Combustion is responsible for providing energy for many applications, especially in propulsion and rocket propellants. Shock tubes provide a controlled, repeatable means of studying combustion characteristics; although, most of these studies require the fuel in a mixture to exist in pure gas-phase. This makes it challenging to test low-vapor-pressure fuels that tend to remain in condensed form. Low-vapor-pressure fuels are commonly used in many combustion applications, making combustion studies of these fuels important. A method to study low-vapor-pressure fuels using a shock tube approach is to inject the fuel into the shock tube as tiny, uniformly-sized aerosol droplets. The sub-micron-sized aerosol droplets remain uniformly suspended in the shock tube prior to running the experiment. An incident shock wave vaporizes the liquid fuel droplets, then the reflected shock wave initiates ignition of the mixture. This study presents the characterization of an aerosol fuel injection method to the shock tube to study the combustion of low-vapor-pressure fuels. An aerosol generator was used to produce repeatable, uniformly-sized fuel droplets, and flow controllers were used to control and measure oxygen and argon dilution gas injected into the shock tube. A technique was developed to ensure consistent and repeatable aerosol fuel production rates over which calibration curves were found. This study presents the ignition delay times for C7H16 (ϕ = 1.0) at a pressure of 2.0 atm for temperatures from 1220 - 1427 K, C7H8 (ϕ = 1.0) at 1.9 atm over a temperature range of 1406 – 1791 K, and C12H26 (ϕ = 0.3) at 3.0 atm for the temperature range of 1293 – 1455 K. The ignition delay times for heptane and toluene were compared to the literature values at the same conditions and were found to be in good agreement. Laser extinction (visible laser at 632nm) was used to verify the presence of aerosol fuel droplets inside the shock tube for dodecane, but showed the heptane aerosol vaporized upon injection into the shock tube. Initial laser absorption (3.39 µm) measurements were also taken. This aerosol technique was found to successfully evaluate combustion effects of low-vapor-pressure fuels; however, was limited by the range of possible fuel concentrations. Further work needs to be performed on the verification of aerosol spatial uniformity and obtaining higher fuel concentrations.

The preionization of a shock tube's gas before the shock passes through it is called the precursor effect. An experimental and theoretical study has been carried out on precursor ionization in an electromagnetic shock tube. The precursor ionization was detected with different types of electric probes and also with photoraaltipliers. Extensive experiments indicated that the ionization was not due to diffusion of particles from the discharge in the shock tube driver. The ionization is primarily caused by radiation from the discharge of wavelengths less than 2000 Å. Radiation from the shock front makes a negligible contribution to the ionization. Langmuir double probe measurements indicated that the gas was about 0.1% ionized and that the electrons in the precursor were not in thermal equilibrium with the gas atoms and ions. The time interval between detection of ionization at two stations was independent of the shock tube gas (air, argon, helium), and corresponded to a propagation speed greater than 1/20 the speed of light. The precursor had a main component lasting about 50 microseconds with ionization proportional to the square of the discharge current. There was also a weaker component which lasted for about 500 microseconds. The experimental results can be understood in terms of a theoretical model based on black body radiation. Considering the driver to act as an infinite slab radiator, whose temperature is a function of the discharge parameters, an expression is derived for the number of photons emitted in some frequency interval. Assuming the electron density to be proportional to radiation absorption from such a radiator, the electron density variation with distance from the driver can be adequately understood. When the shock tube is considered to behave as a transmission line, whose resistance per unit length is proportional to the electron density, one can explain the variation of the shock tube's gas potential both with distance from the driver and with time. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The feasibility and characterization of a novel diaphragmless shock tube was examined at the National Aerothermochemistry Laboratory at Texas A&M University. The goal was to design a facility that reliably produces shock waves through air in a repeatable manner sufficient for statistical analysis. The device is modular, automated, and compact. The proposed diaphragmless shock tube uses a shock wave generating mechanism that consists of a rotating door and locking cam-shaft system. The facility produced the desired driver gas pressures repeatedly to within 0.31% at low-duty-cycle of 6 seconds. The driven gas pressure profiles within the test-section suggest that shock waves may be forming within test section for a driver gas pressure of 200 psig and above, which corresponds to shock wave Mach numbers of 1.7 to 2.0. The measured wave speeds were within 3.1% of that predicted by ideal shock tube theory; however, the induced driven gas pressures within the constant pressure region were approximately half that expedited from ideal shock tube theory.

