Alternative Refrigerants in the Liquid Phase: Thermal Conductivity of Binary and Ternary Mixtures

Refrigerant mixtures seem the most promising candidates as replacement fluids in refrigeration devices and heat pumps. In this paper two prediction methods are proposed for th~ evaluation of the thermal conductivity of refrigerant mixtures in their saturated liquid state and in the subcooled region. The first method is based on the use of mixing rules applied to pure refrigerant data evaluated by means of a prediction method originally developed by present authors. The second method approaches azeotropic refrigerant mixtures as pure refrigerants. It requires the knowledge of the molar mass and the critical temperature only showing very good accuracy and conformity with experimental data. The methods were tested against experimental data and show typical average absolute deviations which are less than 4%, with maximum absolute deviations usually less than 9% in the reduced temperature range 0.30 to 0.95. INTRODUCTION Refrigerant mixtures are becoming the most promising candidates for the replacement of many working fluids in refrigeration devices and heat pumps. In fact, the necessity to replace many, widely used, compounds, combined with the usual requirements for effectiveness and safety, compelled scientific and engineering communities to study and develop new products at a constantly increasing rate. As a result, the number of binary and ternary mixtures introduced as new refrigerants by major industrial companies greatly increased during past years. While many new effective alternatives have been proposed to replace most of working fluids already banned or becoming illegal in few years, the new bottle neck is represented by the testing of these compounds. In most of cases, the mixtures proposed by industrial companies are composed of refrigerants whose characteristics are well known. Thus the central problem becomes the evaluation of the properties of mixtures by means of reliable and effective mixing rules. However, a general equation, able to evaluate transport properties of mixtures as a sole function of their thermophysical properties, would be exceptionally useful. In principle tbis new approach would be possible but no proposal seems to have been published. According to these concepts we stated to approach the evaluation of transport properties of refrigerant mixtures both by testing and refining the usual techniques based on the use of mixing rules and by introducing a new general equation as a sole function of mixture thermophysical properties. For what concerns the first issue, in this paper various alternatives for the evaluation of liquid thermal conductivity of mixtures composed of organic compounds are presented. The latter study, based on the new approach, was, at the moment, peifonned on azeotropic mixtures only. This is mainly due to the fact that their composition is fixed and their equilibrium properties are welllrnown. In both cases the mean and maximum deviations between evaluated and experimental liquid thermal conductivity data are usually less than 4% and 9% respectively, in temperature ranges which go from the normal melting point near to the critical point THE PREDICTION MEmOD FOR PURE REFRIGERANTS Both our prediction methods for the liquid thennal conductivity of refrigerant mixtures are based on the semi-empirical equation already proposed by present authors for pure refrigerants [Latini, 96] and for other organic compounds [Latini, 95b]: