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.

a b s t r a c tA detailed analysis of the flow-flame interactions associated with acoustically coupled heat-release rate fluctuations was performed for a 10 kW, CH 4 /air, swirl stabilized flame in a gas turbine model combustor exhibiting self-excited thermo-acoustic oscillations at 308 Hz. High-speed 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 relevant 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 (308 Hz), a helical precessing vortex core (PVC) circumscribed the burner nozzle at 515 Hz, and the PVC underwent axial contraction and extension at the thermo-acoustic frequency. The global heat release rate fluctuated at the thermo-acoustic frequency, while the heat release centroid circumscribed the combustor at the difference between the thermoacoustic and PVC frequencies. Hence, the three-dimensional location of the 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 include flames stabilized between the inflow and inner recirculation zone, large-scale flame wrap-up by the PVC, radial deflection of the inflow by the PVC, and combustion in the outer recirculation zones. Large oscillations in the flame surface area were observed at the thermo-accoustic frequency that significantly affected the total heat-release oscillations. By filtering the instantaneous reaction layers at different scales, the importance of the various flow-flame interactions affecting the flame area 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.

The dynamics of major flow structures were studied in a gas turbine model combustor for perfectly premixed swirlstabilized flames under a variety of reacting and non-reacting conditions using high-repetition-rate laser diagnostics. The studied combustor is a target case for the International Workshop on Advanced Measurement Techniques and Computational Methods for Premixed and Partially Premixed Combustion. Measurements were taken of the three-component velocity field, OH planar laser induced fluorescence, and OH* chemiluminescence at a rate of 10 kHz for nine different flow conditions, covering a range of thermal powers (P th = 10 − 35 kW) and equivalence ratios (φ = 0.65 − 0.8). Under all non-reacting conditions, the dominant flow structure was a helical vortex core (HVC) that rotated around the burner at a frequency represented by a constant Strouhal number StH,NR = 0.78. However, igniting the burner significantly altered the flow structures. At most conditions, the strength and frequency of the HVC increased relative to the corresponding non-reacting case. The HVC frequency in such cases was once again represented by a constant Strouhal number of StH,R = 0.88, irrespective of the thermal power or equivalence ratio. The HVC frequency was considerably higher than the frequency of the self-excited thermo-acoustic oscillations exhibited by the burner. However, at other conditions, combustion prevented formation of the HVC. In such cases, the dominant flow structure dynamics were periodic shear layer oscillations and shedding of toroidal vortices at the thermo-acoustic frequency. Cases in which combustion prevented formation of the HVC included those at low thermal powers (P th ≤ 15 kW) and the highest equivalence ratio (φ = 0.8). A distinct relationship was found between the flow structure geometry and the pressure oscillation amplitude, with cases having an HVC resulting in higher pressure oscillations. emissions [1][2][3]. However, lean premixed combustion systems have a tendency to self-excite large amplitude pressure and heat release oscillations, or thermo-acoustic instabilities. These oscillations lead to corresponding oscillations in the mechanical and thermal loads on various engine components, which can cause lifetime reduction and/or premature failure. Thermo-acoustic instabilities also can cause reduced combustion efficiency, increased pollutant emissions, flame blow-off, and flame flash-back [2,3].Unfortunately, the physical phenomena that couple heat release and pressure oscillations to excite thermo-acoustic instabilities are very complex. One of the most important mechanisms is flow-field coupling, wherein thermo-acoustically induced velocity oscillations lead to periodic changes in the heat release rate [4][5][6][7][8][9]. In swirl-stabilized combustors particularly, very complex flow structures can form and interact with the flame [10][11][12][13][14][15][16][17][18][19]. For example, swirl induced vortex breakdown leads to the formation of a large central recirculation zone (CRZ) downstream of t...

The structure and stabilization of heated hydrogen jet flames in heated cross-flows was experimentally investigated in a configuration that is analogous to terrestrial gas turbine components. Three flames, with jet velocities ranging from 100-200 m/s, were investigated using particle image velocimetry and OH planar laser induced fluorescence in a total of 11 x − y and y − z planes. Additionally, laser Raman scattering was performed in the 200 m/s jet to characterize the thermo-chemical state. In all cases, the flame along the jet centerline plane consisted of two branches, one stabilized in the jet lee and one lifted above the jet trajectory. The positional stability of the lee-stabilized branch was greater in the higher jet velocity cases due to the larger and stronger recirculation zones created downstream of the injection point. The lifted flame branch was much more dynamic, with measured flame base axial positions ranging from the jet near field to the flame tip. This flame branch instantaneously resided downstream of regions with high extensive principal strain-rate, and the strain-rate significantly affected the thermo-chemical state. The Raman measurements indicated that the base of the lifted flame branch existed in locations where both tribrachial and/or stratified premixed flame behaviors are expected, depending on the instantaneous flame location. Accurately modeling these complex flame structures and flow-flame interactions therefore is necessary to properly simulate jet flames in cross-flows.

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. Nomenclature a = proper orthogonal decomposition temporal coefficient ap = doubly-phase-resolved mean oscillation D = dissipation rate of acoustic energy f = frequency M = proper orthogonal decomposition spatial eigenmode P th = thermal power p = pressure _ q = heat-release rate S = swirl number t = excess stochastic turbulent fluctuation _ v = volume flow rate = thermoacoustic phase shift = total thermoacoustic energy transfer # = local thermoacoustic energy transfer = azimuthal angle through helical vortex cores = proper orthogonal decomposition eigenvalue = flame surface density = phase angle = total doubly-phase-resolved thermoacoustic coupling = local doubly-phase-resolved thermoacoustic coupling ! = vorticity $ = long time average $ap = sum of long average and doubly-phase-resolved oscillation Subscripts a = acoustic h = helical vortex core qc = heat-release centroid

ii Acknowledgements The fact that the cover page of this thesis reads Vincent R. Caux-Brisebois is a misleading proposition that the content of this work is the product of a single man's endeavour.Although nothing would please me more than adding the names of all those who made this possible, the imposed front-to-back limit of 100 pages would hardly allow the table of contents to be published. Nevertheless, I will try to show my appreciation to all those who made this possible in the short space that is available to me.The second name that would go on the cover page of this document is without doubt Prof. Adam M. Steinberg's. In fact, if the degree of MASc. were not conditional to this dissertation, I would likely insist that it be written above mine. Prof. Steinberg is one of the few supervisors that I have known to always work with his door wide open. At any time during his long working hours, any student can walk in and ask any question knowing he or she will be well received, treated fairly, and perhaps most importantly, get the help they need. The work presented in this thesis would not have been possible without his valuable input, guidance, and hard work. Though I could go on about why Prof. Steinberg is a model supervisor, I have to cut this discussion short for the sake of space, and take this opportunity to thank the second reader of this dissertation, Prof. C. P. T. Groth, for taking some time out of his already-full schedule to review this work.

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Experimental analysis of thermo-acoustic instabilities in a generic gas turbine combustor by phase-correlated PIV, chemiluminescence, and laser Raman scattering measurements Experiments in Fluids 56 (2015)