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The generation and turbulent transport of temporal equivalence ratio fluctuations in a swirl combustor are experimentally investigated and compared to a one-dimensional transport model. These fluctuations are generated by acoustic perturbations at the fuel injector and play a crucial role in the feedback loop leading to thermoacoustic instabilities. The focus of this investigation lies on the interplay between fuel fluctuations and coherent vortical structures that are both affected by the acoustic forcing. To this end, optical diagnostics are applied inside the mixing duct and in the combustion chamber, housing a turbulent swirl flame. The flame was acoustically perturbed to obtain phase-averaged spatially resolved flow and equivalence ratio fluctuations, which allow the determination of flux-based local and global mixing transfer functions. Measurements show that the mode-conversion model that predicts the generation of equivalence ratio fluctuations at the injector holds for linear acoustic forcing amplitudes, but it fails for non-linear amplitudes. The global (radially integrated) transport of fuel fluctuations from the injector to the flame is reasonably well approximated by a one-dimensional transport model with an effective diffusivity that accounts for turbulent diffusion and dispersion. This approach however, fails to recover critical details of the mixing transfer function, which is caused by non-local interaction of flow and fuel fluctuations. This effect becomes even more pronounced for non-linear forcing amplitudes where strong coherent fluctuations induce a non-trivial frequency dependence of the mixing process. The mechanisms resolved in this study suggest that non-local interference of fuel fluctuations and coherent flow fluctuations is significant for the transport of global equivalence ratio fluctuations at linear acoustic amplitudes and crucial for non-linear amplitudes. To improve future predictions and facilitate a satisfactory modelling, a non-local, two-dimensional approach is necessary. [ABSTRACT FROM AUTHOR]

A measurement technique for determination of the global and local equivalence ratios from the flame chemiluminescence for a swirl-stabilized lean premixed combustion of natural gas and kerosene is presented. First, we conducted spectrally resolved chemiluminescence studies using an imaging spectrometer to correlate the ratio of individual chemiluminescence signals to the equivalence ratio. Flame spectra were recorded for a multitude of different lean operating conditions for natural gas and kerosene combustion. The spectra show that, without background correction, the CH*/CO2* ratios for both natural gas and kerosene combustion exhibited a monotonic relationship to the equivalence ratio in the investigated range. Subsequently, bandpass-filtered images of CH* and CO2* chemiluminescence were acquired simultaneously on one camera chip using an image doubler to investigate the local relationship of the CH*/CO2* ratio with the equivalence ratio. The ratio images corroborate the monotonic relationship of the CH*/CO2* ratio to the equivalence ratio. Furthermore, the ratio was found to be influenced by the local reaction zone temperature. The presented technique allows high temporal resolution determination of the local equivalence ratio in lean premixed natural gas and kerosene flames and can thus be applied to quantify equivalence ratio oscillations during unstable combustion.

The combined effects of velocity fluctuations (VF) and equivalence ratio fluctuations (ERF) on thermoacoustic flame oscillations are studied experimentally in a gas turbine model combustor. The analysis is based on time-resolved simultaneous measurements of flow field, flame structure and fuel distribution using combined PIV, OH-PLIF and tracer-PLIF at a rate of 10 kHz. In order to separate the effects of VF and ERF, the combustor has been operated in two configurations: a technically (i.e. partially) premixed (TP) configuration where both ERF and VF occur, and a perfectly premixed (PP) configuration where only VF are present. A particular operating condition is selected where the VF are similar for both premixing modes. The corresponding fluctuations of heat release, by contrast, exhibit considerably different spatial patterns for the two modes. In particular, periodic widening and narrowing as well as extinction and re-ignition of the flame base, and a convective motion of the flame zone are observed in the TP flame but not in the PP case. An evaluation of periodic variations of flux rates and heat release shows that the feedback loop of the TP flame includes an additional convective delay of 50°. The time-resolved measurements reveal that this delay leads to a coincidence of fuel-lean unburned gas and high velocity that induces local flame extinction and narrowing of the flame base. When the equivalence ratio later increases, the flame base re-ignites and a widened zone of increased heat release forms that is convected downstream. For the PP case, by contrast, reaction at the flame base is stable and accordingly the flame responds earlier to VF compared to the TP case. The results show that interacting VF and ERF have a strong and complex impact on the thermoacoustic response of turbulent swirl flames, and this impact depends largely on their time-delay. [ABSTRACT FROM AUTHOR]

