Self-excited instability regimes of a confined turbulent jet flame at elevated pressure

Abstract

The dynamic response of a confined, premixed turbulent jet flame is investigated at high thermal power densities (∼25 MW/m2/bar) and turbulence levels (Rejet∼ 5 ×105). As equivalence ratio and inlet jet velocity are varied at these conditions, multiple instability modes coexist: a low-amplitude instability (p′/pC≲ 9%) with longitudinal-mode fluctuations (1L and 2L) and two high-amplitude (p′/pC≲ 20%), high-frequency, transverse instability modes. While the axial modes are ubiquitous across the operational envelope, the transverse mode selection is sensitive to the equivalence ratio and reactant jet velocity. A linear stability analysis (LSA) of the confined base flow is performed to explore the flame perturbation in response to the density and temperature gradients, and the shear-layer instabilities in the flow. The high-frequency combustion instabilities are associated with a combined azimuthal hydrodynamic mode of the reactant jet, (1) at the combustion chamber near field and, (2) downstream in the fully developed region of the combustor. An excellent agreement was observed between the convectively unstable modes identified by the temporal LSA and the self-excited combustion instabilities in the experiment.