THERMAL EQUILIBRATION OF FLUIDS NEAR THE LIQUID-VAPOR CRITICAL-POINT - HE-3 AND HE-3-HE-4 MIXTURES

We study the temperature-equilibration process of fluids at constant volume in a thermal conductivity cell, where an initial temperature gradient relaxes to zero. The calculation is performed in the linear approximation for a pure fluid and a binary mixture. Near the critical point of the pure fluid, the adiabatic heating process, which takes place at constant volume V, causes equilibration to proceed four times faster when CP/CV≫1 than for the process at constant pressure P. For the mixtures, the relaxation rate enhancement at constant V compared with constant P is restricted to a temperature region where the coupling between temperature and mass diffusion is small. The predictions are compared with experimental results for3He and for two3He-4He mixtures along their critical isochores. Finally, we discuss the thermal relaxation in the two-phase (liquid-gas) and one-phase (gas) regimes at the critical density, as measured with a conductivity and a calorimetry cell. The contrasting behavior for3He and a3He-4He mixture in these two regimes and under these different constraints is pointed out and discussed. © 1990 Plenum Publishing Corporation.