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,, 0.60, ,text {and}, ,0.75. It is shown that such a type of forcing significantly changes the lift-off height (L_h) 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 L_h and size of the flames were found to be a function of the type of forcing and its frequency. The minimum value of L_h 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.
Highlights
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
Recent LES (Large Eddy Simulations) and DNS (Direct Numerical Simulations) works of Tyliszczak A.5,6 and Gohil et al.[7] showed that for carefully selected frequencies of combined axial and helical excitation one can obtain multiarmed jets, i.e., the jets characterized by 5, 7 or 11 or even 20 separate branches, closely reminding the blooming jets reported by Reynolds and Parekh[8]
The acoustic excitation is more often used and is added by loudspeakers mounted upstream of the nozzle exits[13–17]. The influence of this type of excitation on the reduction of pollution emissions in a lean premixed lifted flames and flame stability was demonstrated by Chao et al.[13,14], among others. They found that the excitation significantly alters the flame dynamics and can be used as a “tool” suppressing or amplifying the stabilization process
Summary
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 (forcing) is introduced as a component of the velocity prescribed at the inlet as u(x, t) = umean(x) + uturb(x, t) + uexcit (x, t) , where umean(x) is the mean velocity profile corresponding to the fully developed pipe flow (1/7 profile) and uturb(x, t) = 0.05Uj represents turbulent fluctuations computed applying a digital filtering method proposed by Klein et al.[32].
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