Centre mode instability of a dilute particle-laden swirling jet in a swirl flow combustor

Abstract

Linear stability of a locally parallel annular swirling jet laden with particles in a swirl flow combustor is considered. At low Stokes numbers, the eigenspectra of the particle-laden jet with uniform particle concentration shows three unstable modes namely centre, sinuous and varicose modes. As the Stokes number is increased to unity, the growth rates of the centre and shear layer modes reduces compared to that of the unladen swirling jet. The magnitude of the velocity eigenmodes peaks in the vortex core and decays radially outward. The variation in particle concentration occurs mostly in the vortex core and almost none in the shear layer. The strength of flow reversal at the jet centreline is given by the backflow parameter. An increase in the backflow parameter increases the growth rate of the centre mode. Non-uniformity in the base-state particle concentration is introduced using a Gaussian function varying in the radial direction and a reduction in the growth rate of the centre mode is seen compared to the uniform particle concentration profile. When the location of the peak of the base-state particle concentration profile is inside the vortex core, the centre modes are stable. Linearized vorticity budget analysis reveals that this is accompanied by a decrease in the net generation of perturbation vorticity in the axial direction and increased radial and azimuthal perturbation vorticity.