Structure conditioned velocity statistics in a high pressure swirl flame

Abstract

A piloted, partially premixed, liquid-fueled swirl burner is operated at high pressure (1 MPa). High-speed (6 KHz) stereoscopic PIV is used to investigate the characteristics of the stagnation line separating the pilot jet and the central recirculation zone (CRZ) with varying pilot-main ratio and global equivalence ratio. The mean curvature of the stagnation line displayed a large spatial scale pattern that was present for all operating conditions. All three components of velocity, in-plane shear, and swirling strength are conditioned upon the instantaneous stagnation line. Mean distributions of the velocity normal to the stagnation line show that velocity is oriented towards the CRZ when the stagnation line is found nearer the centerline of the combustor. The conditioned out-of-plane velocity (w) shows a distinct concentration of large mean and fluctuation RMS values towards the center of the measurement domain. Varying fuel flow does not significantly change this spatial structure, only the magnitudes of the w statistics. The in-plane shear stress was the largest for the pilot biased condition as a stronger shear layer develops. For the leanest flame, large fluctuation RMS values of shear stress were confined to a region where the pilot jet begins to interact more heavily with the main jet. Operating with less pilot fuel flow enhanced the mean conditional swirling strength indicating that the pilot shear layer was shedding more intense eddies. Disregarding spatial relations, a scatter plot of w, shear stress, and swirling strength displayed trends between the variables. The largest swirling strength values coincide with highest magnitude shear stresses and the widest range of w. These conditioned statistics highlight how certain aspects of the combustor flow field are invariant with fuel distribution. This is desirable for aeropropulsive combustors that must maintain stable ignition from a range of conditions from landing/take-off to cruise.