Key parameters governing the precessing vortex core in reacting flows: An experimental and analytical study

Steffen, Thomas; Oberleithner, Kilian; Paschereit, Christian Oliver

doi:10.1016/j.proci.2014.07.035

Cited by 76 publications

(49 citation statements)

References 23 publications

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“…Numerous previous investigations have shown that the PVC encountered in reacting and non-reacting swirling jets is a manifestation of a super-critical Hopf bifurcation to a global mode [31,32,[34][35][36][37]40,41]. Generally, a global instability mode is characterized by a single oscillation frequency and shape function, it is self-sustained, and it is robust to external forcing [51,52].…”

Section: The Pvc As a Global Hydrodynamic Instability Modementioning

confidence: 99%

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Formation and flame-induced suppression of the precessing vortex core in a swirl combustor: Experiments and linear stability analysis

Oberleithner

Stöhr

et al. 2015

Combustion and Flame

190

152

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a b s t r a c tThe 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.

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Section: The Pvc As a Global Hydrodynamic Instability Modementioning

confidence: 99%

“…Yet, from fundamental flow studies, it is known that the density field has a significant impact on the flow stability [38,39]. In particular, it was shown that the suppression of the PVC at certain operating conditions can be related to the change of the density field [40,41].…”

Section: Introductionmentioning

confidence: 99%

Formation and flame-induced suppression of the precessing vortex core in a swirl combustor: Experiments and linear stability analysis

Oberleithner

Stöhr

et al. 2015

Combustion and Flame

190

152

View full text Add to dashboard Cite

show abstract

“…There are several modes with high harmonic correlation (diameter and colour of the points) and high energy content K. The spatial shape of the low-frequency mode (label 1) shows clear spatial symmetry and a limited spectral bandwidth (St ≈ 0.1). This mode is frequently observed in swirl-stabilized combustors and it is associated with a global hydrodynamic instability (Terhaar, Oberleithner & Paschereit 2015). From the four additional outstanding modes between St = 0.3 and St = 0.8, we choose two for further investigation.…”

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confidence: 99%

Spectral proper orthogonal decomposition

2016

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The identification of coherent structures from experimental or numerical data is an essential task when conducting research in fluid dynamics. This typically involves the construction of an empirical mode base that appropriately captures the dominant flow structures. The most prominent candidates are the energy-ranked proper orthogonal decomposition (POD) and the frequency-ranked Fourier decomposition and dynamic mode decomposition (DMD). However, these methods are not suitable when the relevant coherent structures occur at low energies or at multiple frequencies, which is often the case. To overcome the deficit of these 'rigid' approaches, we propose a new method termed spectral proper orthogonal decomposition (SPOD). It is based on classical POD and it can be applied to spatially and temporally resolved data. The new method involves an additional temporal constraint that enables a clear separation of phenomena that occur at multiple frequencies and energies. SPOD allows for a continuous shifting from the energetically optimal POD to the spectrally pure Fourier decomposition by changing a single parameter. In this article, SPOD is motivated from phenomenological considerations of the POD autocorrelation matrix and justified from dynamical systems theory. The new method is further applied to three sets of PIV measurements of flows from very different engineering problems. We consider the flow of a swirl-stabilized combustor, the wake of an airfoil with a Gurney flap and the flow field of the sweeping jet behind a fluidic oscillator. For these examples, the commonly used methods fail to assign the relevant coherent structures to single modes. The SPOD, however, achieves a proper separation of spatially and temporally coherent structures, which are either hidden in stochastic turbulent fluctuations or spread over a wide frequency range. The SPOD requires only one additional parameter, which can be estimated from the basic time scales of the flow. In spite of all these benefits, the algorithmic complexity and computational cost of the SPOD are only marginally greater than those of the snapshot POD.

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“…PVCs are always encountered at isothermal conditions in gas turbine combustors [5][6][7][8]. In reacting conditions PVCs do not encountered in some particular combustors, or flame operating conditions [9,10].…”

Section: Introductionmentioning

confidence: 99%

Detached eddy simulation of non-reacting swirling flow in a vortex burner

Mansouri¹,

Boushaki²,

Aouissi³

2017

IJHT

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An unconfined high turbulent swirling flow is investigated using Detached Eddy Simulation (DES) in a premixed vortex burner. The burner operates with propane-air mixture at high swirl number Sn = 1.05 under atmospheric pressure. In this analysis, the focus of the investigation is the isothermal flow and its expansion at the burner exit. Comparisons of experimental data show that the DES results are capable of predicting the unsteady flow structure and mean velocity profiles. DES results show that the swirling flow is responsible of the formation of a recirculation zone (CRZ) in the center of the burner exit. A helical precessing vortex core (PVC) is detected in the inner shear layer (ISL). Phase-angle analysis of the instantaneous flow field shows the presence of unsteady stagnation points strongly associated with PVC. In addition, phase-angle analysis of the radial profiles of the instantaneous velocities (u, w) is employed to clarify better the instantaneous flow field. The vortex core inside the PVC is analyzed using two slices perpendicular to its axis. It is found that the vortex core is characterized by the limits of its circumferential velocity u(R) and its radius R.

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Key parameters governing the precessing vortex core in reacting flows: An experimental and analytical study

Cited by 76 publications

References 23 publications

Formation and flame-induced suppression of the precessing vortex core in a swirl combustor: Experiments and linear stability analysis

Formation and flame-induced suppression of the precessing vortex core in a swirl combustor: Experiments and linear stability analysis

Spectral proper orthogonal decomposition

Detached eddy simulation of non-reacting swirling flow in a vortex burner

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