LES-CMC study of an excited hydrogen flame

The large eddy simulation/conditional moment closure approach has been used to study excited, attached and lifted flames in a stream of hot co-flow air. The excitation is obtained by adding a flapping forcing term with amplitude of 15% of fuel velocity and frequency corresponding to Strouhal numbers St = 0.175, 0.25, 0.325 to an inlet velocity profile. It is shown that such a type of forcing dramatically changes the global flame shape, resulting in a flame occupying very large volume or in bifurcating flames or even triple flames. We observed that the flame which is initially circular may transform downstream into a quasi-plane flame. The analysed cases include configurations with a small and large co-flow velocity. In the former case, the flame shape strongly depends on forcing frequency while in the latter substantial differences are seen only in an internal part of the flame. Both the lift-off heights of the flames and their spreading angles were found to be a function of the forcing frequency. The computational results are validated based on the solutions obtained for non-excited flames for which experimental data are available. The results obtained for various co-flow temperatures and velocities show that both auto-ignition times and lift-off heights are predicted correctly and exhibit trends observed experimentally.