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Daniel Guggenheim School of Aerospace Engineering
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ItemTurbulence-chemistry interactions for lean premixed flames(Georgia Institute of Technology, 2018-10-09) Dasgupta, DebolinaTurbulent combustion, particularly premixed combustion has great practical importance due to their extensive industrial usage in gas turbines, internal combustion engines etc. However, the physics governing the inherent multi- scale interactions of turbulence, flow-field and chemistry is not yet well established. A complete understanding of each of these interactions and their coupling is essential for the development of models that can aid simulations of realistic engines (using Large Eddy Simulations (LES) or Reynolds averaged Navier-Stokes equations (RANS). Particularly, understanding the flame structure and its stabilization requires an understanding of the turbulence-chemistry interactions. This can manifest itself in many different forms. For example, flame wrinkling gives rise to flame stretch that can modify the local temperature and species concentrations in turn altering the local chemistry. Also, the smaller eddies in a turbulent flow can penetrate into the preheat and reaction zones changing the species’ gradients within the flame. The influence of turbulence on chemistry can be analyzed in two different ways: firstly, a “global” analysis which investigates the direct impact of turbulence on the chemical pathways (a series of elementary reactions involved in the fuel oxidation process) and secondly, a “local” analysis which investigates the influence of turbulence on the chemical flame structure (i.e. species and reaction rate profiles). To understand these influences of turbulence, this work performs Direct Numerical Simulations (DNS) for lean premixed flames involving three fuels: hydrogen, methane and n-dodecane. A “global” analysis using different metrics such as heat release and species consumption/production is performed to quantify the changes in the chemical pathways. This analysis is performed for the metrics averaged over the entire flame and conditioned on local flame features such as fuel consumption, curvature etc. The results are also compared and contrasted with simple laminar flame models such as unstretched flames, stretched flames and perfectly stirred reactors. In general, the laminar models provide a good estimate for the chemical pathways for these key metrics suggesting turbulence does not have a significant impact on the fuel oxidation pathways. However, this is not true for the reaction rate and species profiles across the flame. Conditional means of these quantities are calculated to identify the “local” influence of turbulence on chemistry. These conditional means are also compared with laminar unstretched and stretched flames to identify regions of good agreement and deviation. The laminar calculations are performed using two different transport models; firstly, the mixture-averaged transport wherein every species diffuses into the mixture with its molecular diffusivity and secondly, Le=1 transport wherein the mass diffusivity of every species is equal to the thermal diffusivity of the mixture eliminating effects of preferential and differential diffusion. Le=1 is considered the theoretical limit of transport where turbulent mixing governs the transport process opposed to molecular diffusivity. For lean hydrogen/air flames (Le<1), the behavior of the profiles is similar to the evolution of the laminar profiles with increasing stretch. However, for the lean methane/air flames (Le~1), with increasing turbulence intensity, the flame profiles deviate from the evolution of laminar profiles with increasing stretch and align more closely with the Le=1 transport model laminar flame profiles. For n-dodecane/air flames (Le>1), the evolution of the turbulent flame profiles, with slight increase in turbulence intensity, replicates the behavior of stretched flames. However, with a further increase, a deviation is seen from the stretched flame profiles. Additionally, these profiles significantly deviate from the Le=1 transport model suggesting the inadequacy of stretched flames and a simple Le=1 model to replicate the behavior of stretched flames. In order, to identify the effect of increased diffusivity due to turbulence, a new transport model is implemented for unstretched and stretched flames wherein a constant is added to the mass diffusivity of the species obtained from the mixture-averaged transport. This constant covers multiple orders of magnitude mimicking the effect of increased turbulence diffusivity. For the lean hydrogen flames(Le<1), the turbulent flame profiles are seen to evolve similar to the laminar profiles with increasing stretch and not similar to the laminar profiles with increasing diffusivity. This suggests mixtures containing a highly diffusive fuel does not need the aid of turbulence to enhance transport. For the lean methane flames (Le~1), the turbulent flame profile evolution is similar to the effect of increasing diffusivity for unstretched flames suggesting a significant effect of diffusivity on the flame structure. For the lean n-dodecane flames (Le>1), the turbulent flame profiles evolve similar to the effect of increased diffusivity on stretched flames. This further emphasizes the necessity to include diffusivity in laminar models used to replicate turbulent flame structure. Overall, this work helps identify the key players in turbulence-chemistry interactions which need to be considered for modeling real combustors.
