Speed of Sound and Derived Virial Coefficients of Difluoromethane (R32) + trans -1,3,3,3-Tetrafluoropropene (R1234ze(E)) and Difluoromethane (R32) + Hexafluoropropylene (R1216) Binary Mixtures in Vapor Phase

Binary mixtures of difluoromethane (R32) with hydrofluoroolefins (HFOs) possess excellent thermodynamic performance and low global warming potential, supporting the reduction of hydrofluorocarbons (HFCs) usage. In this work, the gaseous speed of sound was measured for R32 + trans-1,3,3,3-tetrafluoropropene (R1234ze(E)) and R32 + hexafluoropropylene (R1216) at temperatures (303.15 to 363.15) K and pressures up to 1063 kPa, with relative expanded uncertainties (k = 2) of 0.022% and 0.026%, respectively. For R32 + R1234ze(E), comparisons with two dedicated models revealed opposite trends: one model systematically overpredicted the speed of sound (positive bias), whereas the other underpredicted it (negative bias). This implies that both models require refinement for gas-phase thermodynamic properties, especially for the gaseous speed of sound. A universal acoustic truncation virial equation was developed using critical parameters and molecular structure parameters. For R32 + R1234ze(E), within the experimental range, this acoustic equation outperformed the two dedicated models. For R32 + R1216, deviations between the acoustic virial equation and experimental data increased with pressure, suggesting inaccuracies in the critical parameters and molecular structure parameters of R1216. Finally, acoustic virial coefficients were derived from the experimental speed of sound. These results provide essential benchmarks for developing and improving dedicated models for these mixtures.