Thermodynamic properties of air and mixtures of nitrogen, argon, and oxygen from 60 to 2000 K at pressures to 2000 MPa

A thermodynamic property formulation for standard dry air based upon availa ble experimental p-rho-T, heat capacity, speed of sound, and vapor-liquid e quilibrium data is presented. This formulation is valid for liquid, vapor, and supercritical air at temperatures from the solidification point on the bubble-point curve (59.75 K) to 2000 K at pressures up to 2000 MPa. In the absence of reliable experimental data for air above 873 K and 70 MPa, air p roperties were predicted from nitrogen data in this region. These values we re included in the determination of the formulation to extend the range of validity. Experimental shock tube measurements on air give an indication of the extrapolation behavior of the equation of state up to temperatures and pressures of 5000 K and 28 GPa. The available measurements of thermodynami c properties of air are summarized and analyzed. Separate ancillary equatio ns for the calculation of dew and bubble-point pressures and densities of a ir are presented. In the range from the solidification point to 873 Kat pre ssures to 70 MPa, the estimated uncertainty of density values calculated wi th the equation of state is 0.1%. The estimated uncertainty of calculated s peed of sound values is 0.2% and that for calculated heat capacities is 1%. At temperatures above 873 K and 70 MPa, the estimated uncertainty of calcu lated density values is 0.5% increasing to 1.0% at 2000 K and 2000 MPa. In addition to the equation of state for standard air, a mixture model explici t in Helmholtz energy has been developed which is capable of calculating th e thermodynamic properties of mixtures containing nitrogen, argon, and oxyg en. This model is Valid for temperatures from the solidification point on t he bubble-point curve to 1000 K at pressures up to 100 MPa over all composi tions. The, Helmholtz energy of the mixture is the sum of the ideal gas con tribution, the real gas contribution, and the contribution from mixing. The contribution from mixing is given by a single generalized equation which i s applied to all mixtures used in this work. The independent variables are the reduced density and reduced temperature. The model may be used to calcu late the thermodynamic properties of mixtures at various compositions inclu ding dew and bubble-point properties and critical points. It incorporates t he most accurate published equation of state for each pure fluid. The mixtu re model may be used to calculate the properties of mixtures generally with in the experimental accuracies of the available measured properties. The es timated uncertainty of calculated properties is 0.1% in density, 0.2% in th e speed of sound, and 1% in heat capacities. Calculated dew and bubble-poin t pressures are generally accurate to within 1%. (C) 2000 American Institut e of Physics. [S0047-2689(00)00103-3].