Search

The mechanisms of flame blowout under pressure gradient effects are explored for a bluff-body stabilized flame. The blowout process is induced through equivalence ratio reduction from a lean stabilized flame to complete blowout. Simultaneous high-speed particle image velocimetry (PIV) and C2*/CH* chemiluminescence imaging diagnostics are used to obtain the instantaneous flame structure, vorticity field, equivalence ratio, and local strain rate during the extinction process. The goal is to elucidate the effect of flame-generated vorticity on lean flame extinction. Three test-sections configured as a nozzle, a rectangular duct, and a diffuser, are used to alter the downstream pressure gradient yielding high, nominal, and low magnitudes of flame-generated baroclinic torque. For all three configurations, the flame brush narrows and the shear layer vorticity expands in the transverse direction resulting in flame-shear interactions and extinction. The flame-shear layer interaction increases the strain rate along the flame; however, the strong flame-generated vorticity for the nozzle case delayed the strain rate increase by keeping the flame away from the shear layer the longest. The sharp increase in the Karlovitz number above unity caused by the sudden increase in the strain rate corresponds to the time of flame brush contraction and shear layer width expansion. It is shown that the downstream pressure gradient can either augment or attenuate the time required for the Karlovitz number to reach a critical value of unity, which is associated with local extinctions along the flame. In all of the test-section configurations, the flame-generated vorticity has a weak influence on the Benard von Karman (BVK) instability mode and its harmonics. The Strouhal number during blow-out remained relatively constant in all of the cases showing greater sensitivity to the shear layer length than to the BVK frequency.

Pocket formation is an important characteristic of turbulent premixed flames and understanding pocket behavior is key to developing high-fidelity numerical combustion models. In this study, a dual-burner experiment is used to study pockets in single- and dual-flame configurations and synchronized high-speed OH-planar laser-induced fluorescence and stereoscopic-particle image velocimetry imaging techniques are implemented to track flame pockets and the surrounding flow field. Statistical analysis of pocket origin and fate is performed using a novel tracking algorithm incorporating non-rigid image registration. Results show that pocket formation rates increase as a function of increasing inlet turbulence level; reactant pocket formation increases as a function of downstream distance, whereas product pocket formation decreases. Tracking reactant pocket lifetime shows that a majority of these pockets burn out and displacement speeds are characterized. Product pockets usually merge with the main flame surface, which could have an impact on local flame structure and propagation. Results presented in this study show that pocket behavior in turbulent flames can change local flame dynamics and it is important to capture these effects in sub-grid scale combustion models to accurately predict flame behavior.

Flame–flame interaction events occur frequently in turbulent premixed flames and change the local structure and dynamics of flames. It is essential to understand these flame–flame interaction events to develop high-fidelity combustion models for use in modern combustion devices. In this study, we experimentally investigate the topology of flame–flame interaction events in single- and multi-flame configurations. A dual-burner experiment is probed with high-speed OH-planar laser-induced fluorescence and stereoscopic-particle image velocimetry to obtain simultaneous flame front locations and velocity fields. A non-rigid image registration technique is implemented to track the topological changes occurring in these flames. In both single- and dual-flame configurations, small-scale interactions occur more frequently compared to large-scale interactions, and statistics show that most of the reactant-side interactions contribute to large flame surface destructions than the product-side interactions. It is also found that turbulence length- and velocity-scales can play an important role in facilitating the interaction events and pocket formations from these events. Filamentarity is used to quantify the two-dimensional shape of these interactions and comparisons are made between the orientation and shape of interaction events and the local turbulence in the flowfield. Alignment between the orientation of the interaction shapes and the principal strain rates show that compressive fluid forces drive both types of interaction events.

The PRECCINSTA GTMC was studied at elevated pressure and power density with 6 kHz stereoscopic particle image velocimetry (SPIV), OH* chemiluminescence (CL), and 100 kHz dynamic pressure measurements. This technically premixed, swirl stabilized flame exhibited self-excited thermoacoustic oscillations with limit-cycle behavior. A helical precessing vortex core (PVC) was detected within the inner shear layer, between the central recirculation bubble (CRB) and the reactant jets. The PVC was found to be the delineating flow feature for combustion dynamics even at elevated pressure. Sparse dynamic mode decomposition (DMD) of the velocity fields deconvolved the dynamics into a thermoacoustic and PVC mode. The precession of the PVC was at a non-harmonic frequency to the thermoacoustic oscillations, and at least twice that of findings at atmospheric conditions. Nevertheless, the continuous and persistent structure of the PVC allows it promote unsteady heat release to sustain the thermoacoustic cycle. The three dimensional structure of the reactant jets, central recirculation bubble, and PVC was reconstructed by double phase conditioning the reconstructed velocity field. The surface of the CRB was observed to transition between asymmetric and symmetric states depending on the thermoacoustic phase. Analysis of the swirling strength values on the CRB surface indicates the interaction strength between the hydrodynamic structures of the PVC and CRB. When this coupling is large, the heat release determined by the mean OH*-CL intensity is maximum. These findings indicate a critical role of the PVC and CRB interaction on combustion in unstable swirl flames at conditions closer to those found in a modern gas turbine engine.

A dual burner experiment is used to investigate how flow interactions affect local flame-flame interaction in turbulent premixed flames. The presence of adjacent flows influences the local structure of these flames and understanding the sensitivity of these flames to adjacent flows is essential for multi-nozzle combustion devices. To study this sensitivity, a high-aspect-ratio Bunsen flame operating at a constant flow velocity is placed adjacent to an identical burner with non-reacting flow at varying velocities. High-speed OH-planar laser induced fluorescence and stereoscopic-particle image velocimetry measurements are performed to capture flame-front locations and velocity fields. A non-rigid image registration technique is used to calculate the local flame-area variations that occur due to topological differences, and conditional statistics are extracted to relate the local behavior to changes observed in the global behavior of the flames. Extracted flame curvatures and time-averaged progress variables conditioned on flame-flame interactions show differences existing in the inner and outer flame branches near the flame-attachment region. Statistics of these results are presented and compared for all test cases.

Single-laser-shot temperature measurements at 5 kHz were performed in a gas turbine model combustor using femtosecond (fs) coherent anti-Stokes Raman scattering (CARS). The combustor was operated at two conditions; one exhibiting a low level of thermoacoustic instability and the other a high level of instability. Measurements were performed at 73 locations within each flame in order to resolve the spatial flame structure and compare to previously published studies. The measurement procedures, including the procedure for calibrating the laser system parameters, are discussed in detail. Despite the high turbulence levels in the combustor, signals were obtained on virtually every laser shot, and these signals were strong enough for spectral fitting analysis for determination of flames temperatures. The spatial resolution of the single-laser shot temperature measurements was approximately 600 µm, the precision was approximately ±2%, and the estimated accuracy was approximately ±3%. The dynamic range was sufficient for temperature measurements ranging from 300 K to 2200 K, although some detector saturation was observed for low temperature spectra. These results demonstrate the usefulness of fs-CARS for the investigation of highly turbulent combustion phenomena. In a companion paper, the time-resolved fs CARS data are analyzed to provide insight into the temporal dynamics of the gas turbine model combustor flow field.

A detailed analysis of the flow–flame interactions associated with acoustically coupled heat-release rate fluctuations was performed for a 10 kW, CH4/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 thermo-acoustic 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.