Stephan Schlimpert

Thermoacoustical noise induced by laminar flame annihilation at varying flame thicknesses

The thermoacoustic response of a harmonically excited laminar lean-premixed flame is investigated with respect to artificially increased flame thicknesses. The flame dynamics is described by solutions of the Navier-Stokes equations plus a combined G-equation progress variable modeling approach based on an unstructured Cartesian method. The acoustic propagation is investigated using the acoustic perturbation equations. Strong acoustic source mechanisms caused by flame annihilation effects can be identified inducing nonlinear acoustic responses of the laminar flame front. The impact of flame thickening was found to be negligible for the used combustion model and parameter space.

Hydrodynamic instability and shear layer effects in turbulent premixed combustion

A turbulent premixed plane jet flame is analyzed by large-eddy simulations. The analysis shows that the flame front wrinkling is strongly influenced by the shear layer effect when the gas expansion effects are small leading to larger flame front amplitudes at the flame base than at high gas expansion ratios. However, the hydrodynamicinstabilityeffect induces a continuously increasing flame front amplitude which yields an enhanced flame pocket generation at the flame tip. Both phenomena influence the magnitude of the turbulent burning area and burning area rate response through the flame front deflections which are determined by the contribution coefficient. This coefficient represents the mutual interaction between the flame and the flow. At low gas expansion ratios, the total heat release rate spectra of the turbulentflame are wider in terms of dominant modes at Strouhal numbers which are linked to the mean flame height oscillations. Thus, at low gas expansion ratios, the vortex-flame interaction is less damped by the flame in the sense that vortices can perturb the flame front stronger. The total heat release rate trend of St−2.2 previously found for a round jet flame is also determined for the current slot jet at realistic gas expansion ratios indicating a general tendency to transfer energy from large to small flame structures. At high gas expansion ratios, an increasing Markstein length leads to an energy transfer between neighboring dominant modes in the low frequency range 1 < St < 10 and the burning area rate response becomes more important for the total heat release rate spectra of the turbulent slot flames which agrees with recent findings for a laminar premixed plane flame.