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Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

# Spatiotemporal Characterization of a Conical Swirler Flow Field Under Strong Forcing

[+] Author and Article Information
A. Lacarelle1

Institut für Strömungsmechanik und Technische Akustik, Technische Universität Berlin, Müller-Breslau-Strasse 8, 10623 Berlin, Germanyarnaud.lacarelle@tu-berlin.de

T. Faustmann, C. O. Paschereit, O. Lehmann, D. M. Luchtenburg, B. R. Noack

Institut für Strömungsmechanik und Technische Akustik, Technische Universität Berlin, Müller-Breslau-Strasse 8, 10623 Berlin, Germany

D. Greenblatt

Faculty of Mechanical Engineering, Technion-Israel Institute of Technology, Technion City, Haifa 32000, Israel

1

Corresponding author.

J. Eng. Gas Turbines Power 131(3), 031504 (Feb 06, 2009) (12 pages) doi:10.1115/1.2982139 History: Received March 31, 2008; Revised April 21, 2008; Published February 06, 2009

## 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.

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Copyright © 2009 by American Society of Mechanical Engineers
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## Figures

Figure 1

Principle sketch of the water test rig illustrating the excitation mechanism

Figure 2

Mounted burner in the test rig, side (a) and top views ((b) and (c)). The figures show the PIV and LDA measurement positions. S1 and S2 are the two slots of the burner. The hydrophone h was fixed on S1. For the LDA measurement in the slot, the angle θ was set equal to −20 deg.

Figure 3

Scatter plot of the phase-sorted azimuthal velocity ut, bin averaging of the velocity (left) and comparison of the bin averaging with the Fourier estimation of the bin averaged velocity (right), uF,t for the unforced flow

Figure 4

Contour plot of the mean streamwise velocity u¯x in m/s with the vector plot of the velocity in the (xb,zb)-plane, unforced flow

Figure 5

Frequency spectra of the ux,fluc (left) and ut,fluc (right) components for the unforced flow, in the shear layer at the burner outlet (G position, x=D/4, y=0, and z=−50 mm)

Figure 6

Phase-averaged color maps of the azimuthal vorticity Ω (in s−1) in the (xb,zb)-plane of the unforced flow, which evidences the natural helical mode. Four phases of the hydrophone signal (top right) were used to trigger the acquisition. The arrows represent the velocity vectors.

Figure 7

Relative oscillation amplitude of the natural frequency for ut over the axial position for the unforced flow. Spectrum of the signal at the different locations (E, D, B, and shear layer G) is shown. A best fit power four polynomial is applied to the data.

Figure 8

Phase relationship between the tangential velocity ũt and the hydrophone signal h within Slots 1 and 2 of the burner depending on the axial position x/D for the unforced flow

Figure 9

Phase angle ∠h1h2 of the hydrophone signals as a function of the angle β

Figure 10

Contour plot of the mean streamwise velocity u¯x in m/s with the vector plot of the velocity in the (xb,zb)-plane, forced flow at Stf≈0.92

Figure 11

Line plot of the mean streamwise velocity u¯x (left) and radial velocity u¯r (right), in m/s, for the unforced (Stf=0,−) and forced (Stf≈0.92,●) cases

Figure 12

Phase-averaged azimuthal vorticity Ω (in s−1) in the (xb,zb)-plane of the forced flow at Stf≈0.92, which shows the axisymmetric structure. Four phases of the hydrophone signal (top right) were used to trigger the acquisition. The arrows represent the velocity vectors.

Figure 13

Phase relationship between the tangential velocity ũt and the hydrophone signal h within slot 1, for three forcing amplitudes at Stf≈0.92

Figure 14

Relative oscillation amplitude of ũt depending on the axial position x/D for three forcing amplitudes at Stf≈0.92

Figure 15

Impact of the excitation on the mean flow velocity u¯t in slot 1 at Stf≈0.92

Figure 16

Spectra of ux,fluc and ut,fluc in the shear layer (G) and in Slot (B) for Stf≈1.3 and Fa=10%. The spectrum of the hydrophone is shown in the right plot.

Figure 17

Repartition of the turbulent kinetic energy within the POD modes for the three with PIV investigated cases

Figure 18

Contour plot of the azimuthal vorticity Ω resulting from the POD analysis of the snapshots in the (xb,zb)-plane. The mean vorticity and the first four (1–4) dominant modes for all three investigated cases are presented and show the dominant coherent flow structures. The limits of the gray scale are set to emphasize the vorticity sign of the structures, as absolute vorticity values are not of interest. Superposed are streamline representations of the corresponding modes.

Figure 19

Sensitivity of the POD to the number M of PIV snapshots (63, 125, 250, and 500) taken for the analysis. Modes 1, 3, 7, and 9 of the unforced flow are presented.

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