The turbulence-chemistry interaction model developed by Kong et al. (Kong, Eckhoff et al. 1995 [3]; Kong and Reitz 2002 [4]) has been previously applied to simulations of Diesel engines. This model is included in KINetics as an option and is available for simulating turbulence effects on combustion kinetics in transient combustion simulations. This new model is activated by passing two additional variables, iTurbulenceModel and dTurbulenceMixTime via new interfaces, KINSetSolverOptions (where iTurbulenceModel should be set to 1) and KINCalculateCellWithTurbulence.

This mixing time-scale model considers that the combustion chemistry should be partly controlled by the breakup of turbulent eddies due to the imperfect mixing of fuel and oxidizer in an actual engine process. The model assumes every species moves towards its local equilibrium values with a time scale of and thus the effective (or actual) production rate of species can be expressed as

(2–6)

where the effective time scale is related to the chemical time scale (to reach chemical equilibrium) and the turbulent scalar mixing time scale as

(2–7)

The turbulent scalar mixing time scale can be obtained from the local turbulent kinetic energy and dissipation rate:

(2–8)

The progress variable is given by

(2–9)

where denotes the mass fraction of all combustion products. Before the start of combustion, there is no product present (that is, = 0), and this model assumes that ignition in a CFD cell is entirely controlled by chemical kinetics. As the combustion process advances, more combustion products are formed and, accordingly, the influence of turbulent mixing becomes stronger.

A relationship between the effective species production rate , and the kinetic-only species production rate can be derived from Equation 2–6 as

(2–10)

where is the species mass fraction. Although the concept is the same, the implementation of this mixing time-scale model in Chemkin-CFD takes a slightly different form than that originally described by Kong et al. [3]; Kong and Reitz 2002 [4]. In the original model, updates to the species mass fractions are based on the obtained from kinetic integration, that is, from Equation 2–10, as

(2–11)

For the Ansys Chemkin-CFD/API implementation, we use the effective species production rates in kinetic integration directly as

(2–12)

so that both the gas composition and temperature will be consistent at the end of integration.

The turbulence-kinetics interaction model is invoked at the beginning of the simulation by setting the appropriate flag during the KINetics initialization call, using the KINSetSolverOptions routine, as described in the Ansys KINetics API documentation on the Ansys Developer Portal at https://developer.ansys.com/.