The species’ heat capacity, Cp, the molar enthalpy, h, and the molar entropy, s, are computed using the polynomial format of Gordon and McBride (Ref. 1). This manual also refers to these expressions as NASA polynomials:

Here, Cp,i denotes the species’ heat capacity (SI unit: J/(mol·K)), T the temperature (SI unit: K), and Rg the ideal gas constant, 8.314 J/(mol·K). Further, hi is the species’ molar enthalpy (SI unit: J/mol), Δh is an additional enthalpy contribution (SI unit: V), F is Faraday’s constant (SI unit: C/mol), and si represents its molar entropy (SI unit: J/(mol·K)), at standard state. The additional enthalpy should include the internal energy of the species, if it is not already included, so that the enthalpy of the reaction can account for it. This is important when computing the heat source related with electron impact reactions.

In these equations, hi and si are the species’ molar enthalpy (SI unit: J/mol) and entropy (SI unit: J/(mol·K)), respectively. The heat source of the reaction (SI unit: W/m3) is given as:

where Δεj is the electron energy loss from reaction j (SI unit: V) and F is the Faraday constant (SI unit: C/mol). The last term in the equation above is only added for electron impact reactions to account for the energy loss or gain by the electron. Note that the electron enthalpy is set to zero and does not contribute to Hj. For electron impact reactions resulting in excitation and ionization Δεj corresponds to the energy of the excited state being excited/deexcited or ionized, for attachment Δεj is set to zero, and for elastic collisions

where me and mk are the electron and heavy species mass in kg, Te is the electron temperature in eV, and Tgas is the gas temperature in K.

For electron impact reactions of the type e+Ar=>e+Ar*, it is often assumed that the heavy species don’t gain or loose thermal (translational) energy (only its internal energy changes). To properly account for the reaction enthalpy, the enthalpy of the excited state (Ar* in this example) must include its internal energy. If this is not done, the energy lost or gained by the electron appears as a heat source or sink for the gas, leading to unphysical heating or cooling. For example, if the same enthalpy is used for both Ar and Ar*, an additional enthalpy contribution corresponding to the internal energy of Ar* must be added to ensure the net heat release of the reaction is zero. In a reaction such as Ar*+Ar =>Ar+Ar, the internal energy of Ar* is typically assumed to convert into translational energy, resulting in heating of the gas. In this case, the added enthalpy of Ar* directly contributes to gas heating.

The sum of Qj over all reactions is the total heat source due to volume reactions. This heat source is responsible for heating the background gas and appears as an option in the General source list in the Heat Source node in the Heat Transfer in Fluids interface called Heat source for gas when computed in the Plasma interface or Heat source for gas, period averaged when computed in the Plasma, Time Periodic interface. For the heat source computed in the Plasma, Time Periodic interface, some quantities like electron density and temperature exist along the period and are period averaged before being used in the Heat Transfer in Fluids interface.