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ItemExperimental investigation of transverse acoustic instabilities(Georgia Institute of Technology, 2017-11-09) Smith, Travis EdwardThis work presents 5 kHz stereo PIV and OH PLIF measurements as well as OH* and CH* chemiluminescence measurements of transversely forced swirl flames. The presence of transverse forcing on this naturally unstable flow both influences the natural instabilities, as well as amplifies disturbances that may not necessarily manifest themselves during natural oscillations. By manipulating the structure of the acoustic forcing field, both axisymmetric and helical modes are preferentially excited away from the frequency of natural instability. Additionally, forced and self-excited transverse acoustic instability studies to date have strong coupling between the transverse and axial acoustic fields near the flame. This is significant, as studies suggest that it is not the transverse disturbances themselves, but rather the induced axial acoustic disturbances, that control the bulk of the heat release response. The work first presents a method for spatially interpolating the phase locked r-z and r-θ planar velocity and flame position data, extracting the full three-dimensional structure of the helical disturbances. These helical disturbances are also decomposed into symmetric and antisymmetric disturbances about the jet core, showing the subsequent axial evolution (in magnitude and phase) of each of these underlying disturbances. Then experiments performed with essentially the same transverse acoustic wave field, but with and without axial acoustics, show that significant heat release oscillations are only excited in the former case. The results show that the axial disturbances at the nozzle exit are the dominant cause of the heat release oscillations. These observations support the theory that the key role of the transverse motions is to act as the “clock” for the instability, setting the frequency of the oscillations while having a negligible direct effect on the actual heat release fluctuations. They also show that transverse instabilities can be damped by either actively canceling the induced axial acoustics in the nozzle (rather than the much larger energy transverse combustor disturbances), or by passively tuning the nozzle impedance to drive an axial acoustic velocity node at the nozzle outlet.
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ItemFlow characterization of lifted flames in swirling, reacting flows(Georgia Institute of Technology, 2017-05-15) Chterev, Ianko PavlovSwirl stabilized combustors are commonly used in gaseous fueled land-based gas turbines and liquid fueled aerospace combustors to achieve simultaneously high efficiency, low emissions, wide operability limits, and low thermal and mechanical hardware loadings. Flame shape and location are critical to successful design, and are, therefore, the general focus of this work. In premixed swirl combustion aerodynamically stabilized flames are sometimes observed and desirable as they potentially reduce hardware heat loadings. However, their understanding is largely phenomenological and geometry specific. First, aerodynamically stabilized flames are subject to flow perturbations such as a precessing vortex core (PVC), and therefore, this thesis studies how a precessing flow field affects time-averaged quantities such as flame location. Second, in swirling flowfields with no interior time-averaged stagnation point, flames are sometimes aerodynamically stabilized by instantaneous stagnation points created by large scale structures such as the PVC. Since this places the flame in a time-averaged reverse flow, natural questions are what the flame and flow characteristics are at the flame stabilization location, such as flame stretch, and why the flame does not flash back. Experiments in high pressure, multi-phase, hydrocarbon fueled, reacting flows are highly complex, and quantities such as liquid and gas phase fuel distribution, heat release and flowfield are difficult to obtain. Thus, another focus of this work is experimental development to study the internal physics. First, this thesis finds that precession in radial-axial planar measurements can result in the time-averaged stagnation point to be located in a highly negative region of the flow. Since the time-averaged flow field is often used to determine the flame location, these findings indicate that time-averaged treatments may lead to erroneous results. Precession can also alter the general flow field topology by inducing asymmetries and can cause time-averages to converge slower. Second, the local flow field of a flame aerodynamically stabilized by instantaneous stagnation points is characterized using planar velocity and flame location measurements, conditioned using a line-of-sight technique to capture the flame global leading in the imaging plane. The flame stretch is measured, indicating that the stretch the flame experiences has a high dependence on nozzle velocity. However, the scaling is not understood, and further study is proposed. The time-averaged flame stretch is much higher than opposed diffusion flame extinction stretch rate calculations, which also requires further study. Furthermore, the stretch is not correlated strongly with location and flow velocity. Last, a simultaneous stereo-PIV and fuel/OH-PLIF technique is developed using a single PLIF laser (and a PIV laser) to characterize the spray distribution, flame shape and location, and dual phase flow field for two different jet fuels, at pressures from 2 to 5 bar. Two different flame shapes are observed, with a stability behavior different than with gaseous fuel. Furthermore, the flames extends into the annular jet core, a phenomenon not observed in premixed systems, and mentioned as needing verification in the liquid fueled combustion literature.
