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Thermophysical Fluid Models

Thermophysical models define how the energy, heat, and physical properties of the fluid are calculated. These variables are then determined based on the analysis type and the selected models. The models are relations of a pressure-temperature equation system using which the required fluid properties and variables are then calculated.\(^1\)

Thermophysical fluid models can be selected under Materials after a material has been selected from the library. They are only available for compressible flow simulations. Hence, convective and conjugate heat transfer simulations need to have compressibility toggled on.

thermophysical fluid models panel in simscale
Figure 1: Thermophysical fluid models can be accessed under the Materials tab after a material has been assigned. These models are only available for compressible flow simulations with or without heat transfer.

Important

The thermophysical models are currently applicable to Newtonian fluids only.

These settings are briefly described as follows:

1. Thermo type

The thermo type is distinguished by the approach used to calculate the flow thermal variables. The available types on the SimScale platform are the psiThermo and rhoThermo types that are briefly detailed below:

He psi thermo

This model is for fluids with a fixed chemical composition. It uses a variable ‘psi’, \(\psi\) that determines the compressibility as follows:

$$ \psi = \left ( RT \right )^{-1} \qquad \tag{1}$$

where \(R\) is the gas constant and \(T\) is temperature. Based on this compressibility and the changes in pressure, the density is then determined by the chosen equation of state.

Important

The He psi thermo thermo type is applicable for compressible flow analysis only

He rho thermo

The He rho thermo type is also for fluids with fixed chemical composition. This type calculates the basic thermodynamic properties based on density variation of the fluid, \(\rho\). This is used in heat transfer analyses where the density of the fluid is changing due to changes in its temperature.

Important

The He rho thermo thermo type is applicable for convective and conjugate heat transfer analyses involving compressibility effects.

2. Mixture

It specifies the fluid mixture composition. Generally, thermophysical models without chemical reactions are categorized as pure mixtures, which represents a mixture with fixed chemical composition. Currently, pureMixture is the only available mixture model and is selected by default.

3. Composition

Next, the composition of the constituents is specified. As currently, a single constituent is available, so parameter values for one constituent are required.

Amount of Substance

This is the number of moles of the component. This parameter has a default value of 1 and generally is not required to be changed.

Molar Mass

Molar mass \((M_m)\) represents the molecular weight of the component per mole in units of \(kg/kmol\) or \(lb/kmol\) and is dependent on the molecular structure of the fluid material.

4. Transport Model

The transport model relates to the calculation of the transport variables dynamic viscosity \((\mu)\), thermal conductivity \((\kappa)\), and thermal diffusivity \((\alpha)\) for energy and enthalpy equations.\(^1\) Depending on the problem, the following types of transport models are available:

Const (Constant parameters)

The Const type will assume a constant dynamic viscosity, \(\mu\) and Prandtl number, \(P_{r}= c_{p}\ \mu / \kappa\) . These parameters are then specified in the panel.

Sutherland

For the Sutherland type, the dynamic viscosity \((\mu)\) is not constant, and changes with temperature \((T)\). The dynamic viscosity is then calculated as a function of temperature given by the relation:

$$ \mu = (A_{s}\sqrt{T})/(1+T_{s}/T) \qquad \tag{2}$$

where, \(A_{s}\) is the Sutherland coefficient with SI units \(kg/(m s K^{0.5})\) and \(T_{s}\) is Sutherland temperature with units of \(K\).

5. Thermo Models

Thermo models are used to calculate the specific heat at constant pressure \((c_{p})\) for the fluid, from which the other properties are then derived. The following methods are available for the evaluation of \(c_{p}\).

hConst

This option assumes a constant value for specific heat at constant pressure, \(c_{p}\) and the heat of fusion, \(H_{f}\).

eConst

This option does not assume a constant value for \(c_{p}\). Rather, it assumes the specific heat at constant volume, \(c_{v}\) and the heat of fusion, \(H_{f}\) to be constant.

6. Equation of State

An equation of state is a thermodynamic relation describing the interconnection between various macroscopic properties of a fluid. In OpenFoam solver, it describes the relation between density \((\rho)\) of a fluid and the fluid pressure \((P)\) and temperature \((T)\).\(^1\)

Based on the thermophysical model type, the following equations of state can be used :

Rho Const

In this case, the fluid density \((\rho)\) is kept constant and does not change by pressure \((P)\) or temperature \((T)\)

$$ \rho = constant \tag{3}$$

Perfect Gas

For the case of perfect gas, the fluid is assumed to be an ideal gas and obey the ideal gas law, that is the equation of state as given by the following relation:

$$ \rho = P/(R_{specific}T) \qquad \tag{4}$$

where, \(P\) is the pressure, \(R_{specific}\) is the specific gas constant and \(T\) is the temperature.

Furthermore, the specific gas constant is given by the following equation:

$$ R_{specific} = R/M \qquad \tag{5}$$

where \(R\) is the universal gas constant, and \(M\) is the molar mass of the gas, defined under the Materials tab.

Incompressible Perfect Gas

In this case, the fluid is assumed a perfect gas that is only incompressible with respect to changes in pressure, \(P\). The equation of state is then given as:

$$ \rho = P_{ref}/(R_{specific}T) \qquad \tag{6}$$

where, \(P_{ref}\) is the reference pressure, \(R_{specific}\) is the specific gas constant and \(T\) is the temperature. The specific gas constant is calculated using equation 5.

Important

For incompressible perfect gas, the gas density \((\rho)\) can vary due to changes in temperature \((T)\) .

Perfect Fluid

In the perfect fluid case, the equation of state takes the form as given below:

$$ \rho=P/(RT) \ +\ \rho_{0} \qquad \tag{7}$$

where, \(\rho_{0}\) is the density at \(T = 0\). Then the density of the fluid can change both due to pressure and temperature.

Important

This option is recommended to model natural convection in liquids, e.g., water, due to changes in temperature.

Adiabatic Perfect Fluid

Adiabatic perfect fluid follows the following equation of state:

$$ \rho = \rho_0 \left(\frac{P+B}{P_0+B}\right)^{1/\gamma} \tag{8}$$

where \(P_0\) is the reference pressure, \(B\) is offset pressure for a stiffened fluid, and \(\gamma\) is the isentropic constant.

7. Energy

Under Energy, two options are available for the form of energy to be used in the solution. One is the sensible internal energy and the other is sensible enthalpy. The difference between absolute and sensible energy is the absence of heat of formation in the latter.

For example, absolute enthalpy \((h)\) is related to sensible enthalpy \((h_s)\) for a single specie as follows:

$$ h=h_s+c\ \Delta h_{f} \qquad \tag{9}$$

where, \(c\) is the molar fraction and \(\Delta h_{f}\) is the heat of formation.

Important

In most cases it is recommended to use sensible enthalpy, unless energy change due to reactions is expected.

Last updated: March 12th, 2021

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