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Joint numerical and experimental study of thermoacoustic instabilities / Etude conjointe numérique et expérimentale des instabilités thermoacoustiquesBrebion, Maxence 27 January 2017 (has links)
Les instabilités thermo-acoustiques se rencontrent fréquemment au sein des chambres de combustion de toute taille, de la petite chaudière au moteur de fusée. Ces instabilités sont causées par le couplage entre ondes acoustiques et dégagement de chaleur instationnaire. En effet, le passage d'une onde acoustique au travers d'une flamme va moduler son dégagement de chaleur qui, en retour, va générer de nouvelles ondes acoustiques. Lorsqu'une chambre de combustion entre en instabilité, d'importantes variations de pression sont observées ; ces fluctuations peuvent user prématurément le système ou altérer ses performances. L'étude des instabilités thermo-acoustiques a pour but d'améliorer notre compréhension de ces phénomènes complexes afin de les prévenir. L'objectif de ce travail est d'obtenir et d'intégrer au sein de modèles réduits des descriptions précises de la dissipation acoustique – effet stabilisant - et d'interaction flamme/acoustique – effet déstabilisant. Cette étude se décompose en trois axes : La première partie développe le concept de « modèle acoustique réduit » qui permet de prédire les modes acoustiques d'une chambre de combustion. Pour cela, sont prises en compte les dissipations inhérentes à certaines pièces(diaphragmes, injecteurs, ...) ainsi que le couplage flamme/acoustique. Une fois le modèle établi, il convient d'en chercher les solutions à l'aide d'un solveur numérique spécialement conçu pour cette tâche. Dans une deuxième partie, un banc expérimental est utilisé pour caractériser le lien entre perte de charge et dissipation acoustique. Il est montré de manière théorique et expérimentale que la connaissance des pertes de charge au travers d'un élément permet de prédire son comportement acoustique à basse fréquence. La dernière partie concerne le couplage flamme/acoustique et plus spécifiquement l'influence de la température de l'accroche-flamme :une flamme pauvre pré-mélangée air/méthane est stabilisée sur un cylindre dont la température peut être contrôlée. Ainsi, il est montré que l'influence de la température du cylindre sur la flamme – position d'équilibre, dynamique et stabilité - est remarquable. / From small scale energy systems such as domestic boilers up to rocket motors, combustion chambers are often prone to combustion instabilities. These instabilities stem from the coupling of unsteady heat release rate and acoustic waves. This coupling is two sided: flame front perturbations generate acoustic waves while acoustic waves impinging on flame holders can disturb flames attached on them. Important pressure and velocity oscillations can be reached during unstable regimes, that can alter its efficiency or even damage the entire combustion chamber. One major challenge is to understand, predict, and prevent from these combustion instabilities. The objectives of this thesis are twofold: (1) take into account acoustic dissipation and (2)analyze flame/acoustic coupling to obtain Reduced Order Model (ROM) for combustion instabilities. This work is divided into three parts. First, the concept of ROM that gives the acoustic modes of a combustion chamber is introduced. This modeling strategy is based on the acoustic network theory and may take into account flame/acoustic coupling as well as acoustic dissipation. An efficient numerical algorithm dedicated to solve ROMs was designed on purpose and validated on several academical configurations. Second, an experimental rig was commissioned to study mean and acoustic pressure losses across a diaphragm and two swirl injectors. Results show that these two phenomena are linked and can be simply incorporated into ROMs. Finally, flame/acoustic coupling is investigated by using both direct numerical simulations and experiments: a lean premixed V-shaped laminar flame is anchored on a cylindrical bluff-body and we show that its temperature greatly influences the flame mean shape as well as its dynamics.
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Empirical study of acoustic instability in premixed flames: measurements of flame transfer functionHojatpanah, Roozbeh 08 1900 (has links)
Indiana University-Purdue University Indianapolis (IUPUI) / In order to conform to pollutant-control regulations and minimize NOx emissions,
modern household boilers and central heating systems are moving toward premixed
combustors. These combustors have been successful with regards to emissions along with
efficiency. However, their implementation has been associated with acoustical instability
problems that could be solved through precise optimization in design rather than trial and
error experimentation.
This thesis introduces an experimental apparatus, which is designed to investigate
the acoustic instability problem at the flame level. The goal is an experimental
determination of the flame transfer function and comparison of the experimental data
with a theoretical model of the flame. An experimental procedure is designed to diagnose
the origins of the combustion instabilities by measurement of the flame transfer function.
This research is carried out in three steps. The first step is to understand the
acoustic instability problem through study of the theoretical models of the flame transfer
function and selection of a model, which is most functional in industrial applications. A
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measurement technique for the flame transfer function is developed according to the
required accuracy in measurements, repeatability, and configurability for a wide range of
operating conditions. Subsequently, an experimental apparatus is designed to
accommodate the flame transfer function measurement technique. The components of the
acoustic system are carefully sized to achieve precise measurement of the system
parameters such as flows, pressures, and acoustic responses, and the apparatus is built.
The apparatus is operated to measure the flame transfer function at several operating
conditions.
The experimentally measured flame transfer function is compared with a
theoretical model for further verification. The experimental apparatus provides an
improved assessment of the acoustic instability problem for industrial applications.
