A. Lacarelle 1 e-mail: arnaud.lacarelle@tu-berlin.de T. Faustmann 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 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 Spatiotemporal Characterization of a Conical Swirler Flow Field Under Strong Forcing 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 an- emometry (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 St f  0.92) and two higher frequencies (St f  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 ampli- tudes 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 domi- nant 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. DOI: 10.1115/1.2982139 Keywords: swirling flows, PIV, LDA, POD, coherent structures, flow instability, flow forcing 1 Introduction For the past decades, lean premixed combustion has become a standard feature in gas turbine engines and is expected to be implemented in aircraft engines. The main advantage of this com- bustion technique is that the low fuel/air ratio results in a lower burning temperature and produces relatively low NO x emissions. One of the main disadvantages of lean premixed combustion is that it is susceptible to combustion instabilities that produce large- amplitude pressure oscillations that can damage the combustor and turbine. The mechanisms leading to thermoacoustic instabili- ties are numerous and closely related to each other: fuel/air ratio oscillations 1, acoustics boundary conditions 2, and flame sur- face oscillations induced by coherent structures 3,4. These coherent flow structures are present in most combustors jet flames, bluff body, and swirling flows and lead to periodical oscillations of the velocity and mixing profile. This results in os- cillations of the flame, which can excite acoustic modes of the combustor and in turn generate combustion instabilities. Particu- larly in swirling combustors, different coherent structures have been identified in experimental and numerical studies of isother- mal flows 5–10. Most of these investigations revealed a precess- ing vortex core PVC and a helical mode. The typical frequencies of these two phenomena were generally not related but some in- vestigations showed that the frequency of the PVC was very closed or equal to the frequency of the helical mode 7 and par- ticularly in a downscaled model of the burner used in this inves- tigation 10. In the shear layer of the flow, Kelvin–Helmholtz instabilities have also been observed 3,5,9. The influence of the burner geometry, swirl number, expansion ratio, and boundary conditions on the flow field have also been investigated by many authors 11–13. To avoid combustion instabilities induced by coherent struc- tures, it is essential to understand their generation mechanism, as well as their evolution in the case of forced flows. The forcing, which is applied in the current investigation, approximates the impact of axial acoustic modes of combustion instabilities on the flow field in the burner. Numerous experimental investigations on the excitation of simple jets have been reported in literature 14,15. The forcing of those jets with different excitation modes generated axisymmetric or helical structures. Literature dealing with the experimental forcing of swirling jets is rare, and even rarer are studies carried out on swirl burners. One of the main reasons is that actuators may not have sufficient authority to excite the flow with an am- plitude that is comparable to the oscillation amplitude that is at- tained under full-scale operating conditions 10% of the mean flow velocity. Nevertheless, Paschereit et al. 3 investigated the flow in re- acting and non-reacting flows on a model premixed burner. They observed that axisymmetrical and helical modes could be excited by changing the boundary conditions downstream of the combus- tion chamber. Cold flow investigations of the same burner in a water test rig showed that the helical mode found in the reacting experiments corresponded to a helical mode of the burner flow. Using the same burner type, Lacarelle et al. 16 showed that forcing at the frequency of the helical mode led to an increase in scalar mixing at the burner outlet. Phase-averaged measurements 1 Corresponding author. Contributed by the International Gas Turbine Institute of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received March 31, 2008; final manuscript received April 21, 2008; published online February 6, 2009. Review conducted by Dilip R. Ballal. Paper presented at the ASME Turbo Expo 2008: Land, Sea and Air GT2008, Berlin, Germany, June 9–13, 2008. Journal of Engineering for Gas Turbines and Power MAY 2009, Vol. 131 / 031504-1 Copyright © 2009 by ASME Downloaded 16 Jul 2009 to 130.149.47.15. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm