Formation and flame-induced suppression of the precessing vortex core in a swirl combustor: Experiments and linear stability analysis

Abstract

a b s t r a c tThe precessing vortex core (PVC) is a coherent flow structure that is often encountered in swirling flows in gas turbine (GT) combustors. In some swirl combustors, it has been observed that a PVC is present under non-reacting conditions but disappears in the corresponding reacting cases. Since numerous studies have shown that a PVC has strong effects on the flame stabilization, it is desirable to understand the formation and suppression of PVCs in GT combustors. The present work experimentally studies the flow field in a GT model combustor at atmospheric pressure. Whereas all non-reacting conditions and detached M-shaped flames exhibit a PVC, the PVC is suppressed for attached V-shaped flames. A local linear stability analysis is then applied to the measured time-averaged velocity and density fields. For the cases where a PVC appeared in the experiment, the analysis shows a global hydrodynamic instability that manifests in a single-helical mode with its wavemaker located at the combustor inlet. The frequency of the global mode is in excellent agreement with the measured oscillation frequency and the growth rate is approximately zero, indicating the marginally stable limit-cycle. For the attached V-flame without PVC, strong radial density/temperature gradients are present at the inlet, which are shown to suppress the global instability. The interplay between the PVC and the flame is further investigated by considering a bi-stable case with intermittent transitions between V-and M-flame. The flame and flow transients are investigated experimentally via simultaneous highspeed PIV and OH-PLIF. The experiments reveal a sequence of events wherein the PVC forms prior to the transition of the flame shape. The results demonstrate the essential role of the PVC in the flame stabilization, and thereby the importance of a hydrodynamic stability analysis in the design of a swirl combustor.

Coherent structures in a swirl injector at Re = 4800 by nonlinear simulations and linear global modes

“…It has also performed well in swirling flow studies using local spatio-temporal techniques in injector flows (Oberleithner et al 2015). Finally, Camarri et al (2013) obtained a good agreement with the experimental structural sensitivity region for a porous cylinder flow using only molecular viscosity (the quasi-laminar approach), and similarly Mettot et al (2014b) for a D-shaped cylinder.…”

“…Similar injector flows (Juniper 2012;Oberleithner et al 2015) have been studied by local stability analysis in the combustion chamber region, excluding the nozzle region. For the precessing vortex core, a region of absolute instability was identified very close to the chamber inlet.…”

“…Firstly, we note that different eddy viscosity models extracted from experimental data were tried in a local absolute instability analysis of a similar injector flow (Oberleithner et al 2015). All eddy viscosity models resulted in a nearly neutral growth rate.…”

“…To approximate the effect of turbulent dissipation and its modeling on the linear global modes in the next section, we have extracted an approximate eddy viscosity distribution from our DNS data using the simple Newtonian eddy model, as very recently used by Oberleithner et al (2015), and also suggested by Mettot et al (2014b). The idea is based on a triple decomposition of the turbulent flow field:…”

“…The mean flow stability was observed to produce good results in all regions where harmonics had low amplitudes, and less good results in the regions where harmonics had large amplitudes (Oberleithner et al 2014b). Finally, the effect of eddy viscosity models was considered in Oberleithner et al (2015). While eddy viscosity models did not change the absolute frequencies, they influenced the absolute growth rates, and by doing this, could alter the streamwise location where the global mode frequency was selected.…”