze an experimental case of heating in reduced gravity onboard Mir, using the Alice 2 facility between 1996 and 1999 (Cassiopea to Perseus missions). We have measured the characteristic time of the PE as a function of the critical temperature distance, and investigated the crossover behavior from the pure adiabatic regime for a surrounding wall of infinite conductivity, to the bottleneck conductivity regime induced by a real cell. Close to Tc, we have evidenced the key influence of the geometry of the cell and of the thermal properties of the wall materials.