Impact of a centrebody on the unsteady flow dynamics of a swirl nozzle: Intermittency of PVC oscillations

Gupta, S and Hemchandra, S and Shimura, M and Shanbhogue, S and Ghoniem, A (2021) Impact of a centrebody on the unsteady flow dynamics of a swirl nozzle: Intermittency of PVC oscillations. In: ASME Turbo Expo 2021: Turbomachinery Technical Conference and Exposition, GT 2021, 7 June 2021 through 11 June 2021.

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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 centrebody diameter (Dc) on the PVC in a nonreacting flow in a single nozzle swirl combustor. Time resolved high speed stereoscopic PIV (sPIV) 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 centrebody). The bulk flow velocity at the nozzle exit plane is kept constant as Ub = 8 m/s for all cases (Re ~ 20;000). The centrebody end face lies in the nozzle exit plane. A new modal decomposition technique based on wavelet filtering and proper orthogonal decomposition (POD) provides insight into flow dynamics in terms of global modes extracted from the data. The results show that without a centrebody, a coherent PVC is present in the flow as expected. The introduction of a centrebody makes the PVC oscillations intermittent. These results suggest two routes to intermittency as follows. For S =0:67, the vortex breakdown bubble (VBB) and centrebody wake recirculation zone (CWRZ) regions are nominally distinct. Intermittent separation and merger due to turbulence result in PVC oscillations due to the de-stabilization 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 centrebody. 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. © 2021 American Society of Mechanical Engineers (ASME). All rights reserved.

Item Type:	Conference Paper
Publication:	Proceedings of the ASME Turbo Expo
Publisher:	American Society of Mechanical Engineers (ASME)
Additional Information:	The copyright for this article belongs to American Society of Mechanical Engineers (ASME)
Keywords:	Excited states; Flow velocity; Hydrodynamics; Lagrange multipliers; Nozzles; Oscillating flow; Principal component analysis; Stereo image processing; Stochastic systems; Turbulent flow; Vortex flow; Vorticity; Wavelet decomposition, Flow dynamics; Flow oscillations; Intermittency; Nozzle exit plane; Oscillation state; Precessing vortex core; Self - excited; Swirl nozzles; Time-averaged; Vortex breakdown, Turbulence
Department/Centre:	Division of Mechanical Sciences > Aerospace Engineering(Formerly Aeronautical Engineering)
Date Deposited:	28 Nov 2021 10:16
Last Modified:	28 Nov 2021 10:16
URI:	http://eprints.iisc.ac.in/id/eprint/70316

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