The evolution of local instability regions in turbulent non-premixed flames

摘要

Unsteady turbulent flame evolution in non-premixed combustion has been computationally investigated using large eddy simulations. A simple coaxial combustion chamber, subjected to highly unsteady, turbulent recirculating flow is considered, following the experimental study of Owen et at. (Proc. Combrat. Iasi., vol. 16, 1976, pp. 105-117). Large-scale flame fluctuations, reported in the above experimental study, such as pulsating flames in swirling and non-swirling conditions, were identified here in our computation. New criteria for flame three-dimensional inhomogeneity arc suggested and implemented in the present study, providing the ability to quantify the flame unsteadiness. Using this technique, it is shown that local, large quenched regions develop in the flame's mixing area and rotate continuously, even when swirl is not imposed on the inlet. However, this rotation appears to he disordered, abruptly changing its direction. On the other hand, our study shows that when swirl is imposed on the inlet, a larger quenched region is identified, rotating in steady ordered rotation in the direction of the imposed swirl. In addition, large-scale radial flame fluctuations are increased downstream with the increase of swirl number. Consequently, significant correlations between radial and circumferential flame fluctuation frequencies were retrieved. Proper orthogonal decomposition analysis reveals coherent flame structures of five dominant modes that contain most of the energy in the fluctuating flame. A simplified analytical stability model is derived and implemented here to assess the hydrodynamic contribution to the flame instability; it is shown that radial fluctuations are excited by circumferential perturbations in the mixing region, providing new insight into the mechanism responsible for the onset of radial fluctuations. The computed radial flame fluctuation spectrum is predicted well using the linear stability analysis. Thus, our findings may therefore he applicable to a large class of non-premixed turbulent combustion problems.