Bistability and Transitions between Stable Operation and Thermo-Acoustic Instability in a Staged Liquid-Fuel Combustor

Abstract

Because of increasingly stringent regulations on jet engines pollutant emissions, new ways of reducing them are needed. A promising way is the use of Lean Premixed Prevaporized (LPP) combustion where the liquid fuel is vaporized and mixed with air in excess before burning. In the present work, a model gas turbine combustor fed with liquid dodecane is studied experimentally. It is equipped with two fuel injection stages: a pressurized nozzle called the pilot and a multipoint device. When the split of the fuel injection between the stages is changed, several phenomena can be observed. The flame shapes can change drastically and present different behaviors, some of them showing a strong acoustic activity. In particular, two different flame shapes can be obtained for the exact same operating conditions depending on the burner history. The first flame, named sV, is stabilized thanks to an internal reaction zone that modifies the air flow compared to non-reacting conditions. The second state, associated with a Lifted flame, exhibits a strong thermo-acoustic instability linked to the quarter-wave mode of the chamber. A switch from the sV to the Lifted state can be triggered by air flow rate modulations while the opposite change can occur naturally as the fuel split is changed. These complex phenomena originate from multiple interactions between the fuel spray, the gaseous flow and the flame itself.