Role of turbulence for mixing and soot oxidation for an equivalent diesel gas jet during wall interaction studied with LES.

Paper in proceeding, 2012

Using Large Eddy Simulations (LES), turbulent properties were studied of an equivalent diesel fuel gas jet having a vapour phase penetration equal as for a diesel spray. Half side of the jet was forced to sharply change direction by impinging upon a curved wall. The other side of the jet developed freely acting as a reference case. It was shown how turbulent structures of various scales develop along the jet and after wall impingement. The turbulent eddies contribute strongly to air entrainment into the jet resulting in an increasing rate of dilution of the jet core downstream of the fuel nozzle.

Calculations of the instantaneous fuel concentration were used to study the total surface area and the kinetic energy of the wrinkled stoichiometric zones. Both the resolved kinetic energy and the sub-grid scale turbulent kinetic energy of the stoichiometric zones were found to increase on the wall side. Statistics of turbulent quantities were computed across interesting sections showing relations between velocity gradients, turbulent velocity field, turbulence production and dissipation. 

The findings were used for a reasoning about the role of turbulence for soot oxidation in a real diesel flame.  Previous observations in a high temperature, high  pressure combustion chamber experiment that turbulent eddies sweep fresh gas into the free jet core were confirmed by the simulations. As observed in the experiments, the LES results indicate that also non-fresh gases, especially on the wall side, may be swept into the jet side. As a result, soot oxidation rate can either increase if oxidant radicals are supplied by the turbulent flow or decrease if the in-rushing gases mainly consist of inert combustion products.