Impact of Central Piloting on the Static and Dynamic Stability of Swirl-Stabilized Flames

Abstract

One of the key challenges of lean, low-emissions combustor operation is flame stabilization, including both static and dynamic stabilization. Static flame stability encompasses a range of issues like flame holding, flashback, and blow-off. Dynamic flame stability refers to thermoacoustic combustion oscillations, which are driven by a coupling between combustor acoustics and flame heat release rate oscillations. Pilot flames are used as a passive means of achieving both static and dynamic stability in a number of gas turbine combustor technologies, likely by acting as a source of heat and radical species at the base of the main flame. Previous work used high-speed CH* chemiluminescence imaging to characterize the effect of a central pilot flame on the macrostructure and dynamic stability of a swirled lean-premixed natural gas-air main flame. In this study, the static and dynamic stability of the main flame are controlled by modifying the equivalence ratios of the main and pilot flames to better understand the mechanisms by which pilot flames enhance both static and dynamic stability. High-speed OH planar laser-induced fluorescence (OH-PLIF) is used to capture local instantaneous dynamics of the main and pilot flames across a range of operating conditions and stability outcomes, building upon the line-of-sight chemiluminescence analysis of the previous work. We find that the presence of the pilot flame controls anchoring of a relatively lean main flame. When the pilot flame is added to an unpiloted main flame, the main flame can rapidly change stabilization location, anchoring to the centerbody of the fuel injector. When a piloted main flame has the pilot removed, the flame lingers on the centerbody for a longer duration, likely due to the high-temperature boundary condition at the centerbody anchoring point. Further, the pilot flame mitigates combustion instability for a relatively broad range of operating conditions. Analysis of high-speed OH-PLIF shows that the main and pilot flames do not directly interact, and therefore the stabilizing mechanism of the pilot flame is indirect, as previously suggested.