Density equilibration near the liquid-vapor critical point of a pure fluid .2. Coexisting phases for T<T-c

Measurements were carried out on the density change delta rho(t) in a pure fluid (He-3) after a small step change in the temperature of its container. The sample fluid was kept at constant average densities <Delta(rho)over bar> and in the coexisting liquid and vapor phases below the critical temperature T-c. The measurements were performed via two superposed capacitive sensors. At temperatures far below T-c, the equilibration in the liquid and vapor phases: measured, respectively, by the top and bottom sensors, are found to proceed very differently. As T-c is approached, this difference diminishes; both the measured effective relaxation times level off and join smoothly the data obtained above T-c. This coexisting liquid-vapor system of He-3 is simulated in one dimension. The results are presented for the spatial and temporal evolution of temperature and density in the fluid following a temperature step of the enclosure. The profiles delta rho(t) and their effective relaxation times are compared with the experimental observations in both phases. There is a qualitative agreement between the simulation and experiment for (T-c- T)/T-c less than or similar to 10(-3), and the quantitative differences further away from T-c are discussed. The results of experimental measurements and of computer simulations along isotherms are discussed and lead to complementary information on the equilibration dynamics as a function of average density. The asymptotic relaxation times obtained from simulation and from a formula based on the average thermal diffusivity Of the entire fluid sample are compared and discussed, both for normal gravity, and also under microgravity conditions where both diverge as T-c is approached.