Time Resolved Flowfield, Flame Structure and Acoustic Characterization of a Staged Multi-Injection Burner

International audienceThe present work details the analysis of staging and multipoint injection effects on the flow field, flame structure and acoustic properties of a lean swirled injector, representative of a gas turbine burner. The study is carried out for a constant power and global equivalence ratio while the staging parameter alpha between pilot and multipoint stages is varied. Using PLIF-OH, it is shown that the flame structure consists in a large central recirculation zone of burnt gases surrounded by an annular conical jet of fresh gases. The OH spatial distribution indicates that increasing alpha significantly modifies the flame structure, which becomes more compact and more organized while the mean flame front angle increases. Using High Frequency PIV in the reactive situation, a temporal analysis of the velocity field is carried out, indicating that large coherent structures appear periodically within the combustion chamber. The structure frequencies are determined and compared with acoustic measurements. This comparison indicates that for alpha = 0%, the vortex frequency is slightly lower than the first eigenmode of the chamber, while for alpha = 30%, it corresponds to a secondary peak at a higher frequency of the spectrum for both p′ and q′, with dramatic consequences on the burner behavior in terms of stability. When alpha increases starting from zero, the level of p′ and q′ increases to reach a maximum value for alpha = 20% before decreasing for higher staging. This acoustic behavior can be compared with the evolution of the temperature in the injector, that also reaches a maximum value for alpha = 20%, proving that strong instabilities are associated with flame stabilization within the injector. Increasing the staging factor makes it possible to decrease the flame instability level while keeping the flame compact and robust