Periodic combustion instabilities in a swirl burner studied by phase-locked planar laser-induced fluorescence

Quasi-simultaneous phase-correlated measurements of different species in a turbulent swirl flame with a self-excited instability are presented for the first time. Phase-resolved OH* chemiluminescence and planar laser-induced fluorescence (PLIF) spectroscopy of OH, CH, and H2CO were used to follow the temporal evolution of flame structures in a pulsating swirl-stabilized model injector for gas-turbine applications. H2CO is a suitable indicator for chemical heat release in combination with OH; CH LIF and OH* emission were shown to be suitable indicators for the average shape and location of flame fronts, while OH LIF marked regions with high temperature, both in the flame front and the burned gas regions. The combustor was operated on methane fuel at atmospheric pressure. Lasers and detectors were locked to the phase angle of the self-excited pressure oscillation using a trigger signal derived from a microphone. Measurements at different phase angles were performed by variable delays with respect to the trigger pulse. Due to a high degree of turbulence, a large number of single-pulse measurements at each phase angle had to be performed in order to retrieve phase-sensitive effects from the dominating turbulent fluctuations. Noticeable changes of the flame structure with phase angle, particularly near the injector exit, are indicative of a strong coupling between the flame and a periodically fluctuating flow field.