PVC and instability in swirl combustors

This paper describes the precessing vortex core (PVC) and other instabilities that are present in swirl generators/burners/combustors at high degrees of swirl to the flow and at high Reynolds number. PVC generates a characteristic sinusoidal signal of velocity and pressure and yields very high levels of regular oscillations of pressure, acoustic oscillations, in addition to the high levels of turbulence. A mathematical model is described of the PVC and instability in swirl flows under cold flow conditions by treating the flow to be 2-dimensional, incompressible, and at low Mach number. Relationships are derived for the various forces acting on the central core of the flow and also the conditions under which PVC is accelerated and hampered. Experimental data are obtained from a swirl combustor in which the degree of swirl and mode of gaseous fuel introduction could be changed. Measurements were made of the noise amplitude frequency spectra and PVC frequency under cold and hot flow conditions and the radial distribution of temperatures. Comparative data are presented for the effect of combustion upon PVC. Good agreement is found between the calculated and experimentally measured acoustic frequency of the PVC.