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ItemEnsemble-averaged dynamics of premixed, turbulent, harmonically excited flames(Georgia Institute of Technology, 2017-04-07) Humphrey, LukeIncreasing awareness of the negative impacts of pollutant emissions associated with combustion is driving increasingly stringent regulatory limits. In particular, oxides of nitrogen, generally referred to as NOx, now face strict limits. These restrictions have driven development of cleaner burning combustion systems. Because NOx formation increases significantly at elevated temperatures, one method to reduce NOx emissions is to burn the fuel at lower temperatures. By premixing the fuel and oxidizer prior to combustion significantly lower flame temperatures can be achieved, with corresponding reductions in NOx emissions. Unfortunately, premixed combustion systems are generally more prone to potentially problematic feedback between the unsteady heat release from the flame and unsteady pressure oscillations. This self-excited feedback loop is known as combustion instability. Because these oscillations are associated with unsteady pressure fluctuations they can degrade system performance, limit operability, and even lead to catastrophic failure. Understanding combustion instability is the primary motivation for the work presented in this thesis. The interaction of quasi-coherent and turbulent flame disturbances changes the spatio-temporal flame dynamics and turbulent flame speed, yet this interaction is not fully understood. Therefore, this thesis concentrates on identifying, understanding, and modeling these interactions. In order to address this topic, two primary avenues of research are followed: development and validation of a flame position model and experimental investigations of predicted ensemble-averaged flame speed sensitivity to flame curvature. First, a reduced order modeling approach for turbulent premixed flames is presented, based on the ensemble-averaged flame governing equation proposed by Shin and Lieuwen (2013). The turbulent modeling method is based on the G-equation approach used in laminar flame position and heat release studies. In order to capture the dependence of the ensemble-averaged turbulent flame speed on the ensemble-averaged flame curvature, the turbulent flame model incorporates a flame speed closure proposed by Shin and Lieuwen (2013). Application of the G-equation approach in different coordinate systems requires the inclusion of time-varying integration limits when calculating global flame area. This issue is discussed and the necessary corrections derived. Next, the reduced order turbulent modeling approach is validated by comparison with three-dimensional simulations of premixed flames, for both flame position and heat release response. The reduced order model is the linearized, allowing development of fully analytical flame position and heat release expressions. The use of the flame speed closure is shown to capture nonlinear effects associated with kinematic restoration. Second, the development of and results from a novel experimental facility are described. This facility has the capability to subject premixed flames to simultaneous broadband turbulent fluctuations and narrowband coherent fluctuations, which are introduced on the flame using an oscillating flame holder. Mie scattering images are used to identify the instantaneous flame edge position, while simultaneous high speed PIV measurements provide flow field information. Results from this experimental investigation include analysis of the ensemble-averaged flame dynamics, the ensemble-averaged turbulent displacement speed, the local ensemble-averaged area and consumption speed, and the dependence of both the displacement speed and consumption speed on the ensemble-averaged flame curvature. Finally, the flame speed sensitivity to curvature is quantified through calculation of the normalized turbulent Markstein displacement and consumption numbers. The results show that the amplitude of coherent flame wrinkles generally decreases with both downstream distance and increasing turbulence intensity, providing the first experimental validation of previous isothermal results. The average displacement and consumption speeds increase with downstream distance and turbulence intensity, reflecting the increasing wrinkled flame surface. The ensemble-averaged, phase dependent displacement and consumption speeds demonstrate clear modulation with the shape of the ensemble-averaged flame. Specifically, these turbulent flame speeds increase in regions of negative curvature. For both the displacement and consumption speed, the magnitude of the normalized turbulent Markstein length increases with ratio of the turbulent flame wrinkling length to the coherent wrinkling length when u'/SL0 >2.5 . For u'/SL0 < 2.5 the trends are less clear due to the presence of convecting disturbances which introduce additional fine scale wrinkles on the flame. Together the results presented in this thesis provide a foundation for modeling turbulent flames in the presence of quasi-coherent disturbances. The flame position can be modeled using the ensemble-averaged governing equation with the dynamical flame speed closure, and the corresponding heat release can be calculated from the turbulent consumption speed closure. The turbulent Markstein numbers and uncurved flame speed may be extracted from experimental or numerical data.
