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Daniel Guggenheim School of Aerospace Engineering
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ItemModeling and analysis of chemiluminescence sensing for syngas, methane and jet-A combustion(Georgia Institute of Technology, 2008-06-17) Nori, Venkata NarasimhamFlame chemiluminescence has received increasing attention for its potential sensor and diagnostic applications in combustors. A number of studies have used flame chemiluminescence to monitor flame status, and combustor performance. While most of these studies have been empirical in nature, chemiluminescence modeling has the potential to provide a better understanding of the chemiluminescence processes and their dependence on various combustion operating conditions. The primary objective of this research was to identify and validate the important chemiluminescence reaction mechanisms for OH*, CH* and CO2*. To this end, measurements were performed at various operating conditions, primarily in laminar, premixed flames, fueled with methane, syngas (H2/CO) and Jet-A. The results are compared to 1-d laminar flame simulations employing the chemiluminescence mechanisms. The secondary objective was to use the experiments and validated chemiluminescence reaction mechanisms to evaluate the usefulness of flame chemiluminescence as a combustion diagnostic, particularly for heat release rate and equivalence ratio. The validation studies were able to identify specific mechanisms for OH*, CH* and CO2* that produced excellent agreement with the experimental data in most cases. The mechanisms were able to predict the variation of the chemiluminescence signals with equivalence ratio but not with pressure and reactant preheat. The possible reasons causing this disagreement could be due to the inaccuracies in the basic chemical mechanism used in the simulations, lack of accurate quenching data (for CH*), thermal excitation (for OH*) and radiative trapping (for OH* and CO2*) and interference from the emissions of other species (such as HCO and H2O), for CO2*. Regarding the utility of chemiluminescence for sensing, a number of observations can be made. In syngas-air flames, CO2* is a reasonable heat release rate marker, at least for very lean conditions. OH* shows some advantage in atmospheric-pressure methane and Jet-A flames in general, while CH* is advantageous at high pressure and very lean conditions at atmospheric pressure. The CO2*/OH* intensity ratio is not useful for sensing equivalence ratio in syngas flames, except maybe at very lean conditions. However, the CH*/OH* signal ratio is a promising approach for sensing equivalence ratio at low or very high pressure conditions in hydrocarbon flames. Thermal excitation and self-absorption processes for OH* chemiluminescence can become important for combustors operating at high pressure, high preheat and near stoichiometric conditions. Background subtracted chemiluminescence signals are recommended for sensing purposes.
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ItemAcetone planar laser-induced fluorescence and phosphorescence for mixing studies of multiphase flows at high pressure and temperature(Georgia Institute of Technology, 2008-05-19) Tran, Thao T.An extension of the current acetone Planar Laser-Induced Fluorescence (PLIF) was formulated for mixing studies of fluids at subcritical and supercritical conditions. The new technique, called Planar Laser-Induced Fluorescence and Phosphorescence (PLIFP), employs the difference in the mass diffusivity of the denser (liquid) to the less dense (vapor/supercritical fluid) to delineate the interface where a phase change occurs. The vapor/supercritical acetone fluorescence signal is utilized to measure of the acetone vapor density, the mixture fractions and liquid acetone phosphorescence signal to determine the location of the phase interface. The application of the technique requires the photophysical properties of vapor and liquid acetone to be known. Therefore, a series of controlled experiments were done to determine their photophysics at elevated temperatures and pressures up to T/TC = 1.2 and p/pC =1.25. The demonstration of the techniques shows it was able to provide quantitative measurements of acetone number density and the overall mixture fraction within the test chamber. Also, the size and mass of droplets that have broken off from the main jet were determined as well, though the ability is limited to small droplets (d~100 μm). In addition, the technique was able to delineate the low diffusivity (subcritical)/high diffusivity (supercritical) interface very well.
