The Thermal Diffusivity of the Refrigerants R32, R125, and R143a

The thermal diffusivity of the halogenated fluorocarbons R32, R125, and R143a was systematically measured in a wide region of state around the liquid-vapor critical point using dynamic light scattering as the measuring method. The experimental setup is capable of measuring in homodyne (high light intensity) or heterodyne mode (low light intensity). Especially in the vicinity of the critical point, this method is superior to other techniques since no calibration is necessary and the fluid is held in thermodynamic equilibrium. With high light-scattering intensities in the near-critical region, the uncertainty of the measurements is about 0.5% and increases to up to 5% far from the critical point. Measurements were performed in both coexisting phases, along the critical isochore, and along seven isotherms. The range of application is characterized in terms of the reduced density and pressure by 0.3 < /_c < 2 and 0.5 < p/p_c < 2.5. These limits are defined by low scattering intensities and by the mechanical limits of the apparatus due to high pressures of the fluid. The corresponding temperature range is from 300 to 390 K. When approaching the critical point, the thermal diffusivity drops by orders of magnitude and can be expressed by simple scaling laws depending on the reduced temperature difference = (T–T_c)/T_c. In addition to the thermal diffusivity, the refractive index and the critical parameters T_c, p_c are measured and presented. The density of the fluid is calculated from the refractive index using the Lorentz–Lorenz relation.