R-134a, R-245fa, Mixture, Equation of State, Helmholtz energy
A thermodynamic property model explicit in the Helmholtz energy is presented for the R-134a/245fa mixture. Although some manufacturers have already produced heat pump systems using the mixture, an accurate property model has not been established for the mixture. The model presented in this work can be used to detailed and comprehensive analysis of the heat pumps. The multi-fluid approximation based on pure-fluid Helmholtz energy equations of state forms the framework of the model. The Helmholtz energy of the mixture is expressed as the sum of the ideal gas contribution, the compressibility contribution (real fluid contribution), and the contribution from mixing to represent the deviation from ideal mixtures. The first two contributions are calculated using the most accurate equations of state for R-134a and R-245fa. The independent variables are the temperature, molar volume, and composition. Typical multi-fluid models include five adjustable parameters, which are normally optimized by a nonlinear fitting to experimental data. The fitting in this work determines the parameters using experimental data for the critical parameters and liquid and vapor densities obtained in our previous work, in addition to published vapor-liquid equilibrium data. The numerical data of the critical parameters and densities are also presented. The model can be used to calculate all thermodynamic properties of the mixture at various compositions. The estimated uncertainties in calculated properties from the model are within ±0.5 % for the bubble and dew point pressures and ±0.3 % for the liquid and vapor densities. The critical locus of the mixture is accurately represented with the model. The calculated critical temperature corresponds to experimental values within ±0.3 K.