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ItemExperimental and numerical investigation of laminar flame speeds of H₂/CO/CO₂/N₂ mixtures(Georgia Institute of Technology, 2008-03-12) Natarajan, JayaprakashCoal derived synthetic gas (syngas) fuel is a promising solution for today s increasing demand for clean and reliable power. Syngas fuels are primarily mixtures of H2 and CO, often with large amounts of diluents such as N2, CO2, and H2O. The specific composition depends upon the fuel source and gasification technique. This requires gas turbine designers to develop fuel flexible combustors capable of operating with high conversion efficiency while maintaining low emissions for a wide range of syngas fuel mixtures. Design tools often used in combustor development require data on various fundamental gas combustion properties. For example, laminar flame speed is often an input as it has a significant impact upon the size and static stability of the combustor. Moreover it serves as a good validation parameter for leading kinetic models used for detailed combustion simulations. Thus the primary objective of this thesis is measurement of laminar flame speeds of syngas fuel mixtures at conditions relevant to ground-power gas turbines. To accomplish this goal, two flame speed measurement approaches were developed: a Bunsen flame approach modified to use the reaction zone area in order to reduce the influence of flame curvature on the measured flame speed and a stagnation flame approach employing a rounded bluff body. The modified Bunsen flame approach was validated against stretch-corrected approaches over a range of fuels and test conditions; the agreement is very good (less than 10% difference). Using the two measurement approaches, extensive flame speed information were obtained for lean syngas mixtures at a range of conditions: 1) 5 to 100% H2 in the H2/CO fuel mixture; 2) 300-700 K preheat temperature; 3) 1 to 15 atm pressure, and 4) 0-70% dilution with CO2 or N2. The second objective of this thesis is to use the flame speed data to validate leading kinetic mechanisms for syngas combustion. Comparisons of the experimental flame speeds to those predicted using detailed numerical simulations of strained and unstrained laminar flames indicate that all the current kinetic mechanisms tend to over predict the increase in flame speed with preheat temperature for medium and high H2 content fuel mixtures. A sensitivity analysis that includes reported uncertainties in rate constants reveals that the errors in the rate constants of the reactions involving HO2 seem to be the most likely cause for the observed higher preheat temperature dependence of the flame speeds. To enhance the accuracy of the current models, a more detailed sensitivity analysis based on temperature dependent reaction rate parameters should be considered as the problem seems to be in the intermediate temperature range (~800-1200 K).
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ItemEffects of the reacting flowfield on combustion processes in a stagnation point reverse flow combustor(Georgia Institute of Technology, 2008-01-15) Gopalakrishnan, PriyaThe performance of dry, low NOx gas turbines, which employ lean premixed (or partially premixed) combustors, is often limited by combustor stability. To overcome this issue, a novel design, referred to as a Stagnation Point Reverse Flow (SPRF) combustor, has been recently demonstrated. The SPRF combustor has been shown to produce low NOx emissions with both gaseous and liquid fuels. The objective of this thesis is to elucidate the interactions between the flowfield and combustion processes in this combustor for gas- and liquid-fueled operation. This is achieved with experimental measurements employing various optical diagnostic techniques. These include Particle Image Velocimetry (PIV), chemiluminescence imaging, Planar Laser-Induced Fluorescence (PLIF) of OH radicals and laser scattering from liquid droplets. Velocity measurements in gas-fueled operation show that both nonreacting and reacting flows exhibit a stagnation region with low mean velocity and high turbulence intensities. The high shear between the forward and reverse flows causes significant recirculation resulting in enhanced entrainment and mixing of the returning product gases into the incoming reactant jet for the reacting flow cases, which enables stable operation of the combustor at very lean equivalence ratios. Nonpremixed operation produces a flowfield similar to premixed case except in the near-field region where high turbulence intensities result in significant fuel-air mixing before combustion occurs. Operation of the SPRF combustor with liquid Jet-A is also investigated experimentally. The results indicate that while the overall flow features are similar to the gas-fueled SPRF combustor, the combustion characteristics and NOx performance in liquid operation are strongly controlled by fuel dispersion and evaporation. Injecting the liquid at the exit of the air annulus results in a highly lifted flame, similar to nonpremixed gaseous operation. On the other hand, retracting the fuel injector well inside the air annulus produces a well-dispersed fuel pattern at the reactant inlet leading to a reduction of the equivalence ratio in the fuel consuming reaction zones. Since the effective Dahmkohler number increases with global equivalence ratio, the difference in NOx emissions is more pronounced at higher fuel-air ratios as the retracted injector lowers the relative mixing time compared to the flush case.
