Impact of fuel supply impedance and fuel staging on gas turbine ...

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Introduction The work starts with an overview <strong>and</strong> discussion <strong>of</strong> the available numerical methods (chapter 2) to determine the thermo-acoustic characteristics <strong>of</strong> combustion systems <strong>and</strong> the chosen strategy, which is a hybrid approach using Computational Fluid Dynamics, system identification <strong>and</strong> acoustic network modelling. On this basis, the following chapter deals with the basic equations <strong>of</strong> the acoustics (chapter 3) <strong>and</strong> the simplifications used in this work. Chapter 4 describes the basics <strong>of</strong> Computational Fluid Dynamics, including turbulence <strong>and</strong> combustion models. It is followed by the presentation <strong>of</strong> the system identification method for flame dynamics responding to more than one disturbance signal (chapter 5). The numerical combustion test rig used is then described with two <strong>and</strong> three <strong>fuel</strong> injection stages in chapter 6, including the results <strong>of</strong> the numerical simulations <strong>and</strong> the identification <strong>of</strong> the flame dynamics. Furthermore, the acoustic network model <strong>of</strong> the test rig <strong>and</strong> the parameter study analyzing the different influences on the combustion stability is presented in chapter 7. The work is completed with a short summary <strong>and</strong> conclusion (chapter 8). 1.5 Terminology <strong>and</strong> specification <strong>of</strong> the present work Before the possible numerical calculation methods are evaluated in terms <strong>of</strong> the tasks presented, the terminology <strong>and</strong> the main general specifications are first reviewed. In general, the field <strong>of</strong> gaseous combustion can be subdivided into three different technical processes, which are related to the mixing process <strong>of</strong> <strong>fuel</strong> <strong>and</strong> air: premixed, non-premixed, or partial premixed combustion. A combustion process is called premixed if the <strong>fuel</strong> <strong>and</strong> oxidizer are mixed by turbulence for a sufficient time before the mixture is ignited. In a nonpremixed process, the oxidizer <strong>and</strong> <strong>fuel</strong> are injected separately into the combustion chamber. The mixing <strong>and</strong> combustion process are directly coupled 12
1.5 Terminology <strong>and</strong> specification <strong>of</strong> the present work <strong>and</strong> combustion takes place in regions <strong>of</strong> stoichiometric mixture. The partial premixed process is therefore a combination <strong>of</strong> both. The diesel engine is a good example, where several liquid <strong>fuel</strong> sprays are injected into hot compressed air. Before auto-ignition occurs, the <strong>fuel</strong> evaporates <strong>and</strong> mixes partially with the air. Thus, the beginning chemical reaction can be seen as a premixed process whereas the final burnout occurs mainly under non-premixed conditions. In the present work a so-called ”practical premixed combustion system” is analyzed. It is characterized by inhomogeneous lean premixed conditions, where natural gas is injected at one or multiple locations in the mixing section as shown in Fig. 1.2. As in real gas turbines there is no choke between <strong>fuel</strong> injectors <strong>and</strong> flame. In such a configuration <strong>and</strong> in contrast to perfect premixed conditions, local <strong>and</strong> temporal fluctuations <strong>of</strong> the <strong>fuel</strong> concentration may be induced by acoustic fluctuations <strong>and</strong> must be taken into account. The difference between perfect premixed, non-premixed <strong>and</strong> practical premixed combustion systems is also shown in Fig. 1.3. The <strong>fuel</strong> injectors <strong>of</strong> the practical premixed combustion system considered in the present work are acoustically non-stiff. In other words, the pressure drop between <strong>fuel</strong> plenum <strong>and</strong> mixing section is not sufficient to decouple the acoustics <strong>of</strong> the <strong>fuel</strong> <strong>supply</strong> system from the acoustics <strong>of</strong> the entire combustion system. This means the <strong>fuel</strong> mass flow through the <strong>fuel</strong> injectors may be influenced by the acoustics <strong>and</strong> can vary in time. The flame <strong>of</strong> such a system can therefore respond to equivalence ratio fluctuations, which are caused by acoustic fluctuations in the mixing section at the location <strong>of</strong> <strong>fuel</strong> injection <strong>and</strong> fluctuations in the <strong>fuel</strong> stream in the injector, <strong>and</strong> by mass flow fluctuations at the vicinity <strong>of</strong> the burner exit. The main motivation to develop a design tool <strong>and</strong> design guidelines for practical premixed combustion systems is to demonstrate how <strong>fuel</strong> <strong>staging</strong> <strong>and</strong> differently tuned <strong>fuel</strong> <strong>impedance</strong>s influences the overall thermo-acoustic behavior. Therefore a linear stability analysis is sufficient to accomplish the task. A non-linear analysis, which is required to predict limit cycle amplitudes or non-linear instability limits, is out <strong>of</strong> the scope <strong>of</strong> the present work. 13
Page 1: Technische Universität München In
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Page 7 and 8: Contents 1 Introduction 1 1.1 Techn
Page 9 and 10: CONTENTS 7.2.2 Impact</stro
Page 11 and 12: Nomenclature G stretch factor [-] h
Page 13 and 14: Nomenclature λ air-fuel</s
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Page 17 and 18: 1.2 Thermo-acoustic instabilities 1
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Page 21 and 22: 1.3 Control of the
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3.3 Acoustic network model pressure
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4 Modeling of turb
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4.1 Theory and num
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5.4 A posteriori validation <strong
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5.5 Proof
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5.5 Proof
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5.5 Proof
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6 Numerical simulation MISO model i
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7 Acoustic analysis Once the acoust
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7.2 Acoustic network model results
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8 Summary and Conc
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The flame element of</stron
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Bibliography [1] Verband</s
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BIBLIOGRAPHY [18] L. Crocco. Theore
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BIBLIOGRAPHY [36] A. Giauque, L. Se
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BIBLIOGRAPHY [54] J.J. Keller, W. E
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BIBLIOGRAPHY [72] T. Lieuwen <stron
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BIBLIOGRAPHY Number 98-GT-269 in In
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BIBLIOGRAPHY [112] T. Sattelmayer.
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BIBLIOGRAPHY [130] A. Widenhorn. pr
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A.2 Estimation of
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A.3 Main flow parameters an
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