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ItemAssessment and Analysis of Turbulent Flame Speed Measurements of Hydrogen-Containing Fuels(Georgia Institute of Technology, 2023-08-14) Johnson, HendersonGlobal efforts to reduce greenhouse gas emissions and achieve a carbon-neutral economy have spurred the exploration of integrating hydrogen into various aspects of the global energy infrastructure. This can involve incorporating hydrogen into existing power generation applications or utilizing fuels with significant hydrogen content, such as syngas. However, the introduction of hydrogen poses significant challenges due to its potential to greatly impact the combustion process, with many aspects of its behavior not yet fully understood under practical gas turbine operating conditions. This thesis aims to investigate the influence of thermodynamic, fluid mechanic, and fuel factors on the turbulent global consumption speed, ST,GC, across different fuel types containing up to 90% hydrogen. This parameter represents the average rate of conversion of reactants to products relative to a specific iso-surface. The presented database encompasses three distinct fuel types: H2/CO, H2/CO/CH4/N2, and H2/CH4¬, which represent fuels that are either commonly encountered in practical applications or are of interest for future applications. The latter two fuels are new to the overall Georgia Tech database of turbulent flame speed measurements which increase the amount of high pressure data (up to 20 atm), and add data at preheat temperatures up to 500 K. The addition of this data is of great importance as it allows for further exploration of thermodynamic and fuel effects on ST,GC¬. The analysis of this database reveals several key findings. Firstly, regardless of whether the unstretched laminar flame speed, SL,0, is held constant, higher pressures lead to an increase in ST,GC across all fuel types. The preheat temperature is also shown to increase ST,GC, but when normalized by the laminar flame speed, it demonstrates a decrease. Moreover, the effects of hydrogen addition in H2/CO and H2/CO/CH4/N2 fuel blends are more pronounced compared to those in H2/CH4 fuels. Building upon prior studies that link these observations to mixture stretch sensitivity, the database is analyzed within the framework of a quasi-steady leading points concept model. In this framework, the maximum stretched laminar flame speed, SL,max, serves as the normalizing parameter. This approach proves effective for the H2/CO fuels discussed in this work, as it captures fuel effects at a fixed pressure and preheat temperature. However, a notable limitation arises in its inability to account for systematic differences in pressure and preheat temperature, indicating the need for a second correlating parameter. To identify this second parameter, a systematic investigation of three additional dimensionless numbers, namely the turbulent Reynolds number, Ret, time scale ratio, and acceleration ratio, is presented. Each of these numbers represents a different physical phenomenon that could potentially account for the observed variation in the data reported. The addition of Ret was considered in prior work; however, we identify that is insufficient as an appropriate scaling number due to its inconsistent correlation with preheat temperature. The acceleration ratio was introduced as a novel means of attempting to capture the ability of a flame to accelerate relative to the flow field. Similar to the Reynolds number, this approach showed limited ability to capture both pressure and preheat temperature effects; nevertheless, it does offer a new way to think about turbulence-flame interactions. Ultimately, the time scale ratio emerges as the optimal second correlating parameter due to its lesser degree of scatter compared to the acceleration ratio. This finding is significant, as it aligns with prior analyses that incorporated the time scale ratio to quantify non-quasi-steady chemistry effects at the leading point and demonstrates its promise as an appropriate scaling approach across a wide variety of conditions.
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ItemNonlinear Dynamics of Coupled Thermoacoustic Modes in the Presence of Noise(Georgia Institute of Technology, 2022-07-30) John, TonyThis thesis investigates the dynamics of nonlinearly coupled thermoacoustic modes in the presence of noise. The dynamics of a single linearly unstable thermoacoustic mode has been extensively studied in literature. Typically, practical combustion systems consist of multiple thermoacoustic modes that are linearly stable or unstable at a wide range of frequencies. These modes can express independently or can interact with each other. The interactions between different modes is a strong function of the frequency spacing between them amongst other parameters such as its linear growth/decay rate, mode shapes etc. Studies have shown that, in a configuration with two linearly unstable modes, these modal interactions could lead to the suppression of one of the modes, and under certain conditions the more unstable mode (higher growth rate) can be suppressed. Frequency spacing between the modes particularly influenced the stability and existence of potential limit cycle solutions. In this work, the earlier studies are extended to include the effects of noise in the system, studying how deterministic dynamics change with the addition of noise and the impact of frequency spacing (i.e., closely or widely spaced) on the results. Noise can broaden the distribution of amplitudes (”diffusion”), change both the average limit cycle amplitudes (”drift”), and alter the bifurcation characteristics of the limit cycle solutions. In order to identify these noise-induced features, a local asymptotic analysis is performed to characterize the diffusive effects in the limit of low noise intensity. The width of the distribution is observed to be sensitive to frequency spacing and the variation in width along the limit cycle can be significant for widely spaced modes. Drift effects of noise are characterized by quantifying the shift in the averaged solutions from the deterministic values and the sensitivity of this shift to frequency spacing is explored. Further, bifurcation scenarios that exist due to symmetric/asymmetric coupling as well as those introduced by noise are identified. Examples are presented that show the dynamics of the system in its ensemble averaged state space and numerically obtained probability density functions (PDFs) are used to support the observations in the ensemble averaged state space. For certain frequency spacing and low noise intensity, two fixed points can be observed in the phase space and the most probable solution can be identified from the PDFs as well as by visually observing the domain of attraction for the fixed points. As the noise intensity is increased, changes in the qualitative features of the system are evident in the ensemble averaged state space and PDFs.
