5.2 Thermophysical models
Thermophysical models are used to describe cases where the thermal energy, compressibility or mass transfer is important.
OpenFOAM allows thermophysical properties to be constant, or functions of temperature, pressure and composition. Thermal energy can be described in form of enthalpy or internal energy. The relation can be described with various equations of state or as isobaric system.
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 further layers for the thermodynamic, transport and mixture modelling, as listed in Table 5.1.
Various combinations are available as ‘packages’, specified using, e.g.
20 type heRhoThermo;
21 mixture pureMixture;
22 transport const;
23 thermo hConst;
24 equationOfState perfectGas;
25 specie specie;
26 energy sensibleEnthalpy;
33 molWeight 28.96;
37 Cp 1004.4;
38 Hf 0;
42 mu 1.831e-05;
43 Pr 0.705;
48// ************************************************************************* //
Only certain combinations are predefined. One method to identify the possible combinations from Table 5.1 is to use a nonexistent setting for one of the entries, e.g.banana and execute the solver. OpenFOAM will issue an error message and list all possible combinations to the terminal.
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 transport 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 5.2. 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
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);