219 research outputs found
Premixed flames excited by helical disturbances: flame wrinkling and heat release oscillations
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Flame sheet dynamics of bluff-body stabilized flames during longitudinal acoustic forcing
No full textBluff-body stabilized flames are susceptible to combustion instabilities due to interactions between acoustics, vortical disturbances, and the flame. In order to elucidate these flow-flame interactions during an instability, an experimental and computational investigation of the flame-sheet dynamics of a harmonically excited flame was performed. It is shown that the flame dynamics are controlled by three key processes: excitation of shear layer instabilities by the axially oscillating flow, anchoring of the flame at the bluff body, and the kinematic response of the flame to this forcing. The near-field flame features are controlled by flame anchoring and the far-field by kinematic restoration. In the near-field, the flame response grows with downstream distance due to flame anchoring, which prevents significant flame movement near the attachment point. Theory predicts that this results in linear flame response characteristics as a function of perturbation amplitude, and a monotonic growth in magnitude of the flame-sheet fluctuations near the stabilization point, consistent with the experimental data. Farther downstream, the flame response reaches a maximum and then decays due to the dissipation of the vortical disturbances and action of flame propagation normal to itself, which acts to smooth out the wrinkles generated by the harmonic flow forcing. This behavior is strongly non-linear, resulting in significant variation in far-field flame-sheet response with perturbation amplitude.<br/
Modeling the Response of Turbulent Flames to Harmonic Forcing
No full textThis article analyzes the response of a turbulent, premixed flame to harmonic forcing. This problem has been worked extensively for laminar flames, and the key parameters influencing the flame transfer function are well understood. For turbulent flames, several prior studies have utilized a “quasi-laminar” approach, by utilizing the time-averaged flame position and ensemble-averaged disturbance field, as inputs to what is otherwise identical to the laminar problem. More generally, the manner in which turbulent flames respond to harmonic disturbances is not amenable to analytical solutions because of the nonlinear interactions between stochastic flow disturbances and harmonic flame wrinkling. We utilize a turbulent burning velocity closure proposed by Shin and Lieuwen (2013), who showed that the ensemble-averaged turbulent burning rate for a harmonically forced flame is proportional to the ensemble-averaged flame curvature. Shin and Lieuwen (2013) previously used it to analyze the ensemble-averaged space-time flame wrinkle characteristics. Here, we extend these results to analyze the spatial variation of ensemble-averaged flame surface area and burning rate and then compare these results to computations. These results show that, for low stochastic forcing amplitudes, wrinkling of the front exerts quantitative differences between those predicted by a quasi-laminar and the actual flame response (e.g., reducing peak values of the flame transfer function and eliminating nodes), but does not change the key qualitative features. While this result needs to be considered for strongly turbulent flames, it does suggest why good agreement has been observed between quasi-laminar approaches and experimental data for harmonically excited, turbulent flames. Two results for model problems showing the linearized flame transfer functions are also presented, which explicitly demonstrate qualitative turbulence effects on harmonically excited flames
Partially Premixed and Premixed Aero Engine Combustors
No full textThis chapter reviewed partially premixed and premixed aero engine combustors. Apart from a description of the state of the art, it tried to explain how the design constraints of airborne combustors enforced the breed of premixing combustors that will soon enter service and why the NOx levels emitted from airplanes will continue
to be so different from those of industrial gas turbines. The efficiency required from aero engines of long-range aircraft forces exit temperatures that cannot be sustained
by pure convective cooling with the airworthy materials available today. The airflow for combustion thus being reduced by the necessary film cooling air results in combustion temperatures above the onset of thermal NOx production, which must be limited by a short residence time. As premixing results in poor stability, enforcing piloted combustion, the trade-off between NOx emission at full power and combustion efficiency at cruise defines combustor size and premixed combustion temperature. This strongly favors single annular combustors. Autoignition times of kerosene at high-power conditions preclude full premixing in premixing channels before the combustor that could be safe from flashback. Premixing is therefore completed in a combustor with lifted flames. Flexible internal piloting has to ensure stability and the appropriate flame lift-off. A multitude of operability requirements, including ignition, absence of combustion oscillations, and thermal management, has to be solved before airworthiness is reached. All of those have a potentially detrimental effect on NOx reduction. Together with the moving target of engine pressures and combustor exit temperatures, this explains why it took so long for the technology to mature enough to be introduced replacing optimized RQL combustion with a step change in NOx emission. Having made the transition into service, it can be
expected that further emission reductions will be possible with a further optimization using refined design tools and a penetration of lower thrust classes with premixing combustors can be expected
Development of Acoustic Based, Multi-tasking Sensing and Actuation Capabilities for Gas Turbines
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