Turbulent swirl flames are known to exhibit intermittent transitions between V- and M-shapes at certain operating conditions. A diagnostic apparatus based on a commercial highspeed laser system is developed in this work to investigate the influence of such transient phenomenon on the surface temperature of an atmospheric-pressure gas turbine model combustor. The transition of flame shapes is monitored by planar OH laser-induced fluorescence (OH PLIF) at 1-kHz repetition rate. Simultaneously, the same laser beam is used to perform 1-D surface thermometry on the base plate and the bluff body of the combustor, which are coated with Mg 4 FGeO 6 :Mn, a commonly used thermographic phosphor (TP). Temperature is inferred from the decay of phosphorescence recorded by a highspeed CMOS camera operated at 70 kHz. During transitions between attached (V-) and lifted (M-) flames, pronounced temperature variation as much as 100 K is observed on the tip of the bluff body. In the spectral domain, temperature measured during the M-flame period exhibits a peak at 475 Hz, in good agreement with the measured frequency of the precessing vortex core (PVC) at the same operating condition. This peak does not appear during the V-flame period when PVC is suppressed. The uncertainty of the measurements and the sensitivity of the technique are discussed in detail. [ABSTRACT FROM AUTHOR]

The 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. [ABSTRACT FROM AUTHOR]

Unsteady interactions of flow, fuel–air mixing and reaction in a lean partially-premixed turbulent swirl flame are investigated using simultaneous particle image velocimetry and planar laser-induced fluorescence of OH and acetone with a repetition rate of 10 kHz. The flame is operated with methane and air in a gas turbine model combustor at ambient temperature and pressure. Transport and mixing of fuel is visualized by fluorescence of acetone, which is added with 9% by volume to the methane. The dominant unsteady flow structures are a precessing vortex core (PVC) in the shear layer of the inner recirculation zone and the lower stagnation point (LSP) at the flame root. The measurements show that fuel and air are largely separated at the inlet of the combustion chamber, but are then strongly mixed by the PVC. A well-mixed zone of unburned fuel and air is formed around the vortex, and then ignited by recirculating burned gas. At the flame root, the PVC induces periodic changes in the composition of the unburned gas, which varies between pure air and well-mixed fuel and air. Reaction is locally quenched at the LSP when fresh air without fuel is present, and re-ignition takes place when mixed fuel and air arrive at the boundary of recirculating burned gas. Generally, the results show that the enhancement of fuel–air mixing induced by the PVC contributes significantly to the stabilization of the flame, and that the flame dynamics can only be properly understood when an analysis of transient mixing mechanisms is included. [ABSTRACT FROM AUTHOR]

Abstract: The mechanisms of the interaction between a precessing vortex core (PVC) and a turbulent swirl flame in a gas turbine model combustor are investigated experimentally using simultaneous PIV and OH-PLIF at repetition rates up to 10kHz. Three operating conditions with different flow rates corresponding to Damköhler numbers of 1, 0.5 and 0.3 are studied. The flames are mainly stabilized in the shear layer of the inner recirculation zone, where also the PVC is located. The measurements show that for all conditions, the PVC interacts strongly with the flame. Two main effects have been observed: On the one hand, the PVC leads to roll-up of the reaction zone at the flame base. This enhances the supply of heat and radicals to the unburned gas, and thus favors subsequent ignition of the unburned gas. On the other hand, the PVC causes considerable aerodynamic stretch of the reaction zones. This can lead locally to disruption, quenching or extinction of the flame. The strength of the two effects changes significantly when the Damköhler number changes. For the case Da=1, the flame roll-up due to the PVC leads to a strong increase of heat release around the vortex. The stretch rates are relatively low and do not strongly affect the reaction progress. In the case Da=0.5, the PVC initially also causes flame roll-up at the flame base. Subsequently, however, roll-up and propagation of the flame are affected by local disruptions and quenching of the reaction zone due to the increased vortex-induced aerodynamic stretch. Consequently, part of the heat release is delayed and shifted to locations further downstream. For Da=0.3, aerodynamic stretch rates further increase and the reaction zones are even more affected by local disruptions and quenching. The zone of heat release is therefore even further elongated in the direction of the flow. [Copyright &y& Elsevier]

A measurement technique for determination of the global and local equivalence ratios from the flame chemiluminescence for a swirl-stabilized lean premixed combustion of natural gas and kerosene is presented. First, we conducted spectrally resolved chemiluminescence studies using an imaging spectrometer to correlate the ratio of individual chemiluminescence signals to the equivalence ratio. Flame spectra were recorded for a multitude of different lean operating conditions for natural gas and kerosene combustion. The spectra show that, without background correction, the CH*/CO 2 * ratios for both natural gas and kerosene combustion exhibited a monotonic relationship to the equivalence ratio in the investigated range. Subsequently, bandpass-filtered images of CH* and CO 2 * chemiluminescence were acquired simultaneously on one camera chip using an image doubler to investigate the local relationship of the CH*/CO 2 * ratio with the equivalence ratio. The ratio images corroborate the monotonic relationship of the CH*/CO 2 * ratio to the equivalence ratio. Furthermore, the ratio was found to be influenced by the local reaction zone temperature. The presented technique allows high temporal resolution determination of the local equivalence ratio in lean premixed natural gas and kerosene flames and can thus be applied to quantify equivalence ratio oscillations during unstable combustion. [ABSTRACT FROM AUTHOR]