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 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.
Temporally resolved planar measurements of transient phenomena in a partially pre-mixed swirl flame in a gas turbine model combustor, Combust. Flame 157 (2010)
Lean blowout (LBO) of a partially premixed swirl flame is studied using chemiluminescence imaging and simultaneous stereo-PIV and OH-PLIF measurements at repetition rates up to 5 kHz. The flame, which is operated with methane and air in a gas turbine model combustor at atmospheric pressure, features a pronounced precessing vortex core (PVC) at the inner shear layer. In the first part of the study, the stabilization mechanism of the flame close to LBO is investigated. The fields of velocity and OH show that near LBO there are essentially two regions where reaction takes place, namely the helical zone along the PVC and the flame root around the lower stagnation point. The zone along the PVC is favorable to the flame due to low strain rates in the vortex center and accelerated mixing of burned and fresh gas. The flame root, which is located close to the nozzle exit, is characterized by an opposed flow of hot burned gas and relatively fuel-rich fresh gas. Due to the presence of high strain rates, the flame root is inherently unstable near LBO, featuring frequent extinction and re-ignition. The blowout process, discussed in the second part of the study, starts when the extinction of the flame root persists over a critical length of time. Subsequently, the reaction in the helical zone can no longer be sustained and the flame finally blows out. The results highlight the crucial role of the flame root, and suggest that well-aimed modifications of flow field or mixture fraction in this region might shift the LBO limit to leaner conditions.
A series of measurements was performed on two technically-premixed, swirl-stabilized methaneair flames (at overall equivalence ratios of = 0.73 and 0.83) in an optically accessible gas turbine model combustor. The primary diagnostics used were combined planar laser-induced fluorescence (PLIF) of the OH radical and stereoscopic particle image velocimetry (PIV) with simultaneous repetition rates of 10 kHz and a measurement duration of 0.8 seconds. Also measured were acoustic pulsations and OH chemiluminescence. Analysis revealed strong local periodicity in the thermoacoustically self-excited (or 'noisy') flame ( = 0.73) in the regions of the flow corresponding to the inner shear layer and the jet-inflow. This periodicity appears to be the result of a helical precessing vortex core (PVC) present in that region of the combustor. The PVC has a precession frequency double (at 570 Hz) that of the thermoacoustic pulsation (at 288 Hz). A comparison of the various data sets and analysis techniques applied to each flame suggests a strong coupling between the PVC and the thermoacoustic pulsation in the noisy flame. Measurements of the stable ('quiet') flame ( = 0.83) revealed a global fluctuation in both velocity and heat-release around 364 Hz, but no clear evidence of a PVC.
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
Dynamic processes in gas turbine (GT) combustors play a key role in flame stabilization and extinction, combustion instabilities and pollutant formation, and present a challenge for experimental as well as numerical investigations. These phenomena were investigated in two gas turbine model combustors for premixed and partially premixed CH 4 /air swirl flames at atmospheric pressure. Optical access through large quartz windows enabled the application of laser Raman scattering, planar laser-induced fluorescence (PLIF) of OH, particle image velocimetry (PIV) at repetition rates up to 10 kHz and the simultaneous application of OH PLIF and PIV at a repetition rate of 5 kHz. Effects of unmixedness and reaction progress in lean premixed GT flames were revealed and quantified by Raman scattering. In a thermoacoustically unstable flame, the cyclic variation in mixture fraction and its role for the feedback mechanism of the instability are addressed. In a partially premixed, oscillating swirl flame, the cyclic variations of the heat release and the flow field were characterized by chemiluminescence imaging and PIV, respectively. Using phase-correlated Raman scattering measurements, significant phase-dependent variations of the mixture fraction and fuel distributions were revealed. The flame structures and the shape of the reaction zones were visualized by planar imaging of OH distribution. The simultaneous OH PLIF/PIV high-speed measurements revealed the time history of the flow field-flame interaction and demonstrated the development of a local flame extinction event. Further, the influence of a precessing vortex core on the flame topology and its dynamics is discussed.
The mechanisms of transient formation and oxidation of soot in an aero-engine model combustor at elevated pressure are studied for the first time using a combination of high-speed simultaneous stereo-PIV and OH-PLIF and results from a recent detailed LES. A combined analysis of experiment and LES shows that the highly transient and intermittent evolution of soot in this combustor is governed by an unsteady interplay of distinct pockets of burned gas in the inner recirculation zone (IRZ) with either relatively rich or relatively lean composition. The former originate from reaction of fuel-rich unburned gas, whereas the latter result from additional admixture of secondary air further downstream. The analysis further enables distinction and localization of premixed and diffusion-type flame fronts within the flame zone. The time-resolved complementary measurements of velocity field and flame structure allow accurate tracking of both the burned gas pockets and soot filaments. It is seen that soot generally forms in the rich pockets if their residence time in the IRZ is sufficient, whereas oxidation occurs in the lean zones carrying OH. Correlating the dynamics of flow field and soot indicates that the intermittency of soot is driven by an intermittent flow of lean burned gas into the IRZ that affects the residence time of rich pockets. The results suggest that the formation of soot might be further reduced by a proper adjustment of secondary air injection aiming at a sufficient and more constant recirculation of lean burned gas. A remarkably good agreement of measured and simulated instantaneous flame structures is observed, indicating that flow field and gas-phase reactions are well predicted by the LES. The experimental insights into the transient mechanisms of soot formation and oxidation, on the other hand, may provide useful input for LES soot models where deviations from measurements are generally larger.
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