Computational study of flame characteristics of a turbulent piloted jet burner with inhomogeneous inlets

Recent experimental studies of a piloted turbulent jet burner at Sydney and Sandia with inhomogeneous mixture composition at the inlet have revealed that the flame stability of a partially premixed flame can be significantly increased with a tailored mixture fraction profile at the burner exit. In the present work, large-eddy simulations of this burner are performed with three different levels of mixture inhomogeneity. A multi-regime flamelet model is employed, which accounts for the range of combustion modes found in these flames. The results of the simulations are validated against experimental data for the case with the highest blowoff velocity. Good agreement is observed for velocity, mixture fraction, and temperature fields. The simulations with the multi-regime model provide detailed data of the prevalent combustion regime as well as the heat release. The location of heat release and differences in the combustion modes are analyzed for three cases with different mixture inhomogeneities. The progress variable source term is split up into the individual contributions of the combustion regimes. Substantial differences are found for the contributions of premixed and non-premixed combustion in the different flames as well as the location of the heat release. These results are used to explain the respective flame stabilities. For the inhomogeneous case, which features substantially increased flame stability, hot pilot gases are in contact with reactive mixture directly at the nozzle. Therefore a predominantly premixed zone of strong heat release develops at the jet exit and stabilizes the flame. For both other cases, the heat release at the nozzle is lower leading to smaller blowoff velocities. For the non-premixed case, this is due to air shielding the pilot from the reactive mixture. For the premixed case, the homogeneous mixture issuing from the jet is above the flammability limit and consequently the heat release is diffusion dominated.