Complex Chemistry: Methane-Air Jet Flame
This tutorial demonstrates how to set up and solve a complex chemistry simulation to model finite-rate chemistry in a flame.
The Complex Chemistry model in Simcenter STAR-CCM+ uses a detailed mechanism and a stiff ODE solver to capture the finite-rate kinetics. As solving detailed mechanisms is computationally expensive, Simcenter STAR-CCM+ provides the options of using Clustering and ISAT to reconcile this expense. Clustering groups cells with similar thermal and chemical states before integrating the averaged state for the group. The reaction mapping is then interpolated back to the cells assuming the net reaction rate of all cells in the cluster is the same as the cluster average. Clustering usually provides a substantial speed-up as the number of clusters is less than the number of cells in the simulation. In this tutorial, you use clustering—which is activated by default. The chemistry computation with clustering is reduced to a few thousand points—independent of the number of cells in the simulation. Since a small and simple mesh is used in this tutorial, the speed-up from clustering is not substantial. However, for larger meshes in industrial simulations, the clustering performance increases with the number of cells.
In this tutorial, the CVODE solver is used with a Chemkin mechanism to calculate the reaction rates and solve the complex chemistry calculations.
The two-dimensional axisymmetric geometry consists of the main jet with nozzle diameter of 7.2 mm burning a premixture of 25% methane and 75% dry air by volume. The pilot inlet surrounds the nozzle. The pilot nozzle has an outer diameter of 18.2 mm and is composed of a burnt stoichiometric mixture of the inlet jet and air.
The setup is simplified by using a constant velocity profile at the boundary where gas exits the main and pilot jet nozzles into the domain. The constant velocity and turbulence profiles at the nozzle exits is prone to flame blow-off. You can obtain a more accurate simulation by resolving the flow in the nozzles—by including an upstream mesh, or specifying a boundary layer profile for velocity, turbulent kinetic energy, and turbulent dissipation rate at the exit of the jet nozzle.