The measure of chemiluminescence from the transition of the hydroxyl radical from its electronically excited state (A^2 Sigma^positive) to its ground state (X^2 Pi) is used in many combustion applications for diagnostic purposes due to the non-intrusive nature of the chemiluminescence measurement. The presence of the ultraviolet emission at 307nm is often used as an indicator of the flame zone in practical combustion systems, and its intensity may be correlated to the temperature distribution or other parameters of interest. To date, the measurement of the excited state OH, OH*, is mostly qualitative. With the use of an accurate chemical kinetics model, however, it is possible to obtain quantitative measurements. Shock-tube experiments have been performed in highly diluted mixtures of H2/O2/Ar at a wide range of pressures to evaluate the pressure-dependent rate coefficient of the title reaction. In such mixtures the main contributing reaction to the formation of OH* is, O H M = OH* M. R1 Previous work has determined the reaction rate of R1 at atmospheric conditions and accurately predicts the amount of OH* experimentally produced. At elevated pressures up to 15 atm, which are of interest to the gas turbine community, the currently used reaction rate of R1 (i.e., without any pressure dependence) significantly over predicts the amount of OH* formed. This work provides the pressure dependence of R1. The new reaction rate is able to reproduce the experimental data over the range of conditions studied and enables quantitative measurements applicable to practical combustion environments.

Stemming from a continuing demand for fuel surrogates, composed of only a few species, combustion of high-molecular-weight hydrocarbons (>C5) is of scientific interest due to their abundance in petroleum-based fuels, which contain hundreds of different hydrocarbon species, used for military, aviation, and transportation applications. Fuel surrogate development involves the use of a few hydrocarbon species to replicate the physical, chemical, combustion, and ignition properties of multi-component petroleum-based fuels, enabling fundamental studies to be performed in a more controlled manner. Of particular interest are straight-chained, saturated hydrocarbons (n-alkanes) due to the high concentration of these species in diesel and jet fuels. Prior to integrating a particular hydrocarbon into a surrogate fuel formulation, its individual properties are to be precisely known. n-Nonane (n-C9H20) is found in diesel and aviation fuels, and its combustion properties have received only minimal consideration. The present work involves first measurements of n-C9H20 oxidation in oxygen (O2) and argon (Ar), which were performed under dilute conditions at three levels of equivalence ratio (phi = 0.5, 1.0, and 2.0) and fixed pressure near 1.5 atm using a shock tube. Utilizing shock waves, high-temperature, fixed-pressure conditions are created within which the fuel reacts, where temperature and pressure are calculated using 1D shock theory and measurement of shock velocity. Of interest were measurements of ignition times and species time-histories of the hydroxyl (OH*) radical intermediate. A salient pre-ignition feature was observed in fuel-lean, stoichiometric, and fuel-rich OH* species profiles. The feature at each equivalence ratio was observed above 1400 K with the time-of-initiation (post reflected-shock) showing dependence on phi as the initiation time shortened with increasing phi. Relative percentage calculations reveal that the fuel-rich condition produces the largest quantity of pre-ignition OH*. Ignition delay time measurements and corresponding activation energy calculations show that the phi = 1.0 mixture was the most reactive, while the phi = 0.5 condition was least reactive.

To ensure safety regarding dust explosion hazards, it is important to study the dust lifting process experimentally and identify important parameters that will be valuable for development and validation of numerical predictions of this phenomenon. A new shock tube test section was developed and integrated into an existing shock tube facility. The test section allows for shadowgraph or laser scattering techniques to track dust layer particle motion. The test section is designed to handle an initial pressure of 1 atm with an incident shock wave velocity up to Mach 2 to mimic real world conditions. The test section features an easily removable dust pan and inserts to allow for adjustment of dust layer thickness. The design allows for the changing of experimental variables including initial pressure, Mach number, dust layer thickness and characteristics of the dust itself. A separate vacuum manifold was designed to protect existing equipment from negative side effects of the dust. A study was performed to demonstrate the capabilities of the new facility and to compare results with experimental trends formerly established in the literature. Forty-micron limestone dust with a layer thickness of 3.2 mm was subjected to Mach 1.22 and 1.38 shock waves, and a high-speed shadowgraph was used for flow visualization. Dust layer rise height was graphed with respect to shock wave propagation. Dust particles subjected to a Mach 1.38 shock wave rose more rapidly and to a greater height with respect to shock wave propagation than particles subjected to a Mach 1.22 shock wave. These results are in agreement with trends found in the literature, and a new area of investigation was identified.

The use of chemiluminescence measurements to monitor a range of combustion processes has been a popular area of study due to their reliable and cost-effective nature. Electronically excited carbon dioxide (CO2*) is known for its broadband emission, and its detection can lead to valuable information; however, due to its broadband characteristics, CO2* is difficult to isolate experimentally, and the chemical kinetics of this species is not well known. Although numerous works have monitored CO2* chemiluminescence, a full kinetic scheme for the species has yet to be developed. A series of shock-tube experiments was performed in H2-N2O-CO mixtures highly diluted in argon at conditions where emission from CO2* could be isolated and monitored. These results were used to evaluate the kinetics of CO2*, in particular, the main CO2* formation reaction, CO + O + M CO2* + M (R1). Based on collision theory, the quenching chemistry of CO2* was determined for eleven common collision partners. The final mechanism developed for CO2* consisted of 14 reactions and 13 species. The rate for R1 was determined based on low-pressure experiments performed in two different H2-N2O-CO-Ar mixtures. Final mechanism predictions were compared with the experimental results at low and high pressures, with good agreement seen at both conditions. Peak CO2* trends with temperature as well as overall CO2* species time histories were both monitored. Comparisons were also made with previous experiments in methane-oxygen mixtures, where there was slight over-prediction of CO2* experimental trends by the mechanism.Experimental results and mechanism predictions were also compared with past literature rates for CO2*, with good agreement for peak CO2* trends, and slight discrepancies in overall CO2* species time histories. Overall, the ability of the CO2* mechanism developed in this work to reproduce a range of experimental trends represents an improvement over existing models.

