Operation under stable conditions is an important prerequisite for gas turbine safety. While recent studies have focused primarily on acoustic design to avoid thermoacoustic instabilities, in the present study we shift the focus to improving stability margins by flame transfer function (FTF) modification. The flame transfer function of premixed flames is affected by various mechanisms such as variations in equivalence ratio, swirl fluctuations and shear layer instabilities. These mechanisms can be influenced by modifying parameters such as fuel distribution, injection location, swirl number or gas composition. Based on the Nyquist stability method we formulate criteria for how and at what frequencies the flame transfer function needs to be modified, in order to increase the stability margins of a thermoacoustic system. Gain and phase margin as well as the sensitivity function serve as measures of stability. The criteria are limited to single frequencies, which allows experimental FTF optimization with manageable effort. In the second part of this study it is shown that the Nyquist method can also be used as an efficient and compact way to determine whether the uncertainties of subsystems can affect the overall stability, without requiring eigenvalue calculations.
In gas turbine combustion systems, the reduction of emissions of harmful combustion by-products is a main development goal. This study provides a methodology to model NOX emissions effectively for varying levels of pilot fuel flows at different operational points. It combines one-dimensional flame simulations using detailed chemistry with a stochastic approach for equivalence ratio fluctuations to account for the effect of fuel-air unmixedness. This split allows for computationally fast variations of the gas inlet condition and the consideration of different shares of pilot gas. The generation of emissions is split into a share of prompt formation at the flame front and a slower formation mechanism, occurring within the combustion products in the post flame region. The influence of unmixedness of the fuel-air mixture on both effects is taken into consideration by means of probability density functions (PDFs) of the equivalence ratio. These are modeled on the basis of sampled values from Large Eddy Simulations at the flame front and adapted for different shares of pilot gas. It is shown that with a superposition of Gaussian PDFs the equivalence ratio distribution at the flame front resulting from the main gas supply and the pilot share can be well approximated. Measurement data from experiments in atmospheric conditions as well as emission measurements from high pressure tests are used to evaluate the model. Good agreement is found for atmospheric data, allowing for explanations on the effect of pilot fuel ratio on emissions. For elevated pressure, only qualitative trends could be reproduced. Hypotheses to explain this deviation are made that motivate further research.
A turbulent, swirl-stabilized jet emanating from an unconfined, premixed burner is investigated experimentally by means of optical (OH* chemiluminescence), acoustic (microphone), and laser-optical measurement techniques (Particle Image Velocimetry) for various swirl intensities. It is shown that even in case of an unconfined swirl flame, combustion-induced vortex breakdown (CIVB) occurs which, on one hand, contributes to the stabilization of the flame and stable combustion due to the increased recirculation zone and, on the other hand, can promote flashback if the recirculation zone travels upstream or is extended upstream, especially for high swirl intensity flows. Another effect associated to CIVB is the generation of vortex breakdown for low Reynolds number, reacting flows which corresponding non-reacting, isothermal flows at the some operating conditions do not create a recirculation zone at all. After crossing a threshold of injected momentum, i.e. Reynolds number, normalized flow fields become Reynolds number independent. It is found that noise emissions from the burner grow with increase in different parameters: equivalence ratio of the injected and burnt mixture, Reynolds number, and number of vanes on the swirler disc. All three parameters cause an extended area of heat release which is supposed to generate larger pressure oscillations and, hence, more noise. Nomenclature d = nozzle diameter f = frequency G' x = axial flux of linear momentum G' = axial flux of axial momentum R = radius of the nozzle outlet R H = radius of the swirler hub Re = Reynolds number r = radius r/d = relative radial location with respect to nozzle diameter S = swirl number T = preheat temperature Tu = degree of turbulence/ turbulence level u = axial velocity u bulk = bulk velocity of the jet u/u bulk = normalized axial velocity u RMS = fluctuation of axial velocity 1 Ph.D. student, Chair of Fluid Dynamics, Hermann-Föttinger-Institut, 2 Figure 1. Schematic of a swirler insert (top left), test stand (top right), and sectional view of the outlet section of the test stand (bottom) u = tangential velocity u /u bulk = normalized tangential velocity u ,RMS = fluctuation of tangential velocity v = radial velocity v/u bulk = normalized radial velocity v RMS = fluctuation of radial velocity x/d = relative axial location with respect to nozzle diameter = vane angle = equivalence ratio
Gas turbine combustors are commonly operated with lean premix flames, allowing for high efficiencies and low emissions. These operating conditions are susceptible to thermoacoustic pulsations, originating from acoustic-flame coupling. To reveal this coupling, experiments or simulations of acoustically forced combustion systems are necessary, which are very challenging for real-scale applications. In this work we investigate the possibility to determine the flame response to acoustic forcing from snapshots of the unforced flow. This approach is based on three central hypothesis: first, the flame response is driven by flow fluctuations, second, these flow fluctuations are dominated by coherent structures driven by hydrodynamic instabilities, and third, these instabilities are driven by stochastic forcing of the background turbulence. As a consequence the dynamics in the natural flow should be low-rank and very similar to those of the acoustically forced system. In this work, the methodology is applied to experimental data of an industry-scale swirl combustor. A resolvent analysis is conducted based on the linearized Navier-Stokes equations to assure analytically the low-rank behavior of the flow dynamics. Then, these dynamics are extracted from flow snapshots using spectral proper orthogonal decomposition (SPOD). The extended SPOD is applied to determine the heat release rate fluctuations that are correlated with the flow dynamics. The low-rank flow and flame dynamics determined from the analytic and data-driven approach are then compared to the flow response determined from a classic phase average of the acoustically forced flow, which allow the research hypothesis to be evaluated. It is concluded that for the present combustor, the flow and flame dynamics are low-rank for a wider frequency range and the response to harmonic forcing can be determined quite accurately from unforced snapshots. The methodology further allows to isolate the frequency range where the flame response is predominantly driven by hydrodynamic instabilites.
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