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Oberleithner K.,Institute For Stromungsmechanik Und Technische Akustik | Stohr M.,German Aerospace Center | Im S.H.,German Aerospace Center | Arndt C.M.,German Aerospace Center | And 2 more authors.
Combustion and Flame | Year: 2015

The precessing vortex core (PVC) is a coherent flow structure that is often encountered in swirling flows in gas turbine (GT) combustors. In some swirl combustors, it has been observed that a PVC is present under non-reacting conditions but disappears in the corresponding reacting cases. Since numerous studies have shown that a PVC has strong effects on the flame stabilization, it is desirable to understand the formation and suppression of PVCs in GT combustors. The present work experimentally studies the flow field in a GT model combustor at atmospheric pressure. Whereas all non-reacting conditions and detached M-shaped flames exhibit a PVC, the PVC is suppressed for attached V-shaped flames. A local linear stability analysis is then applied to the measured time-averaged velocity and density fields. For the cases where a PVC appeared in the experiment, the analysis shows a global hydrodynamic instability that manifests in a single-helical mode with its wavemaker located at the combustor inlet. The frequency of the global mode is in excellent agreement with the measured oscillation frequency and the growth rate is approximately zero, indicating the marginally stable limit-cycle. For the attached V-flame without PVC, strong radial density/temperature gradients are present at the inlet, which are shown to suppress the global instability. The interplay between the PVC and the flame is further investigated by considering a bi-stable case with intermittent transitions between V- and M-flame. The flame and flow transients are investigated experimentally via simultaneous highspeed PIV and OH-PLIF. The experiments reveal a sequence of events wherein the PVC forms prior to the transition of the flame shape. The results demonstrate the essential role of the PVC in the flame stabilization, and thereby the importance of a hydrodynamic stability analysis in the design of a swirl combustor. © 2015 The Combustion Institute. Source

Oberleithner K.,Monash University | Paschereit C.O.,Institute For Stromungsmechanik Und Technische Akustik
Journal of Fluid Mechanics | Year: 2014

Spatial linear stability analysis is applied to the mean flow of a turbulent swirling jet at swirl intensities below the onset of vortex breakdown. The aim of this work is to predict the dominant coherent flow structure, their driving instabilities and how they are affected by swirl. At the nozzle exit, the swirling jet promotes shear instabilities and, less unstable, centrifugal instabilities. The latter stabilize shortly downstream of the nozzle, contributing very little to the formation of coherent structures. The shear mode remains unstable throughout generating coherent structures that scale with the axial shear-layer thickness. The most amplified mode in the nearfield is a co-winding double-helical mode rotating slowly in counter-direction to the swirl. This gives rise to the formation of slowly rotating and stationary large-scale coherent structures, which explains the asymmetries in the mean flows often encountered in swirling jet experiments. The co-winding single-helical mode at high rotation rate dominates the farfield of the swirling jet in replacement of the co-and counter-winding bending modes dominating the non-swirling jet. Moreover, swirl is found to significantly affect the streamwise phase velocity of the helical modes rendering this flow as highly dispersive and insensitive to intermodal interactions, which explains the absence of vortex pairing observed in previous investigations. The stability analysis is validated through hot-wire measurements of the flow excited at a single helical mode and of the flow perturbed by a time-and space-discrete pulse. The experimental results confirm the predicted mode selection and corresponding streamwise growth rates and phase velocities. © 2014 Cambridge University Press. Source

Lacoste D.A.,Ecole Centrale Paris | Moeck J.P.,TU Berlin | Moeck J.P.,Institute For Stromungsmechanik Und Technische Akustik | Paschereit C.O.,TU Berlin | And 2 more authors.
Journal of Propulsion and Power | Year: 2013

A convenient way to reduce pollutant emissions or fuel consumption in industrial combustion systems is to burn in lean-premixed mode. A central pin electrode extending from the swirler to the combustion chamber inlet serves as the cathode. The anode is a circular loop with a diameter of 25 mm. It is made of a stainless steel wire with a diameter of 2 mm. This electrode is connected to the high voltage pulser through the burner ground plate with ceramic insulation. With this electrode arrangement, NRP discharge filaments are generated between the central pin electrode and the loop electrode in a disk-shaped planar area at the burner outlet. All measurements were made with the flame present. Voltage and current signals were first synchronized and then multiplied and integrated to obtain the energy deposited per pulse. The average power of the plasma was determined by multiplying the energy deposited per pulse by the repetition frequency of the discharge. Source

Lacarelle A.,TU Berlin | Lacarelle A.,Institute For Stromungsmechanik Und Technische Akustik | Luchtenburg D.M.,TU Berlin | Luchtenburg D.M.,Institute For Stromungsmechanik Und Technische Akustik | And 6 more authors.
AIAA Journal | Year: 2010

A combination of postprocessing tools of OH*-chemiluminescence snapshots is used to characterize the coherent structures of two types of premixed burners: a bluff body and an industrial swirl burner. Two methods are combined to extract the structures: a phase-averaging algorithm and the proper orthogonal decomposition. The first method is based on the estimation of the instantaneous phase of the snapshots relative to a (local) time-resolved signal.Aphasesorting-phase-averaging algorithm then reconstructs the evolution of the flame at a chosen frequency over one cycle. The proper orthogonal decomposition method is used as a filter to smoothen the snapshots. Both methods provide insight into the physical mechanisms of coherent structures in the two premixed flames under consideration. The snapshots of the bluff-body combustion exhibit a symmetric structure. This indicates that the von Kàrmàn vortex street in the cold flow is suppressed by the addition of heat in the shear layer. Three coexisting flame structures of the swirl burner in the combustion chamber could be identified: a natural helical structure of the burner and two axisymmetric modes. Increasing the amplitude of acoustic forcing at the natural flow frequency changes the helical structure to an axisymmetric one. Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Source

Holl T.,TU Berlin | Holl T.,Institute For Stromungsmechanik Und Technische Akustik | Vel Job A.K.,TU Berlin | Vel Job A.K.,Institute For Stromungsmechanik Und Technische Akustik | And 4 more authors.
Journal of Aircraft | Year: 2012

This paper presents a comprehensive numerical study of the actively controlled flow on the flap of a generic threeelement high-lift configuration. An active flow-control mechanism is applied at a Reynolds number of Re = 1 106 to enhance the lift of the airfoil. Both pulsed blowing and a harmonic actuation (zero-net-mass flux) are used and compared. Unsteady Reynolds-averaged Navier-Stokes as well as detached-eddy simulations are conducted. The inhouse code ELAN is used for the computations. The main focus lies on three-dimensional aspects. It is investigated whether it is possible to further increase the lift by dividing the actuation slot into two parts. A phase shift between these two actuation segments is applied, and the segments are varied in their spanwise extension. As a result of the actuation on the flap shoulder, the circulation around the whole airfoil, and therefore the lift, is effectively increased. It can be demonstrated that the excitation of longitudinal vortices on the upper surface of the flap results in reattachment of the flow. Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc. Source

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