Spatiotemporal Characterization of a Conical Swirler Flow Field Under Strong Forcing

Abstract

In this study, a spatiotemporal characterization of forced and unforced flows of a conical swirler is performed based on particle image velocimetry (PIV) and laser Doppler anemometry (LDA). The measurements are performed at a Reynolds number of 33,000 and a swirl number of 0.71. Axisymmetric forcing is applied to approximate the effects of thermoacoustic instabilities on the flow field at the burner inlet and outlet. The actuation frequencies are set at the natural flow frequency (Strouhal number Stf≈0.92) and two higher frequencies (Stf≈1.3 and 1.55) that are not harmonically related to the natural frequency. Phase-averaged measurement are used as a first step to visualize the coherent flow structures. Second, proper orthogonal decomposition (POD) is applied to the PIV data to characterize the effect of the actuation on the fluctuating flow. Measurements indicate a typical natural flow instability of helical nature in the unforced case. The associated induced pressure and flow oscillations travel upstream to the swirler inlet where generally fuel is injected. This observation is of critical importance with respect to the stability of the combustion. Harmonic actuation at different frequencies and amplitudes does not affect the mean velocity profile at the outlet, while the coherent velocity fluctuations are strongly influenced at both the inlet and outlet. On one hand, the dominant helical mode is replaced by an axisymmetric vortex ring if the flow is forced at the natural flow frequency. On the other hand, the natural flow frequency prevails at the outlet under forcing at higher frequencies and POD analysis indicates that the helical structure is still present. The presented results give new insight into the flow dynamics of a swirling flow burner under strong forcing.

Dynamics of Swirling Flames

“…It is known that acoustic instabilities coupled by longitudinal modes change the dynamics of the swirling flow and that a longitudinal acoustic forcing of a rotating cold stream reduces the PVC strength, as demonstrated by Lacarelle et al (2009), but the PVC does not completely vanish in these circumstances, as confirmed numerically by Iudiciani & Duwig (2011). Some recent experiments Vorticity wave: one mode of oscillation supported by the Euler equations; the vorticity mode is convected by the flow and features velocity disturbances in a direction perpendicular to that of the flow Combustion instability: broad range of processes giving rise to oscillations in heat release and resonant coupling between combustion and acoustics and resulting in sound emission, structural vibrations, intensified heat fluxes to the walls, and in extreme cases failure of the system have revealed nonlinear interactions between acoustic instabilities and the PVC , Steinberg et al 2010, confirming that the PVC can be suppressed at highacoustic oscillation amplitudes and that, at intermediate acoustic levels, the velocity signal exhibits three components associated with the PVC: the external forcing and the difference in frequency of the previous two.…”

Effect of High-Amplitude Forcing on Turbulent Combustion Intensity and Vortex Core Precession in a Strongly Swirling Lifted Propane/Air Flame

“…It is also appropriate to note here that small flap actuators can also be efficiently used to control vortex formation and mixing in a nonswirling jet (Suzuki et al, 2004) and weakly swirling jet (Saiki et al, 2011). The literature contains a number of studies on dynamics of a premixed flame under periodic forcing (Ayoola et al, 2009;Balachandran et al, 2005;Bellows et al, 2006Bellows et al, , 2007Giauque et al, 2005;Kang et al, 2007;Kim and Hochgreb, 2011;Kim et al, 2010;Kulsheimer and Buchner, 2002;Lacarelle et al, 2009;Palies et al, 2010Palies et al, , 2011Paschereit et al, 1999;Thumuluru and Lieuwen, 2009, among others). In general, the flame response appears to be linear only for small amplitudes of perturbations and manifests a complex reaction to a strong forcing.…”

A POD-Based Procedure for the Split of Unsteady Losses of an LPT Cascade

“…The largest error in the determination of the TKE production is due to the computation of the time mean velocity gradient while the statistical error in the computation of the Reynolds shear stress per POD mode is very small since the database is constituted by 9000 data and, in the worst condition, at least 90 wake passing periods have been surveyed with 100 snapshots per period. Typically, POD reaches a statistical convergence at the first 10 modes for more than 200 data (e.g., see Lacarelle et al [25]). …”