This study sought to characterize the behavior of exhausted electric thruster xenon ions in the near-Earth magnetospheric environment as functions of various trajectory and particle attributes, neglecting effects of electric fields, plasma waves and particle collisions. This was done via simulation using the AeroTracer program, a software tool which computes ion trajectories within the magnetosphere by applying an adaptive step-size Runge-Kutta technique to the fully relativistic Lorentz equation. Over 3,800 independent simulations were performed, with variables including release position, release energy and direction, ion charge, and orbital phase. Initial release altitude was a major driver in determining whether the ion eventually fell to Earth (“Below Minimum Altitude” or BMA), remained trapped by the simulation’s end (“Maximum Number of Steps” or MNS), or traveled beyond the magnetosphere (“Lost to Space” or LTS). Ions expelled at the highest altitudes investigated - 60,000 km and above - almost invariably were lost to space. Like altitude, increasing inclination and energy were important factors that reduced trapping, affecting the outcome probabilities. Higher charge state produced strong improvement of trapping capability. Effects of orbital phase, day of year and solar cycle phase were also apparent. A transition region was found in the 20,000 km to 60,000 km altitude range, within which the sensitivity of outcomes to parameter variation increased. The ordered sequence MNS> BMA> LTS was found to be consistent with decreasing confinement capability, and it was manifested consistently as parameters were varied.

Recent work by the authors and others has uncovered the need for further chemical kinetic-related modeling and experiments, specifically for NOx kinetics at engine conditions. In particular, data on CH formation at realistic combustion conditions are needed for further refinement of the prompt-NOx kinetics. To this end, a series of shock-tube experiments to obtain CH concentration time histories at elevated temperatures was performed behind reflected shock waves at The Aerospace Corporation using a tuneable laser. This Ti-Sapphire laser was operated in the near infrared at about 854 nm; blue light at 426.9 nm was obtained using an external, frequency-doubling crystal. The resulting light was used in a differential absorption setup with common-mode rejection to measure CH time histories. New measurements in CH4-C2H6-O-2 mixtures highly diluted in argon were performed at temperatures between 1890 K and 2719 K. These new data are compared to several modern, detailed chemical kinetics mechanisms with updated NOx submechanisms. Sensitivity and rate of production analyses at the shock-tube conditions along with a gas turbine model are used to elucidate the current state of affairs in CH prediction by the literature models and its effect on NOx production, particularly through the prompt mechanism. A brief discussion of the chemical kinetics for an important reaction in the production of CH is also presented to emphasize the need for further study and refinement of reactions leading to CH production.

Metal particle contamination is a concern for liquid rocket engines that use enriched O2 at high pressure. It is believed that under some engine conditions contaminant particle impact could release sufficient kinetic energy to initiate combustion, providing an ignition source for engine components (e.g., turbine blades) impacted by the particles, and subsequently a combustion event that eventually consumes structural materials of the engine. It is important that the combustion properties of these candidate metal particles be studied for their propensity to cause ignition under rocket-like conditions, to reduce the risk of engine failure. Laboratory study of such a mechanism under realistic engine conditions is difficult, and data are lacking. Data that reveal the influence of particle mass, kinetic energy, impacted-surface composition, and environmental conditions on ignition propensity are valuable for launch programs involving oxidizer-rich, staged combustion engines.

Hydrogen peroxide was measured in a shock tube via absorption spectra of the ν6 (asymmetric HOO bending) fundamental band using a lead–salt diode laser tuned to 7845.63 nm. At this wavelength, there is minimal interference from water absorption at combustion temperatures and pressures, even for water levels that are more than two orders of magnitude larger than the H2O2 concentration. For obtaining a calibration of the laser absorption technique, a robust method of hydrogen peroxide delivery was employed so that a standard H2O2–H2O mixture could be used as the source, but with high certainty as to the initial concentration of H2O2. The initial amount of hydrogen peroxide was measured using a water absorption laser diagnostic to measure the water concentration in conjunction with the known thermodynamics of the vapor mixture, highly diluted in argon. Measurements were made with argon as the bath gas at temperatures ranging from 292 to 1296 K and a pressure range of 0.05–1.43 atm, resulting in a calibration for the absorption coefficient of H2O2 at 7845.63 nm. To the authors' knowledge, this study marks the first successful measurement of H2O2 concentration in a shock tube using tunable laser absorption in the infrared.

