Adam M. Steinberg

Effects of Flow Structure Dynamics on Thermoacoustic Instabilities in Swirl-Stabilized Combustion

The thermoacoustic coupling caused by dynamic flow/flame interactions was investigated in a gas-turbine model combustor using high-repetition-rate measurements of the three-component velocity field, OH laser-induced fluorescence, and OH* chemiluminescence. Three fuel-lean, swirl-stabilized flames were investigated, each of which underwent self-excited thermoacoustic pulsations. The most energetic flow structure at each condition was a helical vortex core that circumscribed the combustor at a frequency that was independent of the acoustics. Resolving the measurement sequence with respect to both the phase in the thermoacoustic cycle and the azimuthal position of the helix allowed quantification of the oscillatory flow and flame dynamics. Periodic vortex/flame interactions caused by deformation of the helices generated local heat-release oscillations having spatially complex phase distributions relative to the acoustics. The local thermoacoustic coupling, determined by statistically solving the Rayleigh integral, showed intertwined regions of positive and negative coupling due to these vortices. In the quietest flame, the helical vortex created a large region of negative coupling that helped damp the oscillations. In the louder flames, the shapes of the oscillating vortices and flames were such that large regions of positive coupling were generated, driving the instability. From these observations, flame/vortex configurations that promote stability are identified.

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.

Effect of B 3+ -N 3− on YAG:Dy thermographic phosphor luminescence

The use of thermographic phosphors for high-temperature (>1000  K) thermometry currently is limited by loss of signal due to thermal quenching. This work demonstrates a new phosphor generated by substituting tetrahedral site Al(3+)-O(2-) in YAG:Dy with B(3+)-N(3-) to produce YABNG:Dy. Conventional YAG:Dy and YABNG:Dy phosphors were synthesized using identical solgel synthesis techniques. X-ray diffraction measurements showed that both had nearly pure crystalline phases, with a minor secondary yttrium-aluminum-monoclinic (YAM) phase present in the YABNG:Dy. The YABNG:Dy sample had a larger and more spherical primary grain than did the YAG:Dy in scanning electron microscopy images. Tests of the thermal response showed that the YABNG:Dy had much stronger phosphorescence emissions than did YAG:Dy, likely due to the morphological differences. Furthermore, the onset of thermal quenching was delayed by approximately 100 K for YABGN:Dy compared to YAG:Dy, and the rate of signal decrease with temperature was reduced. This resulted in greater signal-to-noise ratios and less uncertainty in the temperature measurements, particularly at high temperatures.

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.

Experimental investigation of a generic, fuel flexible reheat combustor at gas turbine relevant operating conditions

Fuel flexibility in stationary gas turbines (GT) is becoming increasingly important due to the use of a broader spectrum of primary energy sources, particularly H2 -rich fuels derived from the gasification of coal or biomass. GTs also must be able to operate at extremely low emission levels, which is currently achieved with lean-premixed burner designs. To investigate the performance of highly reactive fuels in the reheat combustion concept, mainly with respect to autoignition and flashback limits, a generic reheat combustor with excellent optical access has been developed. The first objective of this work was to carefully characterize the mixing section in order to derive well-defined boundary conditions for the subsequent autoignition studies. Initial autoignition results at T > 1000 K and p = 15 bar are presented for natural gas (NG) and H2 -rich fuels. No autoignition was detected for NG at the investigated operating conditions. For H2 /NG/N2 blends with a constant volumetric N2 concentration of 20% and H2 concentrations higher than 76%, autoignition in the mixing section was detected.Copyright

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.

Analysis of flow-flame interactions in a gas turbine model combustor under thermo-acoustically stable and unstable conditions

Flow-flame interactions in a swirl-stabilized gas turbine model combustor are investigated using doubly-phase-resolved analysis of high-repetition-rate laser and optical measurements. Three flames were studied, each of which exhibited self-excited thermo-acoustic oscillations of different amplitude ranging from fairly stable to highly unstable combustion. High-repetition-rate stereoscopic particle image velocimetry, OH planar laser induced fluorescence, and OH* chemiluminescence measurements were performed at a sustained repetition rate of 5 kHz, which was sufficient to resolve the thermo-acoustic combustor dynamics. Using spatio-temporal proper orthogonal decomposition, it was found that the flow-field contained several simultaneous periodic motions: the reactant flux into the combustion chamber periodically oscillated at the thermo-acoustic frequency, a helical precessing vortex core (PVC) circumscribed the burner nozzle at a frequency set by the global flow rate, and the PVC underwent axial contraction and extension at the thermo-acoustic frequency. The amplitude of the axial PVC dynamics increased with increasing thermo-acoustic oscillation amplitude. The global heat release rate fluctuated at the thermo-acoustic frequency, while the heat release centroid circumscribed the combustor at the difference between the acoustic and PVC frequencies. This latter motion was caused by the axial PVC dynamics. Hence, the three-dimensional location of the heat release and heat release fluctuations depended on the interaction of the PVC with the flame surface. This motivated the compilation of doubly-phase-resolved statistics based on the phase of both the acoustic and PVC cycles, which showed highly repeatable periodic flow-flame configurations. These doublyphase-resolved statistics were used to reconstruct the dynamics of the three-dimensional periodic flow structures and flame surface oscillations over the thermo-acoustic cycle. It was found that the majority of the heat release oscillations at the thermo-acoustic frequency were caused by oscillations in the reaction layer length. By filtering the instantaneous reaction layers at different scales, the importance of the various flow-flame interactions affecting the flame length was determined. The greatest contributor was large-scale elongation of the reaction layers associated with the fluctuating reactant flow rate, which accounted for approximately 50% of the fluctuations. The remaining 50% was distributed between fine scale stochastic corrugation and large-scale corrugation due to the PVC.

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.