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ItemQualitative Comparison of Commercial and Open Source Particle Image Velocimetry Softwares(Georgia Institute of Technology, 2016-07-18) Johnson, Henderson JohnsonComplex chemical and fluid dynamic mechanisms present in combustion systems necessitate the validation of computational fluid dynamics (CFD) models using flow visualization and velocity field measurement techniques. Two cutting edge measurement techniques for reacting flows are planar-laser induced fluorescence (PLIF), which images combustion radicals within the flame, and stereoscopic particle image velocimetry (sPIV), which is a non-intrusive laser-based velocity measurement. These measurements and others are often synchronized and coupled together to gain a complete picture of flow field variation (i.e. pressure, temperature, velocity, etc.). The PIV measurement in particular poses a variety of challenges especially during the data processing phase which can be achieved using either commercial or open-source software packages. This papers explores the qualitative difference between each software with the intent of providing a recommendation on which form of software should be used. By understanding the types of software that should be used, one can develop lab infrastructure that maximizes the effectiveness of using said software.
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ItemDynamics of Harmonically Forced Nonpremixed Flames(Georgia Institute of Technology, 2016-04-19) Magina, Nicholas AThis thesis describes the dynamics, both spatio-temporal and heat release, of harmonically excited non-premixed flames. Analytical, numerical, computational, and, experimental analyses were performed, along with combined analyses methods, to study excitation and evolution of wrinkles on the flame front. Explicit expressions for the dynamics were developed. Wrinkle convection at the mean axial flow speed, and wrinkle dissipation and dispersion were analytically identified in the Pe-->∞ and Pe>>1 limits, respectively. Altered inlet mixture fraction profiles and attachment point dynamics were shown to accompany axial diffusion effects. Some physical effects such as axial diffusion, forcing configuration, and anisotropic diffusion altered the wrinkle interference pattern/waveform characteristics, while others, such as confinement, dimensionality, and differential diffusion, altered the dynamics through modifying the mean flame location. Comparisons to established premixed flame dynamics were made throughout. Despite having similar space-time dynamics, the heat release dynamics of the two differed greatly, having different dominant contributions, as well as different asymptotic trends. Experimental results obtained validated previous findings as well as enabled advanced model development, revealing the importance of accurate mixture fraction field capture, particularly in the near burner exit region. Findings shed light onto model and predictive improvements for future works.