†Initial results were obtained with a miniature, high-pressure shock tube facility, built recently for the study of ignition events produced by the surface impact of metal particles in an oxygen environment. The experiment is designed to simulate particle contamination effects in liquid rocket engines. Non-spherical, sub-mm aluminum particles were seeded in the apparatus and impacted on a target surface. Particles were imaged using continuous backlight and 90-deg strobe reflection configurations, each presenting some advantages. A subset of available test conditions was used to obtain initial data on particle velocity and result of striking the target surface. It was confirmed that a particle-based ignition event can occur under the test condition of the facility. Additional improvements in the imaging technique and a much greater variety of test conditions are planned.

Results from a heterogeneous shock-tube approach recently demonstrated at Texas A&M University, wherein a hydrocarbon fuel is introduced in liquid phase with gaseous oxidizer, are presented. The shock tube has been designed for controlled measurement of ignition delay times, sooting phenomena, radical species concentrations, time-dependent species profiles, and nanoparticle-aided combustion using heavy hydrocarbons which are difficult to study using the traditional shock tube approach. Aerosol is generated in a high-vacuum manifold positioned 4-m from the endwall where optical and pressure-based diagnostics are stationed. The approach reduces the propensity for fuel-film deposition near the endwall avoiding optical and/or kinetic disturbances that could result. The aerosol enters the shock tube initially as a two-phase flow of liquid fuel and gaseous oxidizer/inert gas. Liquid droplets partially evaporate while resident in the shock tube, prior to shock wave generation, and are then completely vaporized behind the incident shock wave. Behind the reflected shock wave, then, resides a pure gas-phase fuel and oxidizer mixture. The primary benefit of the aerosol shock tube approach is the ability to inject fuels of low vapor pressure at high or low concentrations. The classic shock-tube approach introduces gas-phase constituents only, and has difficulty accommodating low vapor-pressure liquids, except when component partial pressures are much lower than what is usually required. In the present work, n-heptane aerosol (C7H16, Pvap, 20 °C ∼ 35 torr), was generated with O2/Ar carrier gas and dispersed in the shock tube in a uniform manner. Stoichiometric ignition delay times with temperature varied from 1240 K to 1600 K and pressure maintained near 2.0 atm are compared to gas-phase data at similar conditions and a chemical kinetic model for heptane combustion. Excellent agreement was found between the two-phase aerosol approach and the classical method involving vapor-phase n-heptane and pre-mixed gases. The measured activation energy for the stoichiometric mixture at 2.0 atm (EA = 42.3 kcal /mol), obtained with the two-phase technique, compares well with the literature value.

A facility has been constructed for the study of ignition events, induced by the surface impact of metal particles in a high-pressure oxygen environment. It is designed to simulate particle contamination effects in liquid rocket engines. The apparatus is essentially an open- ended particle shock tube driven by high-pressure helium gas. The driver gas can be compressed iteratively to a pressure as high as 60,000 psi, using a piston-driven pump powered by a stepper motor. When the diaphragm breaks, a shock wave propagates through the driven-section test gas. The resulting high speed flow of the test gas can entrain small particles placed in a cup on the bottom of the 6-mm bore. The narrow bore exits into an expansion region, where a target assembly is positioned to intercept the high-speed particles, providing a surface on which particle impingement may lead to ignition if the test gas is high-pressure oxygen. Details of the facility construction are presented and analytical predictions are given for apparatus performance over a wide range of conditions. Pressure measurements were made along the length of the driven section, and flow visualization images were obtained in the target region for a wide variety of test conditions to reveal the flow structure. Using these measurements over a wide range of operating points, the performance of the facility was characterized in terms of the exit flow conditions and the conditions at the target region. These results are presented and analyzed.