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ItemKinetic-hydrodynamic instability interactions in near blowoff bluff body flames(Georgia Institute of Technology, 2022-05-02) Manosh Kumar, RaghulThis work examines the dynamics of confined bluff body flames in the process of lean blowoff (LBO) using simultaneous stereo-PIV (particle image velocimetry), OH PLIF (planar laser induced fluorescence) and CH2O PLIF. Flames at high density ratios blow off in at least two distinct stages: stage 1, where intermittent extinction occurs along the flame front, but the flame and flow remain qualitatively similar to stable conditions, and stage 2, where there is permanent downstream flame extinction and large-scale changes in dynamic flow characteristics. This work particularly focuses on stage 2 processes, with the goal of understanding what ultimately leads to irrecoverable flame blowoff. A new test facility was developed with the operational flexibility to achieve two goals: (1) approach LBO by keeping the parameters that influence its hydrodynamic stability approximately constant, particularly flow velocity (u_bulk) and gas expansion ratio (σ), and (2) compare near-LBO dynamics under conditions where, well away from blowoff, the flame is globally stable (high σ case) and globally unstable (low σ case, where the Bénard-Von Karman (BVK) instability of the flow is present). The latter case was of particular interest as most prior detailed diagnostic studies of LBO have been performed at high σ, BVK-suppressed conditions. We find, however, that the transient blowoff process remains largely unchanged in the high and low σ cases, presumably because the BVK instability reappears in either case under conditions very close to LBO. In all cases, blowoff is preceded by permanent downstream extinction that moves progressively closer to the bluff body as LBO is approached. We find that blowoff dynamics are intrinsically 3D, due to both secondary instabilities of the shear layer and confinement effects associated with bluff body-wall interactions. These 3D structures often manifest themselves as burning reactant fingers which are caught in the backflow of the recirculation zone; under very near LBO conditions they impinge on the back of the bluff body and extinguish. At the very edge of blowoff, the recirculation zone is no longer composed of hot products and is unable to autoignite the oncoming reactant flow, leading to global extinction. The characteristic time associated with this feedback between downstream extinction and wake structure alteration causing blowoff is about 2 orders of magnitude larger than the characteristic flow time, D/u_bulk.
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ItemShear layer dynamics of a reacting jet in a vitiated crossflow(Georgia Institute of Technology, 2020-12-01) Nair, VedanthThe jet in crossflow (JICF) is a canonical shear flow that is present in a number of practical configurations including industrial gas turbines. Its complex flow topology, heavily influenced by underlying hydrodynamic instabilities, makes it an attractive configuration to implement when the mixing performance is critical. Past studies analyzing the behavior of non-reacting jets have noted that the overall performance of JICF configurations can be tied to the behavior of the shear layer, which influences both near-field and far-field jet dynamics. As a result, techniques used to manipulate jet mixing and penetration, such as active jet modulation, require an understanding of the dominant instability characteristics of the shear layer. Although this configuration finds extensive use in reacting applications, the hydrodynamics of reacting flows are often fundamentally different from non-reacting flows, and few studies have analyzed the influence of heat release and reactions on JICF dynamics. In addition to varying the momentum flux ratio (J) and the density ratio (S) this study presents a novel method of systematically moving the flame position with respect to the shear layer to gauge its impact on shear layer stability. High speed optical diagnostics including Stereoscopic PIV, OH-PLIF and OH* chemiluminescence were used to quantify the flowfield and infer the behavior of the reaction zone. Moving the flame inside the shear layer was observed to significantly change the jet topology as the shear layer vortices (SLV) were completely suppressed. This was further quantified through a growth rate defined based on tracking the swirling strength of SLV structures. Other structural characteristics including the location of mixing transition were shown to be highly correlated with this extracted growth rate. Time-resolved velocity data was further used to quantify the shear layer spectrum by extracting the dominant instability frequencies and classify the instability behavior as convectively and globally unstable. In order to explain the observed instability behavior, the counter current shear layer (CCSL) model was used to extract an analogous stratification parameter (S’), which along with the counter current velocity (Λ) ratio was shown to capture the stability behavior of both non-reacting as well as reacting configurations.