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REDUCED FIDELITY ANALYSIS OF COMBUSTION INSTABILITIES USING FLAME TRANSFER FUNCTIONS IN A NONLINEAR EULER SOLVERGowtham Manikanta Reddy Tamanampudi (6852506) 02 August 2019 (has links)
<p>Combustion instability,
a complex phenomenon observed in combustion chambers is due to the coupling
between heat release and other unsteady flow processes. Combustion instability
has long been a topic of interest to rocket scientists and has been extensively
investigated experimentally and computationally. However, to date, there is no
computational tool that can accurately predict the combustion instabilities in
full-size combustors because of the amount of computational power required to
perform a high-fidelity simulation of a multi-element chamber. Hence, the focus
is shifted to reduced fidelity computational tools which may accurately predict
the instability by using the information available from the high-fidelity
simulations or experiments of single or few-element combustors. One way of
developing reduced fidelity computational tools involves using a reduced
fidelity solver together with the flame transfer functions that carry important
information about the flame behavior from a high-fidelity simulation or
experiment to a reduced fidelity simulation.</p>
<p> </p>
<p>To date, research has
been focused mainly on premixed flames and using acoustic solvers together with
the global flame transfer functions that were obtained by integrating over a
region. However, in the case of rockets, the flame is non-premixed and
distributed in space and time. Further, the mixing of propellants is impacted
by the level of flow fluctuations and can lead to non-uniform mean properties
and hence, there is a need for reduced fidelity solver that can capture the gas
dynamics, nonlinearities and steep-fronted waves accurately. Nonlinear Euler
equations have all the required capabilities and are at the bottom of the list
in terms of the computational cost among the solvers that can solve for mean
flow and allow multi-dimensional modeling of combustion instabilities. Hence,
in the current work, nonlinear Euler solver together with the spatially
distributed local flame transfer functions that capture the coupling between
flame, acoustics, and hydrodynamics is explored.</p>
<p> </p>
<p>In this thesis, the
approach to extract flame transfer functions from high-fidelity simulations and
their integration with nonlinear Euler solver is presented. The dynamic mode
decomposition (DMD) was used to extract spatially distributed flame transfer
function (FTF) from high fidelity simulation of a single element non-premixed
flame. Once extracted, the FTF was integrated with nonlinear Euler equations as
a fluctuating source term of the energy equation. The time-averaged species destruction
rates from the high-fidelity simulation were used as the mean source terms of
the species equations. Following a variable gain approach, the local species
destruction rates were modified to account for local cell constituents and
maintain correct mean conditions at every time step of the nonlinear Euler
simulation. The proposed reduced fidelity model was verified using a Rijke tube
test case and to further assess the capabilities of the proposed model it was
applied to a single element model rocket combustor, the Continuously Variable
Resonance Combustor (CVRC), that exhibited self-excited combustion
instabilities that are on the order of 10% of the mean pressure. The results
showed that the proposed model could reproduce the unsteady behavior of the
CVRC predicted by the high-fidelity simulation reasonably well. The effects of
control parameters such as the number of modes included in the FTF, the number
of sampling points used in the Fourier transform of the unsteady heat release,
and mesh size are also studied. The reduced fidelity model could reproduce the
limit cycle amplitude within a few percent of the mean pressure. The successful
constraints on the model include good spatial resolution and FTF with all modes
up to at least one dominant frequency higher than the frequencies of interest.
Furthermore, the reduced fidelity model reproduced consistent mode shapes and
linear growth rates that reasonably matched the experimental observations,
although the apparent ability to match growth rates needs to be better
understood. However, the presence of significant heat release near a pressure
node of a higher harmonic mode was found to be an issue. This issue was
rectified by expanding the pressure node of the higher frequency mode. Analysis
of two-dimensional effects and coupling between the local pressure and heat
release fluctuations showed that it may be necessary to use two dimensional
spatially distributed local FTFs for accurate prediction of combustion
instabilities in high energy devices such as rocket combustors. Hybrid
RANS/LES-FTF simulation of the CVRC revealed that it might be necessary to use
Flame Describing Function (FDF) to capture the growth of pressure fluctuations
to limit cycle when Navier-Stokes solver is used.</p>
<p> </p>
<p>The main objectives of
this thesis are:</p>
<p>1. Extraction of
spatially distributed local flame transfer function from the high fidelity
simulation using dynamic mode decomposition and its integration with nonlinear
Euler solver</p>
<p>2. Verification of the
proposed approach and its application to the Continuously Variable Resonance
Combustor (CVRC).</p>
<p>3. Sensitivity analysis
of the reduced fidelity model to control parameters such as the number of modes
included in the FTF, the number of sampling points used in the Fourier
transform of the unsteady heat release, and mesh size.</p>
<p> </p>
<p>The goal of this thesis
is to contribute towards a reduced fidelity computational tool which can
accurately predict the combustion instabilities in practical systems using
flame transfer functions, by providing a path way for reduced fidelity
multi-element simulation, and by defining the limitations associated with using
flame transfer functions and nonlinear Euler equations for non-premixed flames.</p>
<p> </p><br>
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