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ItemDynamics of premixed flames in non-axisymmetric disturbance fields(Georgia Institute of Technology, 2013-07-19) Acharya, Vishal SrinivasWith strict environmental regulations, gas turbine emissions have been heavily constrained. This requires operating conditions wherein thermo-acoustic flame instabilities are prevalent. During this process the combustor acoustics and combustion heat release fluctuations are coupled and can cause severe structural damage to engine components, reduced operability, and inefficiency that eventually increase emissions. In order to develop an engine without these problems, there needs to be a better understanding of the physics behind the coupling mechanisms of this instability. Among the several coupling mechanisms, the “velocity coupling” process is the main focus of this thesis. The majority of literature has treated axisymmetric disturbance fields which are typical of longitudinal acoustic forcing and axisymmetric excitation of ring vortices. Two important non-axisymmetric disturbances are: (1) transverse acoustics, in the case of circumferential modes of a multi-nozzle annular combustor and (2) helical flow disturbances, seen in the case of swirling flow hydrodynamic instabilities. With significantly less analytical treatment of this non-axisymmetric problem, a general framework is developed for three-dimensional swirl-stabilized flame response to non-axisymmetric disturbances. The dynamics are tracked using a level-set based G-equation applicable to infinitely thin flame sheets. For specific assumptions in a linear framework, general solution characteristics are obtained. The results are presented separately for axisymmetric and non-axisymmetric mean flames. The unsteady heat release process leads to an unsteady volume generation at the flame front due to the expansion of gases. This unsteady volume generation leads to sound generation by the flame as a distributed monopole source. A sound generation model is developed where ambient pressure fluctuations are generated by this distributed fluctuating heat release source on the flame surface. The flame response framework is used to provide this local heat release source input. This study has been specifically performed for the helical flow disturbance cases to illustrate the effects different modes have on the generated sound. Results show that the effects on global heat release and sound generation are significantly different. Finally, the prediction from the analytical models is compared with experimental data. First, a two-dimensional bluff-body stabilized flame experiment is used to obtain measurements of both the flow and flame position in time. This enables a local flame response comparison since the data are spatially resolved along the flame. Next, a three-dimensional swirl-stabilized lifted flame experiment is considered. The measured flow data is used as input to the G-equation model and the global flame response is predicted. This is then compared with the corresponding value obtained using global CH* chemilumenescence measurements.
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ItemDynamical characteristics of reacting bluff body wakes(Georgia Institute of Technology, 2013-06-20) Emerson, Benjamin L.Combustion instability plagues the combustion community in a wide range of applications. This un-solved problem is especially prevalent and expensive in aerospace propulsion and ground power generation. The challenges associated with understanding and predicting combustion instability lie in the flame response to the acoustic field. One of the more complicated flame response mechanisms is the velocity coupled flame response, where the flame responds dynamically to the acoustic velocity as well as the vortically induced velocity field excited by the acoustics. This vortically induced, or hydrodynamic, velocity field holds critical importance to the flame response but is computationally expensive to predict, often requiring high fidelity CFD computations. Furthermore, its behavior can be a strong function of the numerous flow parameters that change over the operability map of a combustor. This research focuses on a nominally two dimensional bluff body combustor, which has rich hydrodynamic stability behavior with a manageable number of stability parameters. The work focuses first on experimentally characterizing the dynamical flow and flame behavior. Next, the research shifts focus toward hydrodynamic stability theory, using it to explain the physical phenomena observed in the experimental work. Additionally, the hydrodynamic stability work shows how the use of simple, model analysis can identify the important stability parameters and elucidate their governing physical roles. Finally, the research explores the forced response of the flow and flame while systematically varying the underlying hydrodynamic stability characteristics. In the case of longitudinal combustion instability of highly preheated bluff body combustors, it shows that conditions where an acoustic mode frequency equals the hydrodynamic global mode frequency are not especially dangerous from a combustion instability standpoint, and may actually have a reduced heat release response. This demonstrates the very non-intuitive role that the natural hydrodynamic flow stability plays in the forced heat release response of the flame. For the fluid mechanics community, this work contributes to the detailed understanding of both unforced and forced bluff body combustor dynamics, and shows how each is influenced by the underlying hydrodynamics. In particular, it emphasizes the role of the density-shear layer offset, and shows how its extreme sensitivity leads to complicated flow dynamics. For the flow-combustor community as a whole, the work reviews a pre-existing method to obtain the important flow stability parameters, and demonstrates a novel way to link those parameters to the governing flow physics. For the combustion instability community, this thesis emphasizes the importance of the hydrodynamic stability characteristics of the flow, and concludes by offering a paradigm for consideration of the hydrodynamics in a combustion instability problem.
