Transport Properties of Refrigerants and Refrigerant Mixtures

A. Laesecke, R.A. Perkins, M.O. McLinden, and M.L. Huber

Objectives: To resolve large discrepancies between literature data for the transport properties of pure refrigerants and to provide reliable experimental transport properties data for the refrigerant mixtures to develop advanced property models.

Problem: Viscosity measurements for alternative refrigerants, which were carried out in different laboratories since 1988, exhibited differences up to 35%, far in excess of experimental uncertainty. Lack of experimental transport properties data for alternative refrigerant mixtures impedes model refinement and the use of such mixtures in HVAC equipment.

Approach: The NIST sealed gravitational viscometer with a straight vertical capillary was used for new benchmark measurements of saturated liquid ammonia, R32, and R134a to resolve the disconcerting discrepancies between literature viscosity data. Viscosity and thermal conductivity measurements were carried out on four binary and one ternary blend of R32, R125, R134a, and propane (R290), each at two compositions. Measured conditions included subcritical liquid and vapor as well as supercritical phases. Viscosities were measured in the sealed capillary viscometer and in the torsional crystal viscometer. Thermal conductivities were determined from transient and steady-state measurements in hot-wire instruments. These data are used to develop improved transport property models.

Results and Future Plans: Some of the literature data sets did not properly apply necessary corrections in their analysis. Agreement within the combined experimental uncertainty was achieved after applying these corrections. An improved correction for the radial acceleration in viscometers with coiled capillaries was developed. A need was identified to extend international viscometry standards to sealed gravitational capillary instruments. Present standards cover only open capillary viscometers which cannot be used for measurements of volatile liquids. Transfer of sealed viscometer technology to a manufacturer is underway. The mixture measurements revealed strongly non-ideal composition dependences for transport properties in systems of nonpolar/polar compounds such as R32 + propane. The figure shows that the saturated liquid viscosities of both blends are even lower than the viscosity of propane. These results will be incorporated in improved mixture transport property models. The measurements with the torsional crystal viscometer revealed widely varying electrical conductivities and dielectric permittivities of the mixtures.

Figure 1. Non-ideal viscosity of liquid R32 + Propane (R290) mixtures.

Publications:

Kiselev, S.B., Perkins, R.A., and Huber, M.L. "Transport Properties of Refrigerants R32, R125, R134a, and R125 + R32 Mixtures in and Beyond the Critical Region," Int. J. Refrig. 22, 509 (1999).