Experimental study of vortex-flame interaction in a gas turbine model combustor

doi:10.1016/j.combustflame.2012.03.020

Combustion and Flame

Volume 159, Issue 8, August 2012, Pages 2636-2649

https://doi.org/10.1016/j.combustflame.2012.03.020 Get rights and content

Abstract

The interaction of a helical precessing vortex core (PVC) with turbulent swirl flames in a gas turbine model combustor is studied experimentally. The combustor is operated with air and methane at atmospheric pressure and thermal powers from 10 to 35 kW. The flow field is measured using particle image velocimetry (PIV), and the dominant unsteady vortex structures are determined using proper orthogonal decomposition. For all operating conditions, a PVC is detected in the shear layer of the inner recirculation zone (IRZ). In addition, a co-rotating helical vortex in the outer shear layer (OSL) and a central vortex originating in the exhaust tube are found. OH chemiluminescence (CL) images show that the flames are mainly stabilized in the inner shear layer (ISL), where also the PVC is located. Phase-averaged images of OH-CL show that for all conditions, a major part of heat release takes place in a helical zone that is coupled to the PVC. The mechanisms of the interaction between PVC and flame are then studied for the case P = 10 kW using simultaneous PIV and OH-PLIF measurements with a repetition rate of 5 kHz. The measurements show that the PVC causes a regular sequence of flame roll-up, mixing of burned and unburned gas, and subsequent ignition of the mixture in the ISL. These effects are directly linked to the periodic vortex motions. A phase-averaged analysis of the flow field further shows that the PVC induces an unsteady lower stagnation point that is not present in the average flow field. The motion of the stagnation point is linked to the periodic precession of the PVC. Near this point burned and unburned gas collide frontally and a significant amount of heat release takes place. The flame dynamics near this point is also coupled to the PVC. In this way, a part of the reaction zone is periodically drawn from the stagnation point into the ISL, and thus serves as an ignition source for the reactions in this layer. In total, the effects in the ISL and at the stagnation point showed that the PVC plays an essential role in the stabilization mechanism of the turbulent swirl flames. In contrast to the PVC, the vortices in the OSL and near the exhaust tube have no direct effect on the flame since they are located outside the flame zone.

Introduction

Modern gas turbine (GT) combustors are operated under lean premixed or partially premixed conditions in order to reduce emissions of NO_x and CO. The highly turbulent flames in GT combustors are most often stabilized aerodynamically by swirl-induced vortex breakdown. This leads to recirculation of hot burned gases, which enhances ignition of unburned gas and thus helps to operate the flames under the desired lean conditions. The detailed mechanisms of the stabilization of turbulent swirl flames, however, are based on complex unsteady interactions of flow field and chemistry that are still not well enough understood today [1]. Further improvements of GT combustors with respect to emissions, fuel flexibility, and reliability therefore depend considerably on a better understanding of these interactions.

The flow field of turbulent swirl flames not only contains turbulent velocity fluctuations, but in many cases also features unsteady coherent vortex structures. The most common coherent vortex structure in swirl combustors is the so-called precessing vortex core (PVC) [2]. A literature survey provided in Appendix A shows that PVCs are frequently encountered in GT-typical swirl combustors. Consequently, not only turbulence-chemistry interaction [3], [4], [5] is important in swirl combustors, but also the interaction of coherent vortices with the flame. While flow pattern and frequency of PVCs have been frequently characterized, studies of their interaction with a flame are, however, limited to the pioneering works of Syred and coworkers [6], [7], [8] and a study of Li and Gutmark [9]. Although these works could show some effects of the PVC on mixing and flame shape, the detailed mechanisms of the interaction remain largely unclear.

Many previous works have investigated vortex-flame interaction at a rather fundamental level (see, e.g., Refs. [10], [11], [12], [13] and references therein). In these works, interaction of a single vortex or a vortex pair with a flame front is studied under well-defined conditions. The studies revealed that a vortex can interact with a flame in numerous ways. E.g., the vortex can influence the flame through transport and mixing, leading to changes of local gas composition and temperature. Furthermore, vortical structures may induce an enlarged flame surface or lead to strain-induced local extinction. The magnitude of each effect strongly depends on various parameters like, e.g., local gas composition and temperature, relative size and orientation of flame front and vortex, and time scales of chemistry and flow. Conversely, the flame affects the vortex due to the expansion induced by heat release and leads to changes of flow parameters such as density and viscosity.

