Flame Wall Interaction
The investigation of flames that get in contact with relative cold walls is an important task of our time. This situation occurs in almost all kind of technical combustion applications, e.g. internal combustion engines and aero-engines. Due to the heat losses at the non-adiabatic walls, the chemical reactions within the flame stagnate. This leads to incomplete combustion in the vicinity of the walls, which consequently causes the formation of unburned hydrocarbons and carbon monoxide. The current tendency is towards smaller internal combustion engines, which enable higher pressure ratios and therefore, reach higher efficiencies. However this evolution increases the surface to volume ratio, which leads to growing influence of the wall on the overall combustion process. The objective of this work is the enhancement of the existing Flamelet Generated Manifold (FGM) method in order to describe the effects arising within the flame-wall interaction (FWI).
Method and Theory
In order to determine the underlying flow field the CFD-code FASTEST is used, being developed for incompressible flows and structured grids. The combustion process is modeled by the FGM method, whereby the flame is described only by its CO2 mass fraction evolution. Beside the CO2 mass fraction, the mixture fraction and the enthalpy have to be transported within the computational domain. They are required for the determination of the correct CO2 chemical source term and therefore, have an essential influence on the flame speed.
The FWI manifests basically through the decreasing levels of enthalpy, which in turn reduce the chemical reaction rates. Based on the detailed chemistry simulations, it can be concluded that the chemical source terms of the individual species are not independent of the manner in which the heat is drawn out of the flame. In order to account for these effects, an additional variable has to be transported. The identification of such variable will be done by numerical experiments that include detailed chemistry calculations. Thereby, several parameters need to be analyzed regarding their sensibility to the species source terms. Potential candidate for the additional progress variable can be any linear combination of species mass fractions, quantities of the flow field, and thermodynamic quantities.