THERMAL-DIFFUSIVITY OF FLUIDS IN A BROAD REGION AROUND THE CRITICAL-POINT

Dynamic light scattering is a suitable method for the investigation of transport properties such as the thermal diffusivity of optically transparent fluids. An insufficient production of intensity of scattered light may be a limiting effect. For that reason the vicinity of the gas-liquid critical point represents the classical range of application. In this paper, it is shown that by means of an appropriate choice of experimental apparatus, measurements are also feasible in an extended range of states. Broad regions around critical points of three pure fluids (sulfur hexafluoride, SF//6; ethane, C//2H//6; nitrous oxide, N//2O) over temperature ranges vertical T - T//c vertical of 0. 02 to 50 K and density ranges (p/p//c) of 0. 2 to 2 were investigated. In this region the thermal diffusivity shows great variations with temperature and density and cannot be described by means of ideal-gas behavior or relations for liquids. The measurements were carried out along the coexistence curve for both phases, along the critical isochore and along some isotherms with T less than / greater than T//c. The measured or calculated density, pressure, and thermal diffusivity data as well as some correlations are presented.