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Daniel Guggenheim School of Aerospace Engineering
Daniel Guggenheim School of Aerospace Engineering
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ItemEntrainment, Mixing, and Ignition in Single and Multiple Jets in a Supersonic Crossflow(Georgia Institute of Technology, 2020-08-19) Fries, DanJets in crossflow are a canonical example for three-dimensional turbulent mixing. Here, non-reacting and reacting sonic jets in a supersonic crossflow are studied. The influence of injectant properties on turbulent mixing is investigated. Using pure gases, the molecular weight and specific heat ratio is varied between 4-44 g/mol and 1.24-1.66, respectively. The jets are injected into a Mach 1.71 crossflow with a stagnation temperature ~600 K. Two single jet injectors and two staged jet injectors are designed to characterize potential enhancements in turbulent mixing and combustion processes. Mixture fraction and velocity fields are determined via Mie-scattering off solid particles. Velocity vectors are obtained by processing Mie-scattering image pairs with a correlation technique (particle image velocimetry). To ignite the flow field and enable systematic variation of the ignition location a traversable laser spark system is employed. The reacting flow is probed via CH* chemiluminescence and OH planar laser induced fluorescence visualizing regions containing hot combustion products. A new trajectory scaling improves correlation between all data sets considered, suggesting that the bow shock, boundary layer and momentum flux ratio are the dominant controlling factors. Turbulent mixing rates are highest for injectants with higher molecular weight and lower specific heat ratio. The larger of two jet spacings tested yields the greater enhancement of turbulent mixing rates. Ignition locations on the symmetry plane of the flow field are evaluated for their ability to sustain chemical reactions/heat release. Most favorable ignition locations lie in the windward jet shear layer away from the regions of highest flow strain. The smallest diameter single jet with presumably more boundary layer interaction and moderate strain rates provides the best results with regard to thermal energy release after spark deposition. Trends suggest that moderate compressible strain rates and no flow expansion are advantageous to sustain thermal energy release. Implications for future research directions and opportunities are discussed.
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ItemInvestigation of high-pressure methane and syngas autoignition delay times(Georgia Institute of Technology, 2019-08-28) Karimi, MiadThis thesis reports methane (CH4) and a syngas mixture (H2/CO=95:5) autoignition delay measurements relevant to operating conditions of supercritical carbon dioxide (sCO2) power cycle (100 to 300 bar) combustors. To acquire data at these conditions as part of this thesis, a new high-pressure shock tube is designed, fabricated and commissioned. The experiments are conducted for diluted carbon dioxide environments at 100 and 200 bar and at temperatures within the range of approximately 1100–1400 K. To investigate the chemical effect of CO2 at supercritical conditions, experiments are conducted at similar pressures and temperatures by substituting CO2 with an inert bath gas, Ar (argon). Obtaining ignition delay times in Ar bath gas allows to systematically study the chemical effect of CO2 on ignition chemistry. Methane ignition delay times are compared to several chemical kinetic models, such as Aramco 2.0, FFCM-1, HP-Mech, USC Mech II and GRI 3.0. For the conditions of this study, predictions of the Aramco 2.0 kinetic model show the overall best agreement with experimental measurements. Following the experimental data, brute-force sensitivity analyses and reaction pathway flux analyses are utilized to gain insight into details of the ignition chemistry of the fuels (CH4 and H2/CO=95:5). These analyses indicate that methyl (CH3) recombination to form ethane (C2H6) and oxidation of CH3 to form methoxide (CH3O) are the most important reactions controlling the ignition behavior of methane at temperatures greater than approximately 1250 K. However, at temperatures below approximately 1250 K, an additional reaction pathway for methyl radicals is found through CH3+O2+M=CH3O2+M, which leads to formation of methyldioxidanyl (CH3O2). This reaction pathway plays a distinct role in dictating the ignition trends at lower temperature conditions. Replacing CO2 with argon as the bath gas reveals that CO2 does not have major effects on ignition chemistry of CH4. A similar approach is taken to obtain experimental data at 100 bar and 200 bar for a syngas fuel mixture of 95% H2 (hydrogen) and 5% CO (carbon monoxide) in CO2 and Ar bath gasses. Aramco 2.0 kinetic model, FFCM-1 kinetic model, HP-Mech and USC Mech II show good agreement with the measured ignition delay times. Detailed sensitivity analyses of these kinetic models highlight the importance of the third-body reaction between hydrogen atoms (H) and oxygen molecules (O2) through H+O2+M=HO2+M to form hydroperoxyl (HO2). In both cases, irrespective of the diluents, this reaction is the most influential reaction to hinder ignition. Ignition delay times obtained from both mixtures not only show a similar trend, but also the same magnitude when compared to the CO2 mixture. While this observation may suggest that CO2 has no chemical effect on ignition chemistry, it is found to play a counterbalancing role on syngas ignition at the elevated pressures and temperatures of this study. CO2 increases the OH (hydroxyl) radical production by colliding with hydrogen peroxide (H2O2) through H2O2+M=OH+OH+M. However, it reduces OH production through HO2+H=OH+OH due to a lower amount of H radical production compared to the Ar mixture. Therefore, these two effects cancel out the change of OH productions, and CO2 does not change the ignition delay time of the syngas mixture considered in this study upon comparison with the mixture with Ar bath gas.
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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.