A hybrid finite element/molecular dynamics (FEM/MD) simulation was developed to simulate the collisions of micron sized aluminum particles with a nickel wall in a highpressure liquid oxygen environment such as could be found in a liquid rocket engine. The finite element method was chosen in order to model full sized (100-500 micron diameter) aluminum particles. Molecular dynamics is used to accurately simulate the physics occurring at the contact point between the sphere and the wall: the collision dynamics, aluminum fracture and heating up, and subsequent initiation of aluminum-oxygen reactions on the surface of the sphere. The two simulation regimes are coupled by providing boundary conditions for each other. Initial results show the heating of the aluminum surface due to the collision of the aluminum particle with the wall at an impact velocity of 500 m/s.

Shock-tube experiments and chemical kinetics modeling were performed to further understand the ignition and oxidation kinetics of lean methane-based fuel blends at gas turbine pressures. Such data are required because the likelihood of gas turbine engines operating on CH4-based fuel blends with significant (>10%) amounts of hydrogen, ethane, and other hydrocarbons is very high. Ignition delay times were obtained behind reflected shock waves for fuel mixtures consisting of CH4, CH4∕H2, CH4∕C2H6, and CH4∕C3H8 in ratios ranging from 90/10% to 60/40%. Lean fuel/air equivalence ratios (ϕ=0.5) were utilized, and the test pressures ranged from 0.54 to 30.0atm. The test temperatures were from 1090K to 2001K. Significant reductions in ignition delay time were seen with the fuel blends relative to the CH4-only mixtures at all conditions. However, the temperature dependence (i.e., activation energy) of the ignition times was little affected by the additives for the range of mixtures and temperatures of this study. In general, the activation energy of ignition for all mixtures except the CH4∕C3H8 one was smaller at temperatures below approximately1300K(∼27kcal∕mol) than at temperatures above this value (∼41kcal∕mol). A methane/hydrocarbon–oxidation chemical kinetics mechanism developed in a recent study was able to reproduce the high-pressure, fuel-lean data for the fuel/air mixtures. The results herein extend the ignition delay time database for lean methane blends to higher pressures (30atm) and lower temperatures (1100K) than considered previously and represent a major step toward understanding the oxidation chemistry of such mixtures at gas turbine pressures. Extrapolation of the results to gas turbine premixer conditions at temperatures less than 800K should be avoided however because the temperature dependence of the ignition time may change dramatically from that obtained herein.

Ignition and oxidation characteristics of CO/H 2 fuel blends were studied using both experimental and computer simulation methods. Shock-tube experiments were conducted behind reflected shock waves at intermediate temperatures (890 < T < 1300 K) for three pressure regimes of approximately 1, 2.5, and 15 atm. Results of this study provide the first undiluted fuel-air ignition-delay-time experiments to cover such a wide range of CO/H 2 composition (5-80% H 2 ) over the stated temperature range. Emission in the form of chemiluminescence from the hydroxyl radical (OH*)A 2 Σ + → X 2 Π transition near 307 nm was used to monitor the reaction progress from which ignition delay times were determined. In addition to the experimental analysis, chemical kinetics calculations were completed to compare several chemical kinetics mechanisms with the new experimental results. The models were in excellent agreement with the shock-tube data, especially at higher temperatures and lower pressures, yet there were some differences between the models at the higher pressures and lowest temperatures, in some cases by as much as a factor of 5. Ignition-delay-time and reaction-rate sensitivity analyses were completed at higher and lower temperatures and higher and lower pressures to identify the key reactions responsible for ignition. The results of the sensitivity analysis indicate that the ignition-enhancing reaction H + O 2 = O + OH and hydrogen oxidation kinetics in general were most important, regardless of mixture composition, temperature, or pressure. However, lower-temperature, higher-pressure ignition-delay-time results indicate additional influence from HO 2 - and CO-containing reactions, particularly, the well-known H + O + M = HO 2 + M reaction and the CO + O + M = CO 2 + M and CO + HO 2 = CO 2 + OH reactions. Differences in the rates of the CO-related reactions are shown to be the cause of discrepancies among the various models at elevated pressures. Additional calculations were performed to show that the mixtures used are insensitive to small levels of water vapor, and the disagreement between experiment and model at the lowest temperatures and higher H 2 concentrations cannot be explained by possible impurities.

