NASA7IdealGasFluidProperties

Properties

The equation of state is that of an ideal gas:

where is pressure, is specific volume, is temperature, is the universal gas constant, and is the molar mass of the species. With the dimensionless standard-state heat capacity , enthalpy , and entropy from the polynomials, the specific enthalpy, internal energy, entropy, and heat capacities are

where Pa is the reference pressure of the data. The ratio of specific heats is , and the sound speed is .

The enthalpy includes the enthalpy of formation of the species, and the entropy is the absolute entropy. The enthalpies of different species therefore share a common reference, so a mixture energy balance accounts for the heat of reaction when the composition changes by reaction.

The derivatives of the properties are computed by forward-mode automatic differentiation of the expressions above, so they are exact.

This class provides the properties from pressure and temperature, from temperature and specific volume, from specific volume and internal energy, and from pressure and enthalpy, as well as the pressure from enthalpy and entropy and the internal energy from pressure and density and from specific volume and enthalpy. It does not provide viscosity or thermal conductivity, so it cannot yet be used in flow models that need them, such as the gas-mixture flow channels of the thermal hydraulics module. Mixtures of these gases are described by IdealGasMixtureFluidProperties.

Comparison with IdealGasFluidProperties

Both classes describe ideal gases, but they differ as follows:

Temperature from internal energy or enthalpy

The properties from specific volume and internal energy, and those from enthalpy, require the temperature at which the internal energy or the enthalpy takes a given value. The temperature is found with the 1D Newton solver of the fluid properties utilities, whose relative tolerance and maximum number of iterations are set with "tolerance" and "max_newton_its". Two choices make this inversion robust:

• The iteration starts at the upper end of the temperature range of the data, so this class has no initial-guess parameters. The internal energy and the enthalpy are convex functions of temperature over nearly all of that range, because the heat capacity increases with temperature, so the iterates approach the solution from above. An iteration started below the solution can overshoot far beyond the range, where the extrapolated polynomials eventually make the energy decrease with temperature and produce a second, spurious solution. For the built-in species, the solution is found up to at least 1.7 times the upper end of the range.

• The internal energy and the enthalpy include the enthalpy of formation, so they can be zero: the internal energy of N, for example, vanishes near 417 K. The Newton solver tests convergence relative to the target value, which close to zero would demand accuracy below roundoff. The equation is therefore offset by a constant that makes both of its sides close to (or ), which turns the test into one on the relative error in temperature.

At the midpoint temperature of the data, where the two polynomials do not match exactly, the same internal energy or enthalpy is also reached slightly above the midpoint temperature, by less than K for the built-in species. The inversion may return either temperature.

Temperature range

The polynomials are fitted from 200 K to 3500 K, except for N, whose fit extends from 300 K to 5000 K. Outside its range, the polynomials of a species are still evaluated, and each such evaluation is counted as a solution warning, which appears in the solution invalidity summary. For example, N at room temperature lies slightly below its range. A non-positive or not-a-number temperature is flagged as an invalid solution.

Input Parameters

References