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ItemParticle Vaporization Velocimetry and Quantitative Soot Concentration Measurement in Sooty Flows(Georgia Institute of Technology, 2007-11-15) Yang, PingSoot is a combustion generated pollutant that is both a direct risk to human health and a contributing source to global environmental change. Soot can also be a controlling factor in heat transfer inside combustion systems. Thus there is a growing interest in being able to measure soot and understand its production in practical, turbulent combustion environments. Therefore, the specific objectives of this research work were: (1) developing a way to measure velocity of sooty regions that is compatible with existing methods for measuring temporally and spatially resolved soot concentration fields and (2) using these methods to make quantitative measurements of soot in an unsteady, turbulent-like combustor. The Particle Vaporization Velocimetry (PVV) technique was developed and is compatible with Laser Induced Incandescence (LII), a soot concentration measurement approach. PVV is a flow tagging approach, where a high intensity laser (~2-3 J/cm2) is used to vaporize a small region in the soot field. This approach was demonstrated to produce a long lasting and easily readable flow tag that allows for velocity measurements over a wide range of velocities. LII proved to be the best method for detection the motion of the tag after a fixed delay. PVV and LII were used to measure velocity and two-dimensional soot concentration fields in an acoustically excited burner. In addition, images of soot luminosity were obtained. Both laminar and transitional acetylene diffusion flames were studied. The results reveal that strong acoustic forcing can significantly reduce total flame soot, as well as maximum soot concentrations, while simultaneously increasing the average soot temperature. The influence of acoustically generated vortices on soot formation was studied, and soot and products mixture mostly likely dominant high soot concentration regions. Eventually, these mixtures will be propagated downstream and oxidized as a diffusion flame.
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ItemFlame stabilization and mixing characteristics in a stagnation point reverse flow combustor(Georgia Institute of Technology, 2007-10-10) Bobba, Mohan KrishnaA novel combustor design, referred to as the Stagnation Point Reverse-Flow (SPRF) combustor, was recently developed that is able to operate stably at very lean fuel-air mixtures and with low NOx emissions even when the fuel and air are not premixed before entering the combustor. The primary objective of this work is to elucidate the underlying physics behind the excellent stability and emissions performance of the SPRF combustor. The approach is to experimentally characterize velocities, species mixing, heat release and flame structure in an atmospheric pressure SPRF combustor with the help of various optical diagnostic techniques: OH PLIF, chemiluminescence imaging, PIV and Spontaneous Raman Scattering. Results indicate that the combustor is primarily stabilized in a region downstream of the injector that is characterized by low average velocities and high turbulence levels; this is also the region where most of the heat release occurs. High turbulence levels in the shear layer lead to increased product entrainment levels, elevating the reaction rates and thereby enhancing the combustor stability. The effect of product entrainment on chemical timescales and the flame structure is illustrated with simple reactor models. Although reactants are found to burn in a highly preheated (1300 K) and turbulent environment due to mixing with hot product gases, the residence times are sufficiently long compared to the ignition timescales such that the reactants do not autoignite. Turbulent flame structure analysis indicates that the flame is primarily in the thin reaction zones regime throughout the combustor, and it tends to become more flamelet like with increasing distance from the injector. Fuel-air mixing measurements in case of non-premixed operation indicate that the fuel is shielded from hot products until it is fully mixed with air, providing nearly premixed performance without the safety issues associated with premixing. The reduction in NOx emissions in the SPRF combustor are primarily due to its ability to stably operate under ultra lean (and nearly premixed) condition within the combustor. Further, to extend the usefulness of this combustor configuration to various applications, combustor geometry scaling rules were developed with the help of simplified coaxial and opposed jet models.