High-speed particle dispersion research is motivated by the energy release enhancement of explosives containing solid particles. In the initial explosive dispersal, a dense gas-solid flow can exist where the physics of phase interactions are not well understood. A dense particle flow is generated by the interaction of a shock wave with an initially stationary packed granular bed. The initial packed granular bed is produced by compressing loose aluminum oxide powder into a 6.35 mm thick wafer with a particle volume fraction of 0.48. The wafer is positioned inside the shock tube, uniformly filling the entire cross-section. This results in a clean experiment where no flow obstructing support structures are present. Through high-speed shadowgraph imaging and pressure measurements along the length of the channel, detailed information about the particle-shock interaction was obtained. Due to the limited strength of the Mach 2 incident shock wave, no transmitted shock wave is produced. The initial “solid-like” response of the particle wafer acceleration forms a series of compression waves that coalesce to form a shock wave. Breakup is initiated along the periphery of the wafer as the result of shear that forms due to the fixed boundary condition. Particle break-up starts at local failure sites that result in the formation of particle jets that extend ahead of the accelerating, largely intact, wafer core. In a circular tube the failure sites are uniformly distributed along the wafer circumference. In a square channel, the failure sites, and the subsequent particle jets, initially form at the corners due to the enhanced shear. The wafer breakup subsequently spreads to the edges forming a highly non-uniform particle cloud. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2012-09-25 14:15:37.615

It can be shown that for the complete description of all the physical parameters in the flow behind an imtermediate strength unsteady shock, a knowledge of the particle trajectories within the flow is sufficient. This principle has been applied to determine the variation of the physical parameters throughout the length of a conventional shock tube. The particle trajectories were obtained by the high speed photography of cigarette smoke tracers, placed at 10 cm. intervals along the tube. By applying the conservation of mass equation to the particle trajectory data, the density variation was obtained throughout the flow including the rarefaction wave from the end of the compression chamber and behind the first reflected shock from the closed end of the expansion chamber. By means of the Rankine-Hugoniot relation, the pressures immediately behind the incident and reflected shock fronts were calculated, and by assuming isentropic flow between shocks along any particle trajectory, the complete pressure variation was determined. The temperature and local sound speed were subsequently calculated at all points and the particle velocities were determined from the time derivative of the particle trajectories. A complete mapping of all the parameters in the shock tube was thus obtained using a single photographic technique, which is simpler than previous methods. / Graduate / 0605

The hydroxyl (OH) radical is a common intermediate species in any hydrogen- or hydrocarbon-based flame. Investigating OH at elevated temperatures and pressures is not a trivial task, and many considerations must be made to fully study the molecule. Shock tubes can provide the experimenter with a wide range of temperatures and pressures to investigate a variety of combustion characteristics including, but not limited to, OH kinetic profiles. Described in this dissertation is the diagnostic used to measure OH within a shock tube using UV absorption spectroscopy from an enhanced UV Xenon lamp passed through a spectrometer. OH absorption was made over a narrow range of wavelengths around 309.551 nm within the widely studied OH X→A ground vibrational transition region. Experiments have been performed in the shock-tube facility at Texas A&M University using this OH absorption diagnostic. A calibration mixture of stoichiometric H2/O2 diluted in 98% argon by volume was tested initially and compared with a well-known hydrogen-based kinetics mechanism to generate an absorption coefficient correlation. This correlation is valid over the range of conditions observed in the experiments at two pressures near 2 and 13 atm and temperatures from 1182 to 2017 K. Tests were completed using the absorption coefficient correlation on stoichiometric mixtures of methane, methane and water, ethane, ethylene, and acetylene to compare against a comprehensive, detailed chemical kinetics mechanism which considers up through C5 hydrocarbons. Measurements of methane show good agreement in peak OH formation and ignition delay time when compared with the mechanism. Improvements can be made in the shape of the methane-oxygen OH profile, and sensitivity and rate of production analyses were performed with the mechanism to identify key reactions for tuning. Similar results were found for methane-water-oxygen mixtures with no difference in profile shape or ignition delay time noted. There is room for improvement between the mechanism and measured values of OH for ethane-, ethylene-, and acetylene-based mixtures, although interesting pre-ignition features are nonetheless captured relatively well by the mechanism. Uncertainty in the measurement comes from the inherent noise in the photomultiplier tube signal and is ±25-150 ppm for the 2-atm experiments and ±6-25 ppm for the 13-atm experiments.