Dynamics and stability of lean-premixed swirl-stabilized combustion

The interactions between acoustic waves and a premixed combustion process can play an important role in the characteristicunsteadinessofcombustiondevices.Inparticular,theyareoftenresponsiblefortheoccurrenceofselfexcited, combustion-driven oscillations that are detrimental to combustor life and performance. A tutorial review is provided of current understanding of these interactions. First, the mutual interaction mechanisms between the combustion process and acoustic, vorticity, and entropy waves are described. Then, the acoustic‐ e ame interaction literatureisreviewed,primarily focusingon modeling issues.Thisliteratureisessentially organized into fourparts, depending on its treatment of 1) linear or 2) nonlinear analyses of 1) e amelets or 2) distributed reaction zones. A sizeable theoretical literature has accumulated to model the unsteady response of the laminar e ame structure, for example, the burning rate response to pressure perturbations. However, essentially no serious experimental effort has been performed to critically assess these predictions. As such, it is dife cult to determine the state of understanding in this area. On the other hand, good agreement has been achieved between well-coordinated experiments and theory describing the interactions between inherent e ame instabilities and acoustically induced e ow oscillations. Similarly, both the linear and nonlinear kinematic response of simple laminar e ames to acoustic velocity disturbances appear to be well understood, as evidenced by the agreement between surprisingly simple theory and experiment. Other than kinematic nonlinearities, additional potential mechanisms that introduce heat release‐ acoustic nonlinearities, such as e ame holding, or extinction, have been analyzed theoretically, but lack experimentalverie cation.Unsteadyreactormodelshavebeenusedextensivelytomodelcombustionprocessesinthe distributed reaction zone regimes. None of thesepredictionsappears to have been subjected to direct experimental scrutiny. Itisunlikelythatthismodeling approach willbeusefulforquantitativecombustion responsecalculations, due to their largely heuristic nature and the dife culty in rationally modeling the key interactions between reaction rate and the global characteristics of the combustion region, such as its volume. Several areas in need of work are particularly highlighted. These include e nite amplitude effects, modeling approaches for interactions outside of the e amelet regime, turbulent e ame wrinkling effects, and unsteady vortex‐ e ame interactions.

Modeling Premixed Combustion-Acoustic Wave Interactions: A Review

Recent advances in coherent anti-Stokes Raman scattering spectroscopy: Fundamental developments and applications in reacting flows

In many continuous combustion processes, such as those found in aeroengines or gas turbines, the flame is stabilized by a swirling flow formed by aerodynamic swirlers. The dynamics of such swirling flames is of technical and fundamental interest. This article reviews progress in this field and begins with a discussion of the swirl number, a parameter that plays a central role in the definition of the flow structure and its response to incoming disturbances. Interaction between the swirler response and incoming acoustic perturbations generates a vorticity wave convected by the flow, which is accompanied by azimuthal velocity fluctuations. Axial and azimuthal velocities in turn define the flame response in terms of heat--release rate fluctuations. The nonlinear response of swirling flames to incoming disturbances is conveniently represented with a flame describing function (FDF), in other words, with a family of transfer functions depending on frequency and incident axial velocity amplitudes. The FDF, howev...

Dynamics of Swirling Flames

The Theory of Sound, Two Volumes In One

Linear instability analysis was performed to investigate the origin of the self-sustained oscillations, at St=O(0.1), which have been widely reported in backward-facing step flows. Parametric studies, based on local stability analysis of a family of time-average velocity profiles modeling those observed in recirculating flows, show that the frequency of the absolute mode is determined primarily by the shear layer thickness, and the growth rate of the absolute mode is controlled by the amount of backflow. Given the known streamwise variation of the local velocity profile in the actual flow, this implies that the oscillations are likely to be generated at the middle of the recirculation zone, where the backflow is sufficiently strong, and shear layer thickness is comparable to the step height. The corresponding frequency is determined to be St=O(0.1), because the shear layer thickness is bounded by the step height. To verify this hypothesis, mean velocity profiles obtained from a two-dimensional numerical s...

Self-sustained oscillations and vortex shedding in backward-facing step flows: Simulation and linear instability analysis

