When modeling chemical non-equilibrium, it is convenient to introduce a chemical Damkohler number, Da, defined as the ratio of characteristic flow time to chemistry time. If Da is close to zero, the chemical reactions are negligible, and the flow is chemically frozen. If Da is very large, the chemical reactions take place much faster than the flow speed, and the flow is in chemical equilibrium. For hypersonic flows, the value of Da is often in the order of unity, and therefore the flow is in locally chemical non-equilibrium. Chemical reactions can take place in the shock layer due to the high temperature. For example, for air at 1-atmosphere pressure, the oxygen starts to dissociate at 2000K and completes at 4000K. The nitrogen begins to dissociate at 4000K and finishes at 9000K. At higher temperatures, ions and free electrons can appear due to ionization reactions. The prediction of gas composition is important since it has a large impact on transport properties. Finite Rate Chemistry (Modeling Species Transport and Finite-Rate Chemistry) is suitable for modeling this flow. The Stiff or the None - Direct Source chemistry solver can be selected for the finite rate chemistry model. For the density-based solver, the None - Direct Source solver is recommended due to the strong coupling between the chemistry source terms and species mass equations.
As a prerequisite to using the finite rate chemistry solver, you must first define a set of species and the relevant chemical mechanism. To facilitate the setup of the case, Fluent offers the following predefined mixture materials:
For Earth atmospheric flight or entry:
At the lower range of hypersonic flight and when the flow temperature is below 10,000 K, it is sufficient to use the following air mixtures:
air-2species-nitrogenis a mixture of N2 and N, with Nitrogen dissociation reactions. This mixture is typically used in experiments where the working fluid is pure Nitrogen.air-5species-park93is a mixture of N2, O2, NO, N and O, with the Park 93 chemical mechanisms [113].
When the flow temperature exceeds 10,000K, the flow can be weakly ionized. When ions and free electrons are defined in the mixture, Fluent can account for ionization by modeling the vibrational-electronic energy change due to ionizations and offers transport property methods suitable for ionized flows (for details, see The Two-Temperature Model for Hypersonic Flows in the Fluent Theory Guide). Therefore, it is recommended to import one of the following mixtures:
For Martian and Venus atmospheric entry:
mars-5species-mckenzieis a mixture of CO, O2, C, O and CO2, with the Mckenzie chemical mechanisms.mars-8species-parkis a mixture of CO, O2, C, O, CO2, N2, N and NO, with the Park chemical mechanisms.mars-venus-16species-johnstonis a mixture of CO2, CO, N2, O2, NO, C, N, O, CN, C2, C+, O+, NO+, O2+, CO+, and e- with the Johnston chemical mechanisms [73]. Note thatmars-venus-16species-johnstonshould be used if ionizations are resolved.
For Titan atmospheric entry:
titan-13species-gokcenis a mixture of CH4, CH3, CH2, CH, C2, H2, CN, NH, HCN, N, C, H, and N2 with the Gokcen chemical mechanisms [53].titan-21species-gokcenis a mixture of N2, CH4, CH3, CH2, CH, C2, H2, CN, NH, HCN, N, C, H, Ar, N2+, CN+, N+, C+, H+, Ar+, and e- with the Gokcen chemical mechanisms [53]. Note thattitan-21species-gokcenshould be used if ionizations are resolved.
These mixtures can be utilized for one-temperature and two-temperature models. When the
one-temperature model is selected, the default property method for air mixtures is
gupta-curve-fit, while for other mixtures, it is
kinetic-theory. When the two-temperature model is chosen, the
default property method for all mixtures is blottner-curve-fit and
eucken-relation.