Thermophysical models are concerned with the energy, heat and physical properties.
The thermophysicalProperties dictionary is read by any solver that uses the thermophysical model library. A thermophysical model is constructed in OpenFOAM as a pressure-temperature system from which other properties are computed. There is one compulsory dictionary entry called thermoType which specifies the complete thermophysical model that is used in the simulation. The thermophysical modelling starts with a layer that defines the basic equation of state and then adds more layers of modelling that derive properties from the previous layer(s). The naming of the thermoType reflects these multiple layers of modelling as listed in Table 7.1.
The following is an example entry for thermoType:
The keyword entries specify the choice of thermophysical models, e.g. constant transport (constant viscosity, thermal diffusion), Perfect Gas equationOfState, etc. In addition there is a keyword entry named energy that allows the user to specify the form of energy to be used in the solution and thermodynamics. The energy can be internal energy or enthalpy and in forms that include the heat of formation or not. We refer to absolute energy where heat of formation is included, and sensible energy where it is not. For example absolute enthalpy is related to sensible enthalpy by
where and are the molar fraction and heat of formation, respectively, of specie . In most cases, we use the sensible form of energy, for which it is easier to account for energy change due to reactions. Keyword entries for energy therefore include e.g. sensibleEnthalpy, sensibleInternalEnergy and absoluteEnthalpy.
The basic thermophysical properties are specified for each species from input data. Data entries must contain the name of the specie as the keyword, e.g. O2, H2O, mixture, followed by sub-dictionaries of coefficients, including:
- containing i.e. number of moles, nMoles, of the specie, and molecular weight, molWeight in units of g/mol;
- containing coefficients for the chosen thermodynamic model (see below);
- containing coefficients for the chosen tranpsort model (see below).
The thermodynamic coefficients are ostensibly concerned with evaluating the specific heat from which other properties are derived. The current thermo models are described as follows:
- assumes a constant and a heat of fusion which is simply specified by a two values , given by keywords Cp and Hf.
- assumes a constant and a heat of fusion which is simply specified by a two values , given by keywords Cv and Hf.
- calculates as a function of temperature from a set of coefficients taken from JANAF tables of thermodynamics. The ordered list of coefficients is given in Table 7.2. The function is valid between a lower and upper limit in temperature and respectively. Two sets of coefficients are specified, the first set for temperatures above a common temperature (and below , the second for temperatures below (and above ). The function relating to temperature is:
In addition, there are constants of integration, and , both at high and low temperature, used to evaluating and respectively.
- calculates as a function of temperature by a polynomial of any order. The following case provides an example of its use: $FOAM_TUTORIALS/lagrangian/porousExplicitSourceReactingParcelFoam/filter
The transport coefficients are used to to evaluate dynamic viscosity , thermal conductivity and laminar thermal conductivity (for enthalpy equation) . The current transport models are described as follows:
- assumes a constant and Prandtl number which is simply specified by a two keywords, mu and Pr, respectively.
- calculates as a function of temperature from a Sutherland coefficient and Sutherland temperature , specified by keywords As and Ts; is calculated according to:
- calculates and as a function of temperature from a polynomial of any order.
The following is an example entry for a specie named fuel modelled using sutherlandTransport and janafThermo:
highCpCoeffs (1.63543 0.0100844 -3.36924e-06 5.34973e-10
-3.15528e-14 -10005.6 9.9937);
lowCpCoeffs (5.14988 -0.013671 4.91801e-05 -4.84744e-08
1.66694e-11 -10246.6 -4.64132);
The following is an example entry for a specie named air modelled using constTransport and hConstThermo: