Experimental study of unsteady flame structures of an oscillating swirl flame in a gas turbine model combustor

Abstract

Velocity fields and flame structures of a partially premixed swirl flame in a gas turbine model combustor are measured in axial and transverse sections using simultaneous (Stereo-)PIV and OH-PLIF. The flame, operated under atmospheric pressure with air and methane at a thermal power of 10.3 kW and a global equivalence ratio of ϕ = 0.75, features thermoacoustic oscillations at a frequency f ≈ 295 Hz. The averaged flow field with inner and outer recirculation zones is typical of swirl-stabilized flames, and the instantaneous measurements show the presence of a helical vortex (PVC) located in the inner shear layer. The PVC, which rotates with a different frequency than the thermoacoustic oscillation, leads to an enhanced mixing of burned and unburned gas and thus to stabilization of the flame. Two distinct large-scale structures of velocity and OH are found in the transverse cross-sections. The first type is characterized by a roughly annular region of inflowing unburned gas and distinct inner and outer recirculation zones. In the second type, the region of positive axial velocity forms a spiral, and the recirculation zone consists of an inner region that is connected to the outer parts by a narrow curved zone along the spiral arm. Whereas the first type is a typical scenario of vortex breakdown with a PVC, the transient spiral recirculation zone observed here has, to our knowledge, not yet been reported. A phase-resolved analysis shows that the annular form correlates with low, and the spiral form with high rates of global heat release.

Introduction

Modern gas turbine (GT) combustors are operated under lean premixed or partially premixed conditions in order to achieve low emissions of NO_x. Flames are most often stabilized by inducing a swirling flow of the reactants, which leads to an inner recirculation of hot burned gases and thereby enhances ignition of unburned gas. However, the operation of GT combustors under such conditions is highly susceptible to thermoacoustic oscillations, which may strongly affect lifetime and reliability of the GT. The mechanisms of these oscillations are still not well understood, and their prediction is a critical issue of combustor design.

The formation of the inner recirculation zone (IRZ) results from vortex breakdown of the swirling flow [1], [2], [3]. An important aspect of this phenomenon is the occurence of large-scale coherent structures such as the precessing vortex core (PVC). The PVC is an unsteady vortex located in the shear layer of the IRZ that precesses around the central axis. Several experimental works have found PVCs in isothermal flows in GT combustors for swirl numbers S > 0.5 [4], [5], [6], [7]. The occurence and role of PVCs under combustion conditions is a complex issue and strongly depends on mode of fuel entry, equivalence ratio and combustor geometry [8]. Only few experimental studies have addressed the interaction of flame and PVC, and the mechanisms are largely unclear. Schildmacher and Koch reported the presence of a PVC under isothermal conditions which disappeared for the reacting case with same flow conditions [9]. Syred et al. found a PVC under reacting conditions and discussed its effect of increased mixing [10]. Li and Gutmark proposed that a PVC is the main factor driving the combustion instability in a swirl-dump combustor [11].

Strong efforts are currently undertaken for the numerical simulation of swirl-stablized flames with the intentional use for the design of improved GT combustors. Numerous recent studies have shown that large eddy simulation (LES) is capable of reproducing unsteady swirl-induced vortex structures like the PVC for both isothermal [12], [13], [14] and reacting [15], [16], [17], [18], [19], [20], [21], [22], [23] cases. Some of these studies found a PVC in the isothermal flow that disappeared under reacting conditions [15], [20], [22]. Duwig and Fuchs found a PVC in a swirl-stabilized flame for S = 1.05 and S = 1.35, but not for S = 0.45 [19].

The present work investigates the interaction between a PVC and a GT-typical swirl flame by means of planar laser diagnostics. Velocity fields and flame structures in axial and transverse sections were simultaneously measured using particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) of OH. In a recent study we discussed the potentials and limitations of this experimental technique, and reviewed its application by other groups [24]. The flame is operated with CH₄ and air in a GT model combustor under atmospheric pressure. It has a thermal power of 10.3 kW, a global equivalence ratio of ϕ = 0.75 and a swirl number of S ≈ 0.55. Furthermore it features thermoacoustic oscillations at f ≈ 295 Hz. Several experimental studies have been applied to the same combustor and operating condition: distributions of OH, CH and H₂CO have been measured by PLIF [24], [25], [27], [28], major species concentrations have been determined using Raman spectroscopy [26], [27], [29] and velocities by laser Doppler velocimetry (LDV) and PIV [26], [27], [28], [24]. The focus of this study is on the phenomenology of unsteady large-scale coherent structures in a GT typical swirl flame. A PVC is identified in instantaneous velocity fields and its effect on flame structure is discussed using the corresponding OH-PLIF images. Furthermore large-scale structural changes of flow field and reaction zone during a cycle of the thermoacoustic oscillation are characterized. The observed phenomena are compared to phase-dependent variations of heat release and pressure in the combustion chamber.