RELAXATION OF A SUPERCRITICAL-FLUID AFTER A HEAT PULSE IN THE ABSENCEOF GRAVITY EFFECTS - THEORY AND EXPERIMENTS

We study the response of a fluid in near-critical conditions to a heat pulse, in the absence of gravity effects. The fluid under investigati on is CO2 at critical density. It is enclosed in a thermostated sample cell. We apply a theory that accounts for hydrodynamics and a real eq uation of state. Comparison with experiments performed under reduced g ravity on board the MIR orbital station show quantitative agreement an d demonstrate that the dynamics of relaxation is ruled by two typical times, a diffusion time t(D) and a time t(c) associated to adiabatic h eat transport, the so-called ''Piston effect'' (PE). Three regions are observed in the fluid. First, a hot boundary layer, developing at the heat source, which shows large coupled density-temperature inhomogene ities. This part relaxes by a diffusive process, whose density and tem perature relaxations are slowed down close to the critical point. Seco nd, the bulk fluid, which remains uniform in temperature and density a nd whose dynamics is accelerated near the critical point and governed by the PE time. At the thermostated walls a slightly cooler boundary l ayer forms that cools the bulk also by a PE mechanism. The final equil ibration in temperature and density of the fluid is governed by the di ffusion time t(D), which corresponds to the slowest mechanism. Compari son with a one-dimensional model for temperature relaxation is perform ed showing good agreement with experimental temperature measurements. A brief comparison is given with the situation in the presence of grav ity.