Combustion instability arises mostly due to the coupling between heat-release dynamics and system acoustics. In these cases, the acoustic field acts as the resonator, while the heat-release dynamics, induced due to the impact of pressure or flow velocity perturbations on the equivalence ratio, flame length, residence time, shear layer, etc., supplies energy to support pressure oscillations. In this paper, we discuss another mechanism in which the resonator may not be the acoustic field: instead, it is an absolutely unstable shear layer mode acting as the source of sustained oscillations; which morph as large-scale eddies at a frequency different from acoustic modes. These perturb the flame motion and provide energy to the acoustic field, acting here as an amplifier to support pressure oscillations. We present evidence for the existence of this mechanism from several experiments, in which a pressure spectral peak can not be explained using acoustic analysis of the system, but instead matches the predicted absolute instability mode of the separated shear layer, and from numerical simulations. We also examine the impact of the mean velocity and temperature distribution on the frequency and growth rate to determine the dynamics leading to an absolute instability, and show that absolutely unstable modes are likely to arise at low-equivalence ratios. In some cases, they can also be present at near-stoichiometric conditions, that is, only low- and near-unity stoichiometry can support an absolutely unstable mode. We use numerical simulation to distinguish between different unstable modes in the separating shear layer, the layer mode and the wake mode, and derive a reduced-order model for the shear-driven instability using proper orthogonal decomposition analysis. Estimates of pressure amplitudes of fluid dynamic modes, when applied as forcing functions to the acoustic field, using this analysis compare favorably with experimental data.

/pdf/shear-flow-driven-combustion-instability-evidence-simulation-3jsz9ohcqo.pdf

Shear flow-driven combustion instability: Evidence, simulation, and modeling

Picosecond-laser electronic-excitation tagging (PLEET), a seedless picosecond-laser-based velocimetry technique, is demonstrated in non-reactive flows at a repetition rate of 100 kHz with a 1064 nm, 100 ps burst-mode laser. The fluorescence lifetime of the PLEET signal was measured in nitrogen, and the laser heating effects were analyzed. PLEET experiments with a free jet of nitrogen show the ability to measure multi-point flow velocity fluctuations at a 100 kHz detection rate or higher. Both spectral and dynamic mode decomposition analyses of velocity on a Ma=0.8 free jet show two dominant Strouhal numbers around 0.24 and 0.48, respectively, well within the shear-layer flapping frequencies of the free jets. This technique increases the laser-tagging repetition rate for velocimetry to hundreds of kilohertz. PLEET is suitable for subsonic through supersonic laminar- and turbulent-flow velocity measurements.

Seedless velocimetry at 100 kHz with picosecond-laser electronic-excitation tagging

The present paper addresses real-time prediction of incipient lean blowout in partially premixed, liquid-fueled gas turbine combustors. Near-lean-blowout combustion is characterized by the intensified, low-frequency combustion oscillations (typically, below 30 Hz). Two indices, namely, the normalized chemiluminescence root mean square and the normalized cumulative duration of lean blowout precursor events, are recommended for lean blowout prediction. Both indices are associated with the statistical characteristics of the flame structure, which changes from the normal distribution to the Rayleigh distribution at the approach of lean blowout. Both indices change little within a large range of equivalence ratios and start to shoot up only when lean blowout is approached. To use the two indices for lean blowout prediction, one needs to perform a detailed a priori lean blowout mapping undersimulated engine operating conditions. However, the mapping can be done without running the engines very close to lean blowout.

Real-Time Prediction of Incipient Lean Blowout in Gas Turbine Combustors

The present paper reports flame structure and combustion instability characteristics in a turbulent liquid-fueled swirl-stabilized lean direct fuel injection combustor. Because of the complexities in droplet size and distribution, evaporation, droplets–vapor–air entrainment and mixing, and droplets–flame–turbulence interactions, the flame structure is remarkably different from lean premixed gas-fueled combustion. With the preheat temperature above 423K, heat release is completed within a compact doughnut ring, about 12mmdownstream of the dump plane; with decreasing equivalence ratios at the approach of lean blowout, the doughnut ring shrinks in diameter and gradually converges to the axis; after that, the heat release zone becomes elongated and tilted off the axis. Chemiluminescence only provides qualitative information of the heat release rate, suggesting the limitations of global chemiluminescence measurements for spray combustion. Phase-locked intensified charge coupled device imaging of CH chemiluminescence shows that during combustion instability there aremainly variations in the chemiluminescence intensity rather than in the spatial distribution of heat release. Depending on the working conditions, the one-wave mode, the half-wave mode, or both of the combustion chambers can be excited. Although the combustion chamber is highly blocked at both ends, no pressure antinodes are found at the chamber exit and the chamber inlet. Simultaneous excitation of the one-wave mode and the half-wave mode is intrinsically unsteady and unstable, which can be attributed to the nonlinear response of the reacting swirling shear layer to acoustic oscillations. Both the amplitude and the frequency of thermoacoustic oscillations are constantly time-varying. With decreasing oscillation intensity, the limit-cycle oscillator becomes increasingly vulnerable to external disturbances, as indicated by a larger neighborhood of the state trajectory.

Tongxun Yi

Papers

Self-sustained oscillations and vortex shedding in backward-facing step flows: Simulation and linear instability analysis

Shear flow-driven combustion instability: Evidence, simulation, and modeling

Seedless velocimetry at 100 kHz with picosecond-laser electronic-excitation tagging

Real-Time Prediction of Incipient Lean Blowout in Gas Turbine Combustors

Combustion Instability and Flame Structure of Turbulent Swirl-Stabilized Liquid-Fueled Combustion