Christoph M. Arndt

Flow-Field and Flame Dynamics of a Gas Turbine Model Combustor During Transition Between Thermo-Acoustically Stable and Unstable States

Laser-based and optical measurements of a gas turbine (GT) model combustor undergoing transitions between a thermo-acoustically stable and unstable state are presented. Planar laser-induced fluorescence of the OH radical, OH chemiluminescence and the planar three-component velocity field were simultaneously measured at a sustained repetition rate of 5 kHz. The combustor was operated with a lean, technically premixed CH4 /air flame at ambient pressure that transitioned unpredictably between a thermo-acoustically unstable (‘noisy’) state and a state without pulsations (‘quiet’ state). The transition from the noisy to the quiet state was correlated with the lift-off of the flame from the burner nozzle and a subsequent stabilization of the flame above the nozzle. During the transition from the quiet to the noisy state, the flame reattached to the nozzle. It was observed that the transitions occurred consistently at a particular phase of the thermo-acoustic cycle. The axial velocity fields indicated that the reattachment of the flame was assisted by an increase of the backflow velocity in the inner recirculation zone.Copyright

Experimental Study of Turbulence-Chemistry Interactions in Perfectly and Partially Premixed Confined Swirl Flames

Abstract A gas turbine model combustor (Turbomeca Burner) for premixed methane/air flames has been operated at atmospheric pressure in two different modes of premixing. In the partially premixed mode, fuel was injected into the air flow within the swirl generator shortly upstream of the combustion chamber while in the perfectly premixed mode fuel and air were mixed far upstream. The main objective of this work is the study of the influence of the mode of premixing on the combustion behavior. Stereoscopic particle image velocimetry has been applied for the measurement of the flow field, OH chemiluminescence imaging for the visualization of the flame shapes and single-shot laser Raman scattering for the determination of the joint probability density functions of major species concentrations, mixture fraction and temperature. The mixing and reaction progress and effects of turbulence-chemistry interactions are characterized by scatterplots showing the correlations between different quantities. To isolate effects of mixing from combustion instabilities that were frequently observed in this combustor, operating conditions without thermo-acoustic oscillations or coherent flow structures were chosen. While the mode of premixing had no major influence on the general flame behavior characteristic differences were observed with respect to flame anchoring, the flow field in the inner recirculation zone and the CO concentration level. The results further extend the data base of previous experimental and numerical investigations with this burner.

INFLUENCE OF HEAT TRANSFER AND MATERIAL TEMPERATURE ON COMBUSTION INSTABILITIES IN A SWIRL BURNER

The current work focuses on the large eddy simulation (LES) of combustion instability in a laboratory-scale swirl burner. Air and fuel are injected at ambient conditions. Heat conduction from the combustion chamber to the plenums results in a preheating of the air and fuel flows above ambient conditions. The paper compares two computations: In the first computation, the temperature of the injected reactants is 300 K (equivalent to the experiment) and the combustor walls are treated as adiabatic. The frequency of the unstable mode ( 635 Hz) deviates significantly from the measured frequency ( 750 Hz). In the second computation, the preheating effect observed in the experiment and the heat losses at the combustion chamber walls are taken into account. The frequency ( 725 Hz) of the unstable mode agrees well with the experiment. These results illustrate the impor- tance of accounting for heat transfer/losses when applying LES for the prediction of com- bustion instabilities. Uncertainties caused by unsuitable modeling strategies when using computational fluid dynamics for the prediction of combustion instabilities can lead to an improper design of passive control methods (such as Helmholtz resonators) as these are often only effective in a limited frequency range.

Thermo-acoustic Coupling in Swirl-Stabilized Flames with Helical Vortices

Thermoacoustic coupling produced by owame interactions was studied in a perfectlypremixed swirl-stabilized combustor by means of simultaneous high-repetition-rate stereoscopic particle image velocimetry (S-PIV) and planar laser induced uorescence (PLIF). Nine cases were studied, varying the thermal power between 10 and 35 kW and the equivalence ratio between 0.65 and 0.80. Proper orthogonal decomposition (POD) of the velocity data showed that cases with higher amplitude thermoacoustic oscillations had ow elds containing helical vortex cores (HVC); these cases were further analysed to determine the driving mechanisms of the oscillations. Flow and ame statistics were compiled as a function of both the phase in the thermoacoustic cycle and a phase representing the azimuthal position of the HVC relative to the measurement plane. These data were used to spatially map the thermoacoustic energy transfer eld, as described by the Rayleigh integral. It was found that oscillatory deformation of the HVC caused large-scale ame motions, resulting in regions of positive and negative energy transfer.

Influence of flow-structure dynamics on thermo-acoustic instabilities in oscillating swirl flames

The thermo-acoustic coupling caused by dynamic owame interactions was investigated in a gas turbine model combustor through analysis of high-repetition-rate laser measurements. Planar three-component velocity elds and OH radical distributions, as well as the line-of-sight integrated chemiluminescence from OH*, were measured at a sustained repetition rate of 5 kHz. Three fuel-lean, swirl-stabilized ames were investigated, each of which underwent thermo-acoustic pulsations. The most energetic ow structure at each condition was a helical vortex core that spiraled around the burner axis and circumscribed the combustor at a rate that was independent of the acoustics. By resolving the measurement sequence with respect to both the phase in the thermo-acoustic cycle and the azimuthal position of the helical vortex core, the repeatable oscillatory processes could be reconstructed in three dimensions. Periodic deformations in the helices at the thermoacoustic frequency were found to cause oscillations in the ame surface area. The local ame area oscillated either inor out-of-phase with the acoustic pulsations depending on the relative shapes of the ame and helices. To investigate this further, the local thermoacoustic coupling was determined by statistically solving the Rayleigh integral. In all cases, intertwined regions of positive and negative coupling occurred near the burner nozzle due to the helical vortices. In the quietest ame, the helical vortex created a large region of negative coupling that helped damp the thermo-acoustic oscillations. In the moderately louder ame, the shapes of the helix and ame were such that there was a large helical region of positive thermo-acoustic coupling that contributed energy to the thermo-acoustic pulsations. In the loudest ame, positive thermo-acoustic coupling occurred in both a large helical region and in the outer recirculation zone.

