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Combustion instability is a serious problem limiting the operating envelope of present day gas turbine systems using a lean premixed combustion strategy. Gas turbine combustors employ swirl as a means for achieving fuel-air mixing as well as flame stabilization. However swirl flows are complex flows comprised of multiple shear layers as well as recirculation zones which makes them particularly susceptible to hydrodynamic instability. We perform a local stability analysis on a family of base flow model profiles characteristic of swirling flow that has undergone vortex breakdown as would be the case in a gas turbine combustor. A temporal analysis at azimuthal wavenumbers m = 0 and m = 1 reveals the presence of two unstable modes. A companion spatio-temporal analysis shows that the region in base flow parameter space for constant density density flow, over which m = 1 mode with the lower oscillation frequency is absolutely unstable, is much larger that that for the corresponding m = 0 mode. This suggests that the dominant self-excited unstable behavior in a constant density flow is an asymmetric, m=1 mode. The presence of a density gradient within the inner shear layer of the flow profile causes the absolutely unstable region for the m = 1 to shrink which suggests a possible explanation for the suppression of the precessing vortex core in the presence of a flame.

Combustion instability, the coupling between flame heat release rate oscillations and combustor acoustics, is a significant issue in the operation of gas turbine combustors. This coupling is often driven by oscillations in the flow field. Shear layer roll-up, in particular, has been shown to drive longitudinal combustion instability in a number of systems, including both laboratory and industrial combustors. One method for suppressing combustion instability would be to suppress the receptivity of the shear layer to acoustic oscillations, severing the coupling mechanism between the acoustics and the flame. Previous work suggested that the existence of a precessing vortex core (PVC) may suppress the receptivity of the shear layer, and the goal of this study is to first, confirm that this suppression is occurring, and second, understand the mechanism by which the PVC suppresses the shear layer receptivity. In this paper, we couple experiment with linear stability analysis to determine whether a PVC can suppress shear layer receptivity to longitudinal acoustic modes in a nonreacting swirling flow at a range of swirl numbers. The shear layer response to the longitudinal acoustic forcing manifests as an m = 0 mode since the acoustic field is axisymmetric. The PVC has been shown both in experiment and linear stability analysis to have m = 1 and m = −1 modal content. By comparing the relative magnitude of the m = 0 and m = −1,1 modes, we quantify the impact that the PVC has on the shear layer response. The mechanism for shear layer response is determined using companion forced response analysis, where the shear layer disturbance growth rates mirror the experimental results. Differences in shear layer thickness and azimuthal velocity profiles drive the suppression of the shear layer receptivity to acoustic forcing.

Hydrodynamic instabilities of the flow field in lean premixed gas turbine combustors can generate velocity perturbations that wrinkle and distort the flame sheet over length scales that are smaller than the flame length. The resultant heat release oscillations can then potentially result in combustion instability. Thus, it is essential to understand the hydrodynamic instability characteristics of the combustor flow field in order to understand its overall influence on combustion instability characteristics. To this end, this paper elucidates the role of fluctuating vorticity production from a linear hydrodynamic stability analysis as the key mechanism promoting absolute/convective instability transitions in shear layers occurring in the flow behind a backward facing step. These results are obtained within the framework of an inviscid, incompressible, local temporal and spatio-temporal stability analysis. Vorticity fluctuations in this limit result from interaction between two competing mechanisms — (1) production from interaction between velocity perturbations and the base flow vorticity gradient and (2) baroclinic torque in the presence of base flow density gradients. This interaction has a significant effect on hydrodynamic instability characteristics when the base flow density and velocity gradients are co-located. Regions in the space of parameters characterizing the base flow velocity profile, i.e. shear layer thickness and ratio of forward to reverse flow velocity, corresponding to convective and absolute instability are identified. The implications of the present results on prior observations of flow instability in other flows such as heated jets and bluff-body stabilized flames is discussed.

Kiran M an o h aranD ep a rtm e n t o f A erosp a ce E n g in e erin g, In d ia n In s titu te o f S cience, B an ga lore 5 6 0 0 1 2 , India e -m a il: k ira n m @ a e ro .iis c .e rn e t.in Santosh H em chandra D epartm ent o f A e rospace E n g in e erin g, In d ia n In s titu te o f Science, B a n ga lore 5 6 0 0 1 2 , In d ia e -m a il: h san to s h @ a e ro .iis c .e rn e t.in Absolute/Convective Instability Transition in a Backward Facing Step Combustor: Fundamental Mechanism and Influence of Density GradientHydrodynamic instabilities of the flow field in lean premixed gas turbine combustors can generate velocity perturbations that wrinkle and distort the flame sheet over length scales that are smaller than the flame length. The resultant heat release oscillations can then potentially result in combustion instability. Thus, it is essential to understand the hydrodynamic instability characteristics of the combustor flow field in order to understand its overall influence on combustion instability characteristics. To this end, this paper eluci dates the role of fluctuating vorticity production from a linear hydrodynamic stability analysis as the key mechanism promoting absolute!convective instability transitions in shear layers occurring in the flow behind a backward facing step. These results are obtained within the framework of an inviscid, incompressible, local temporal and spatiotemporal stability analysis. (2) baroclinic torque in the pres ence of base flow density gradients. This interaction has a significant effect on hydrody namic instability characteristics when the base flow density and velocity gradients are colocated. Regions in the space of parameters characterizing the base flow velocity pro file, i.e., shear layer thickness and ratio of forward to reverse flow velocity, correspond ing to convective and absolute instability are identified. The implications of the present results on understanding prior experimental studies of combustion instability in back wardfacing step combustors and hydrodynamic instability in other flows such as heated jets and bluff body stabilized flames is discussed. Journal of Engineering for Gas Turbines and Power FEBRUARY 2 0 1 5 , Vol. 1 3 7 / 021501-3 F ig . 1 0 T ra n s itio n b o u n d a r y b e tw e e n a b s o lu te ly a n d c o n v e ctiv e ly u n s ta b le f lo w s ( r = 1, 5W = 0 .1 ). T h e a r ro w s m a rk e d A U a n d C U , re s p e c tiv e ly , p o in t in to r e g io n s o f a b s o lu te ly a n d c o n v e c tiv e ly u n s ta b le flo w . Vorticity fluctuations in this limit result from interaction between two competing mechanisms-(1) production from interaction between velocity perturbations and the base flow vorticity gradient and