旋流燃烧器中旋进涡核(PVC)的形成和火焰诱导抑制：实验和线性稳定性分析

The 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 consid-ering 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.