Development and evaluation of Gappy-POD for noisy PIV measurements in gas turbine combustors

Gappy proper orthogonal decomposition (GPOD) is assessed here as a data reconstruction technique for particle image velocimetry (PIV) measurements, specifically those applicable to gas turbine combustors (GTC). At practical operating conditions, PIV measurements are plagued with issues, such as low signal-to-noise ratios, that result in significant gaps in data. Four GPOD methods are studied here, including a new method that makes use of a median filter (MF) outlier detection technique to adaptively update reconstructions between iterations. The analysis of the performance of the GPOD methods is done by implementing them on a non-gappy PIV data set. Artificial gaps of varying amounts are added to this non-gappy data set in a manner similar to the gaps found in real experimental data. Additionally, two criteria to check for GPOD convergence are investigated. It was found that the MF method produced the lowest reconstruction error regardless of the amount of gappiness. Furthermore, the MF method was found to be relatively insensitive to the accuracy of the convergence criterion. The MF GPOD therefore is an effective method for filling in missing data in PIV measurements of gas turbine combustor flows.

Measurement of 3D Rayleigh Index fields in helically-perturbed swirl flames using doubly-phase-conditioned chemiluminescence tomography

This paper demonstrates a method for calculating thermoacoustic energy transfer (viz. Rayleigh Index) fields in complex swirl-stabilized flames having asymmetric 3D flow structures using high-repetition-rate OH* chemiluminescence measurements. Measurements were acquired in a variety of perfectly premixed methane-air flames, each of which contained a helical velocity disturbance that was coupled with a precessing vortex core (PVC). The azimuthal position of the PVC and helical disturbance relative to the OH* chemiluminescence viewing angle was determined by tracking the position of the chemiluminescence centroid. Tomographic reconstruction of multiply-phase-conditioned mean chemiluminescence fields then was performed to determine the mean 3D shape of the helically-perturbed heat release field at different phases of the thermoacoustic cycle. These fields, in combination with measured pressured signals, allowed calculation of the thermoacoustic energy transfer distribution. Complex patterns were found, which generally involved considerable energy transfer in the periphery of the burner (i.e. towards the outer recirculation zone). The method described provides a relatively simple and robust diagnostic for determining combustor regions driving thermoacoustic oscillations.

Influence of Air Staging on the Dynamics of a Precessing Vortex Core in a Dual-Swirl Gas Turbine Model Combustor

Detailed laser diagnostic and optical measurements to study temperature, major species concentration, velocity field and flame shape, have been carried out in a partially premixed dual-swirl gas turbine model combustor (GTMC) at atmospheric pressure using methane as fuel. The GTMC features separate air plenums for the inner and outer air stream, thus allowing control of the air split ratio between the inner and outer air stream. In the current study, flames with a thermal power of 22.5 kW and a global equivalence ratio of φ = 0.63 have been studied for air split ratios L between 1.2 and 2.0, with L = 1.6 corresponding to equal pressure loss across both swirlers. Temperature and major species concentrations were measured with laser Raman scattering, the velocity field with particle image velocimetry (PIV) and the flame shape and position with OH* chemiluminescence. For all air split ratios, the flame did not exhibit strong thermo-acoustic oscillations, such that the global heat release rate did not vary with time. However, due to a precessing vortex core (PVC) that was present for all operating conditions, strong variations in the local heat release distribution of the flame could be observed. The frequency of the PVC was at a constant Strouhal number, which was based on the air mass flow through the inner air nozzle, but was independent of the air split ratio. The Strouhal number for the PVC was Srnr = 0.78 for the non-reacting case and Srr = 0.85 for the reacting case. The current paper focuses on (a) providing a detailed data set for the validation of numerical simulations of this combustor and (b) on the influence of the air split ratio on the dynamics of the PVC.

Investigation of Auto-Ignition of a Pulsed Methane Jet in Vitiated Air using High-Speed Imaging Techniques

An experimental arrangement for the investigation of auto-ignition of a pulsed CH4 jet in a co-flow of hot exhaust gas from a laminar lean premixed H2 /air flame at atmospheric pressure is presented. The ignition events were captured by high-speed imaging of the OH* chemiluminescence associated with the igniting flame kernels at a frame rate of 5 kHz. The flow field characteristics were determined by high-speed PIV and Schlieren images. Further, high-speed imaging of laser-induced fluorescence of OH was applied to visualize the exhaust gas flow and the ignition events. Auto-ignition was observed to occur at the periphery of the CH4 jet with high reproducibility in different runs concerning time and location. In each measurement run several hundred consecutive single shot images were recorded from which sample images are presented. The main goals of the study are the presentation of the experimental arrangement and the high-speed measuring systems and a characterization of the auto-ignition events occurring in this system.Copyright