In highly turbulent GT-typical swirl flames, interaction of a vortex with the flame is much more complex than under the well-defined conditions of the fundamental studies referenced above. This is mainly owing to the irregular turbulent velocity fluctuations that are superimposed on the coherent vortex flow structure. The turbulent flow can lead to effects like unsteady strain, mixing and a change of local flame surface orientation that can then compete and interact with the effects of the coherent vortex. Under such conditions, the detailed mechanisms of vortex-flame interaction are not well understood, and further detailed experimental studies are required.

The objective of this work is to elucidate the interaction of large-scale coherent vortices within turbulent swirl flames in a GT-typical combustor by means of modern laser-based methods. In particular, novel high-speed laser techniques are applied for simultaneous measurements of flow field and flame structure with high temporal and spatial resolution, which provide detailed insights into the mechanisms of the interaction. The study uses a GT model combustor, which at certain operating conditions exhibits thermoacoustic oscillations. These oscillations have been subject of numerous previous experimental studies, where several diagnostic techniques have been applied such as Raman spectroscopy, PLIF, and PIV at repetition rates between 5 Hz and 5 kHz. Recent reviews of these works are provided by Boxx et al. [14] and Steinberg et al. [15]. In another work, where the same combustor was operated at a fuel-lean condition, Stöhr et al. have investigated the dynamics of lean blowout [16].

The present study, by contrast, focuses completely on the role of coherent vortex structures. The combustor is therefore operated at conditions where no blowout and virtually no thermoacoustic oscillations occur, while coherent vortices such as a PVC are present. The operating conditions cover a range of thermal power from 10 kW to 35 kW. After a general characterization of the flames, the flow field with the coherent vortices is studied in detail using particle image velocimetry (PIV) and acoustic analysis. An important part of this study is the analysis of the flow field using proper orthogonal decomposition (POD). The usefulness of POD for the analysis of a PVC has been demonstrated in a recent publication [17]. In the second part, the interaction of vortex and flame is studied using simultaneous PIV and planar laser-induced fluorescence of OH with repetition rates up to 5 kHz.

Section snippets

Combustor

A sketch of the GT model combustor is shown in Fig. 1a. Dry air at ambient temperature enters the plenum (diameter 79 mm, height 65 mm) and then separately passes the inner (Fig. 1d) and outer (Fig. 1c) radial swirl generator. The two co-swirling flows enter the combustion chamber through a central circular nozzle (diameter 15 mm) and a surrounding annular nozzle (inner diameter 17 mm, outer diameter 25 mm contoured to an outer diameter of 40 mm). Non-swirling gaseous fuel is supplied through 72

Flame shape

Figure 2 shows averaged OH^∗ chemiluminescence images of the flames with φ = 0.65 and thermal powers from 10 to 35 kW. The OH^∗ signal (integrated along the line-of-sight) is used as a marker of the locations of heat release [20]. At P = 10 kW, the heat release zone lies between y = 5 mm and y = 45 mm in axial direction and within a radius of r < 35 mm from the central axis. When thermal power and thus flow velocities are increased, the upper end of the flame zone gradually moves up to y = 100 mm for P = 35 kW. For

Instantaneous flow fields

Figure 6 shows instantaneous velocity fields in the vertical section (z = 0 mm) for P = 10 kW and P = 35 kW. It can be seen that the instantaneous flow contains distinctive features that are not present in the average flow field. First, both instantaneous flow fields exhibit a zig–zag arrangement of vortices in the ISL marked by black arrows. This arrangement indicates the presence of a coherent 3D helical vortex in the ISL. Such a helical vortex is often found in swirl flames (cf. Appendix A) and is

General remarks

The aim of this section is to characterize the effects of the PVC on the flame and its role in the mechanism of flame stabilization. For all conditions, the flame is stabilized in the ISL, where also the PVC is located. This suggests that the PVC may strongly interact with the flame. In contrast, the vortex in the OSL and the exhaust tube vortex are largely separated from the flame zone, and therefore, presumably, have no direct effect on the flame. In the following, the effect of the PVC is

Summary and conclusions

The interaction of a helical precessing vortex core (PVC) with turbulent swirl flames in a gas turbine model combustor was studied experimentally. The combustor was operated with air and methane at atmospheric pressure and thermal powers from 10 to 35 kW. The flow field was characterized using particle image velocimetry (PIV), and the dominant unsteady vortex structures were determined by means of proper orthogonal decomposition. For all operating conditions, a PVC was detected in the shear

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Experimental study of vortex-flame interaction in a gas turbine model combustor

Abstract

Introduction

Section snippets

Combustor

Flame shape

Instantaneous flow fields

General remarks

Summary and conclusions

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