### 1.4.4. Specific Heat Capacity

The Specific Heat Capacity at constant pressure determines the amount of heat energy required to raise the temperature of a fixed mass of the fluid by 1 K. You can model Specific Heat Capacity using these options:

- By directly setting specific heat.

- Using the built in NASA Format polynomials.

- Using the built in Zero Pressure Polynomial.

- Using the Real Gas option if a real gas equation of state has been selected. For details, see Real Fluid Properties.

- By reading properties from a TASCflow RGP table. For details, see Real Fluid Properties.

#### 1.4.4.1. Value

This is the most flexible option for specific heat capacity. With this option you can set specific heat to:

- A constant (Calorically Perfect)

- Varying with temperature

- Varying with temperature and/or pressure

#### 1.4.4.2. NASA Format

This option defines specific heat as a function of temperature, in two distinct ranges, as a quartic polynomial and is available only when the equation of state is set to Ideal Gas or Real Gas. The format for the polynomial coefficients is also used by CHEMKIN [26]

The Temperature Limit frame expects three temperatures that define two temperature ranges for the upper and lower interval coefficients. In each interval, the same form for the polynomial is used but different coefficients can be used.

The Upper and Lower Interval coefficients are directly entered in their individual frames and correspond to the upper and lower range coefficients in the standard NASA SP-273 format. There are seven coefficients in each interval for a total of 14 coefficients. When entering these coefficients, note that the NASA format states that upper range coefficients are listed first, and lower range coefficients are listed second.

Also note that the format has changed slightly in order to accommodate proper units checking. In CFX the format followed the NASA Format to the letter: the first 7 are for the upper temperature interval, the last 7 for the lower temperature interval. In order to convert your CFX materials, you should need to load only your existing materials file into CFX-Pre and then write out the materials to a new materials file.

For each interval, the coefficients define the thermodynamic properties of the material according to the following formulae for specific heat capacity at constant pressure,

| (1–25)

specific static enthalpy,

| (1–26)

and specific static entropy,

| (1–27)

#### 1.4.4.3. Zero Pressure Polynomial

This option is available for both Ideal Gas equation of state or when the equation state is given by a function of temperature and pressure (Option = Value with a CEL or User Fortran function). In either case the flow solver will build a table of values for enthalpy and entropy. In the former case the enthalpy will be a function only of temperature, and in the latter it will be a function of both temperature and pressure.

For this option, the specific heat is expressed in polynomial form:

| (1–28)

where is the specific gas constant.

The coefficients for are entered on the Material Properties form in CFX-Pre. The minimum and maximum temperature limits can also be supplied and define the range of applicability of the polynomial expression.

The maximum temperature limits for zero pressure polynomials are consistent in the RULES file, CFX-Pre, and the CFX-Solver (= 1000 K). Temperature limits for table generation are set by CFX-Pre: 5000 K for ideal gases, 1000 K for real gases.

#### 1.4.4.4. Real Gas

This option is available only for materials that use a Real Gas equation of state. Ideal gas coefficients must be supplied in the same way as for Zero Pressure Polynomial or NASA Format. Whereas the `Zero Pressure` or `NASA Format` options allow only to be a function of temperature, the `Real Gas` option takes the same coefficients as input to predict as a function of both temperature and pressure.

#### 1.4.4.5. Reference State Properties

##### 1.4.4.5.1. Reference Temperature and Reference Pressure

When defining specific heat capacity, you must set a reference temperature and reference pressure. These quantities are used only to define a reference level from which some derived quantities, such as a change in enthalpy or entropy, are calculated. The defaults for these values are 1 atm and 25°C. However, you should set these values to values that are representative of the average temperature and pressure that will occur in your model. You should also consider table limits as well in that it is better to choose reference values that fall in the valid range for table property generation.

##### 1.4.4.5.2. Reference Specific Enthalpy and Entropy

These values are available for input if the specific heat capacity is not given by NASA coefficients. When NASA coefficients are used, the Reference Specific Enthalpy and Reference Specific Entropy are ignored by the CFX-Solver because they are built into the expressions for enthalpy and entropy through the upper and lower interval coefficient “constant” values.

The reference specific enthalpy is the enthalpy of formation at the specified Reference Pressure and Reference Temperature (often 1 atm, 25°C). The reference specific entropy is also evaluated at the specified reference pressure and temperature.

The reference enthalpy must be accurately set for simulations involving phase change (cavitation, evaporation and so on) or chemical reactions (for example, combustion). For details, see Latent Heat.

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