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ItemMeasurements and modeling of turbulent consumption speeds of syngas fuel blends(Georgia Institute of Technology, 2013-02-19) Venkateswaran, PrabhakarIncreasingly stringent emission requirements and dwindling petroleum reserves have generated interest in expanding the role of synthesis gas (syngas) fuels in power generation applications. Syngas fuels are the product of gasifying organic-based feedstock such as coal and biomass and are composed of mainly H₂ and CO. However, the use of syngas fuels in lean premixed gas turbine systems has been limited in part because the behavior of turbulent flames in these mixtures at practical gas turbine operating conditions are not well understood. This thesis presents an investigation of the influence of fuel composition and pressure on the turbulent consumption speed, ST,GC, and the turbulent flame brush thickness, FBT, for these mixtures. ST,GC and FBT are global parameters which represent the average rate of conversion of reactants to products and the average heat release distribution of the turbulent flame respectively. A comprehensive database of turbulent consumption speed measurements obtained at pressures up to 20 atm and H₂/CO ratios of 30/70 to 90/10 by volume is presented. There are two key findings from this database. First, mixtures of different H₂/CO ratios but with the same un-stretched laminar flame speeds, SL,0, exposed to the same turbulence intensities, u'rms , have different turbulent consumption speeds. Second, higher pressures augment the turbulent consumption speed when SL,0 is held constant across pressures and H₂/CO ratios. These observations are attributed to the mixture stretch sensitivities, which are incorporated into a physics-based model for the turbulent consumption speed using quasi-steady leading points concepts. The derived scaling law closely resembles Damkhler's classical turbulent flame speed scaling, except that the maximum stretched laminar flame speed, SL,max, arises as the normalizing parameter. Scaling the ST,GC data by SL,max shows good collapse of the data at fixed pressures, but systematic differences between data taken at different pressures are observed. These differences are attributed to non-quasi-steady chemistry effects, which are quantified with a Damkhler number defined as the ratio of the chemical time scale associated with SL,max and a fluid mechanic time scale. The observed scatter in the normalized turbulent consumption speed data correlates very well with this Damkhler number, suggesting that ST,GC can be parameterized by u'rms/SL,max and the leading point Damkhler number. Finally, a systematic investigation of the influence of pressure and fuel composition on the flame brush thickness is presented. The flame brush thickness is shown to be independent of the H₂/CO ratio if SL,0 is held constant across the mixtures. However, increasing the equivalence ratio for lean mixtures at a constant H₂/CO ratio, results in a thicker flame brush. Increasing the pressure is shown to augment the flame brush thickness, a result which has not been previously reported in the literature. Classical correlations based on turbulent diffusion concepts collapse the flame brush thickness data obtained at fixed u'rms/U₀ and pressure reasonably well, but systematic differences exist between the data at different u'rms/U₀ and pressures.
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ItemPremixed flame kinematics in a harmonically oscillating velocity field(Georgia Institute of Technology, 2012-11-13) Shin, Dong-hyukAir pollution regulations have driven modern power generation systems to move from diffusion to premixed combustion. However, these premixed combustion systems are prone to combustion instability, causing high fluctuations in pressure and temperature. This results in shortening of component life, system failure, or even catastrophic disasters. A large number of studies have been performed to understand and quantify the onset of combustion instability and the limit cycle amplitude. However, much work remains due to the complexity of the process associated with flow dynamics and chemistry. This thesis focuses on identifying, quantifying and predicting mechanisms of flame response subject to disturbances. A promising tool for predicting combustion instability is a flame transfer function. The flame transfer function is obtained by integrating unsteady heat release over the combustor domain. Thus, the better understanding of spatio-temporal characteristics of flame is required to better predict the flame transfer function. The spatio-temporal flame response is analyzed by the flame kinematic equation, so called G-equation. The flame is assumed to be a thin interface separating products and reactant, and the interface is governed by the local flow and the flame propagation. Much of the efforts were done to the flame response subject to the harmonic velocity disturbance. A key assumption allowing for analytic solutions is that the velocity is prescribed. For the mathematical tools, small perturbation theory, Hopf-Lax formula and numerical simulation were used. Solutions indicated that the flame response can be divided into three regions, referred to here as the near-field, mid-field, and farfield. In each regime, analytical expressions were derived, and those results were compared with numerical and experimental data. In the near field, it was shown that the flame response grows linearly with the normal component of the velocity disturbance. In the mid field, the flame response shows peaks in gain, and the axial location of these peaks can be predicted by the interference pattern by two characteristic waves. Lastly, in the far field where the flame response decreases, three mechanisms are studied; they are kinematic restoration, flame stretch, and turbulent flow effects. For each mechanism, key parameters are identified and their relative significances are compared.