Combustion dynamics of turbulent swirling flames

In the present paper the influence of a periodic excitation of the mass flow rate, varied in frequency and in amplitude, on the isothermal flow field and on the flame characteristics of swirl burners with different swirl intensities S-0,S-th is investigated. At first the fluid dynamical conditions for the formation of ring-vortex structures in the burner near flow field are determined. The results indicate that the minimum level of excitation Of the mass flow rate for vortex formation decreases hyperbolically with increasing frequency of pulsation and characteristic Strouhal number, respectively. In further studies, the dynamical behavior of lean-premixed swirl flames is investigated, whereas the fuel gas/air mixture mass flow rate at the burner exit was modulated sinusoidally. The dynamical behavior of the investigated flames is found to be dominated by two different effects in certain frequency ranges: For moderate pulsation frequencies (f(puls) less than or equal to 50 Hz) the detected periodic heat release rate of pulsated, premixed swirl flames is dominated by an effect that inhibits the strong entrainment of ambient medium in comparison with the corresponding quasi steady-state swirl flames. With increasing frequency (f(puls) greater than or equal to 50 Hz) this effect will be overlaid by the periodical formation of ring-vortices entraining additional ambient medium and interfering with the combustion process. The physical understanding of the frequency-dependent flame dynamics (flame transfer function) on periodic disturbances is indispensable for the prediction of the formation of combustion-driven oscillations that occur in technical combustion systems (e.g., gas turbines, industrial combustors). It is also evident for the development of methods to prevent or suppress periodic combustion instabilities. The results of the flame transfer measurements presented in this paper will lead to a basic understanding of the formation and reaction of large-scale coherent vortex structures in turbulent flames, that are ell known as drivers of combustion instabilities. (C) 2002 by The Combustion Institute.