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ItemCharacterizing High-Energy-Density Propellants for Space Propulsion Applications(Georgia Institute of Technology, 2007-04-05) Kokan, Timothy SalimThere exists wide ranging research interest in high-energy-density matter (HEDM) propellants as a potential replacement for existing industry standard fuels for liquid rocket engines. The U.S. Air Force Research Laboratory, the U.S. Army Research Lab, the NASA Marshall Space Flight Center, and the NASA Glenn Research Center each either recently concluded or currently has ongoing programs in the synthesis and development of these potential new propellants. In order to perform conceptual designs using these new propellants, most conceptual rocket engine powerhead design tools (e.g. NPSS, ROCETS, and REDTOP-2) require several thermophysical properties of a given propellant over a wide range of temperature and pressure. These properties include enthalpy, entropy, density, viscosity, and thermal conductivity. Very little thermophysical property data exists for most of these potential new HEDM propellants. Experimental testing of these properties is both expensive and time consuming and is impractical in a conceptual vehicle design environment. A new technique for determining these thermophysical properties of potential new rocket engine propellants is presented. The technique uses a combination of three different computational methods to determine these properties. Quantum mechanics and molecular dynamics are used to model new propellants at a molecular level in order to calculate density, enthalpy, and entropy. Additivity methods are used to calculate the kinematic viscosity and thermal conductivity of new propellants. This new technique is validated via a series of verification experiments of HEDM compounds. Results are provided for two HEDM propellants: quadricyclane and 2-azido-N, N-dimethylethanamine (DMAZ). In each case, the new technique does a better job than the best current computational methods at accurately matching the experimental data of the HEDM compounds of interest. A case study is provided to help quantify the vehicle level impacts of using HEDM propellants. The case study consists of the National Aeronautics and Space Administrations (NASA) Exploration Systems Architecture Study (ESAS) Lunar Surface Access Module (LSAM). The results of this study show that the use of HEDM propellants instead of hypergolic propellants can lower the gross weight of the LSAM and may be an attractive alternative to the current baseline hypergolic propellant choice.
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ItemCombustor Exhaust Temperature Nonuniformity Sensing Using Diode Laser Absorption(Georgia Institute of Technology, 2007-02-12) Palaghita, Tudor I.This thesis describes the development of a sensing technique for temperature nonuniformity along the line of sight through combustion exhaust, geared for gas turbine applications. Tunable diode laser absorption spectroscopy is used to measure three absorption lines and compute a variable to characterize the level of temperature nonuniformity along the laser path. Nonuniformity information is obtained from one line of sight sensor because the absorption has a nonlinear dependence on temperature. This dependence is analyzed to determine the behaviour, shape, and response of absorption lines measured through mediums with nonuniform temperature profiles. Based on this analysis a new line selection process for nonuniformity sensing is developed. A sensor for temperature nonuniformity is proposed and demonstrated through computer simulations and experiments in the exhaust of a laboratory-scale combustor. The nonuniformity variable, U, is shown to monotonically track the level of temperature nonuniformity along the laser path. The capabilities of this sensing technique are determined based on a comprehensive analysis of errors and their effect on sensor performance. Methods to mitigate these errors are described, and the overall sensor capability is determined based on the characteristics of state of the art diode laser and absorption sensor technology. Such a sensor is capable of measuring minimum temperature deviations of 17% or more, which is well within the needed capabilities for industrial applications. Furthermore, the results and knowledge presented in this thesis apply to other absorption based sensing techniques.