A numerical study of a lifted H 2/ N 2 flame excited by an axial and flapping forcing

Abstract

The large eddy simulation method combined with the Eulerian stochastic field approach has been used to study excited lifted hydrogen flames in a stream of hot co-flow in a configuration closely corresponding to the so-called Cabra flame. The excitation is obtained by adding to an inlet velocity profile three types of forcing [(1) axial; (2) flapping; (3) combination of both] with the amplitude of 15% of the fuel jet velocity and the frequency corresponding to the Strouhal numbers St=0.30,0.45,0.60and0.75. It is shown that such a type of forcing significantly changes the lift-off height (Lh) of the flame and its global shape, resulting in the flame occupying a large volume or the flame, which transforms from the circular one into a quasi-planar one. Both the Lh and size of the flames were found to be a function of the type of forcing and its frequency. The minimum value of Lh has been found for the case when the axial and flapping forcing were combined and acted at the forcing frequency close to the preferred one in the non-excited configuration. The impact of the flapping forcing manifested through a widening of the flame in the flapping direction. It was shown that the excitation increases the level of temperature fluctuations caused by an intensified mixing process. The computational results are validated based on the solutions obtained for the non-excited flame for which experimental data are available.