Gupta, S and Shanbhogue, S and Shimura, M and Ghoniem, A and Hemchandra, S (2022) Impact of a Centerbody on the Unsteady Flow Dynamics of a Swirl Nozzle: Intermittency of Precessing Vortex Core Oscillations. In: Journal of Engineering for Gas Turbines and Power, 144 (2).
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Abstract
The precessing vortex core (PVC) is a self-excited flow oscillation state occurring in swirl nozzles. This is caused by the presence of a marginally unstable hydrodynamic helical mode that induces precession of the vortex breakdown bubble (VBB) around the flow axis. The PVC can impact emissions and thermoacoustic stability characteristics of combustors in various ways, as several prior studies have shown. In this paper, we examine the impact of centerbody diameter (Dc) on the PVC in a nonreacting flow in a single nozzle swirl combustor. Time-resolved high-speed stereoscopic PIV measurements are performed for combinations of two swirl numbers, S = 0.67 and 1.17 and Dc = 9.5 mm, 4.73 mm, and 0 (i.e., no centerbody). The bulk flow velocity at the nozzle exit plane is kept constant as Ub = 8 m/s for all cases (Re ~ 20; 000). The centerbody end face lies in the nozzle exit plane. A new modal decomposition technique based on wavelet filtering and proper orthogonal decomposition provides insight into flow dynamics in terms of global modes extracted from the data. The results show that without a centerbody, a coherent PVC is present in the flow as expected. The introduction of a centerbody makes the PVC oscillations intermittent. These results suggest two routes to intermittency as follows. For S = 0.67, the VBB and centerbody wake recirculation zone regions are nominally distinct. Intermittent separation and merger due to turbulence result in PVC oscillations due to the destabilization of the hydrodynamic VBB precession mode of the flow. In the S = 1.17 case, the time averaged VBB position causes it to engulf the centerbody. In this case, the emergence of intermittent PVC oscillations is a result of the response of the flow to broadband stochastic forcing imposed on the time averaged vorticity field due to turbulence.
| Item Type: | Journal Article |
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| Publication: | Journal of Engineering for Gas Turbines and Power |
| Publisher: | American Society of Mechanical Engineers (ASME) |
| Additional Information: | The copyright for this article belongs to American Society of Mechanical Engineers (ASME). |
| Keywords: | Combustors; Flow velocity; Harmonic analysis; Hydrodynamics; Nozzles; Oscillating flow; Principal component analysis; Stereo image processing; Stochastic systems; Turbulence; Velocity measurement; Vortex flow, Flow dynamics; Flow oscillations; Intermittency; Nozzle exit plane; Oscillation state; Precessing vortex core; Self - excited; Swirl nozzles; Time-averaged; Vortex breakdown, Vorticity |
| Department/Centre: | Division of Mechanical Sciences > Aerospace Engineering(Formerly Aeronautical Engineering) |
| Date Deposited: | 27 Jun 2022 05:49 |
| Last Modified: | 27 Jun 2022 05:49 |
| URI: | https://eprints.iisc.ac.in/id/eprint/73903 |
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