Impact of a Centrebody on the Unsteady Flow Dynamics of A Swirl Nozzle: Intermittency of PVC Oscillations

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 non-reacting 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.

Impact of precessing vortex core dynamics on the thermoacoustic instabilities in a swirl-stabilized combustor

“…The mode also shows helical rollup along the inner shear layer. This spatial SPOD mode structure is characteristic of a PVC oscillation seen in past studies of this instability (Tammisola & Juniper 2016;Manoharan et al 2020;Datta et al 2021;Gupta et al 2021). The coherent rollup along the shear layer is due to the helical forcing imposed on the shear layers, as has been argued in recent studies (Oberleithner et al 2013;Manoharan et al 2020).…”

“…2021; Gupta et al. 2021). For the reacting results presented in this paper, SPOD results have been determined by dividing the time record of 10 000 velocity field snapshots into 38 ensembles of 512 snapshots each with 50 % overlap between ensembles.…”

“…2021; Gupta et al. 2021). The coherent rollup along the shear layer is due to the helical forcing imposed on the shear layers, as has been argued in recent studies (Oberleithner et al.…”

“…Towne et al (2018) also show rigorously that SPOD is an optimized dynamic mode decomposition (Schmid 2010) method for stationary flows. We have used the SPOD technique in several recent studies to extract spectrally resolved coherent modes corresponding to PVC oscillations (Manoharan et al 2020;Datta et al 2021;Gupta et al 2021). For the reacting results presented in this paper, SPOD results have been determined by dividing the time record of 10 000 velocity field snapshots into 38 ensembles of 512 snapshots each with 50 % overlap between ensembles.…”

Spectral analysis of OH-PLIF snapshots of a bluff body wake

“…Another application, involving chemiluminescence images, is seen in (Wickersham et al, 2014). More recently, other applications involving the continuous wavelet transform for the analysis of high-speed flame imagery are seen in the literature (Datta et al, 2021;Gupta et al, 2021;Karmarkar et al, 2021;Nakaya et al, 2021).…”