To study combustion chemistry at low temperatures in a shock tube, it is of great importance to increase experimental test times, and this can be done by tailoring the interface between the driver and driven gases. Using unconventional driver-gas tailoring with the assistance of tailoring curves, shock-tube test times were increased from 1 to 15 ms for reflected-shock temperatures below 1,000 K. Provided in this paper is the introduction of tailoring curves, produced from a one-dimensional perfect gas model for a wide range of driver gases and the production and demonstration of successful driver mixtures containing helium combined with either propane or carbon dioxide. The He/CO2 and He/C3H8 driver mixtures provide a unique way to produce a tailored interface and, hence, longer test times, when facility modification is not an option. The tailoring curves can be used to guide future applications of this technique to other configurations. Nonreacting validation experiments using driver mixtures identified from the tailoring curves were performed over a range of reflected-shock temperatures from approximately 800 to 1,400 K, and some examples of ignition-time experiments that could not have otherwise been erformed are presented.

Shock-tube ignition delay time experiments and chemical kinetics model calculations were performed for several fuel blends of carbon monoxide and hydrogen in air at elevated pressures. Due to the interest in coal-derived fuels, namely syngas, these data are important for characterizing the ignition and oxidation of possible fuel blends used in gas turbines and for the validation of chemical kinetics models. Three lean, CO/H2 (80/20%, 90/10%, and 95/5% by volume) fuel blends in air were studied behind reflected shock waves at temperatures between 929 and 1304 K and pressures ranging from 1.7 to 15 atm. Ignition delay times were monitored using chemiluminescence emission from excited hydroxyl radicals. Results exhibit the second-explosion limit behavior from hydrogen oxidation kinetics at low temperatures and high pressures for all mixtures. In addition, comparisons of modeling results and experimental data show good agreement for the entire temperature range at high pressure and poor agreement with the data at low temperatures in the intermediate pressure regimes. Ignition and reaction sensitivity analyses indicate that the H + O2 + M = HO2 + M termination reaction is important at all conditions herein, and the early formation of HO2 suppresses the growth of the ignition-enhancing radicals H and OH.

*† ‡ A technique for uniformly dispersing solid particles into the driven section of a shock tube was demonstrated using aluminum powder. The technique uses a particle injector in a secondary vessel to produce suspended Al particles in the carrier gas, which is then introduced into the shock tube. Laser extinction measurements were made to examine the performance of the injection process. Using this approach, heterogeneous shock-tube experiments were demonstrated for 20-µm aluminum particles in air. These experiments were conducted behind reflected shock waves for temperatures ranging between (2320 < T < 3172 K) and pressures near 1 atmosphere. Visible emission from the transition of the intermediate species AlO near 488 nm was used to monitor reaction progress. + + Σ →

When considering the shock-induced ignition of common fuels, it is well known that methane produces the longest ignition delay times of all the hydrocarbons (Lifshitz et al., 1971; Petersen et al., 1996). In contrast, hydrogen produces some of the shortest ignition delay times of all fuels, even when compared to higher-order hydrocarbons. For applications involving phenomena such as supersonic combustion or detonation waves, short ignition delay times are desirable. In situations where hydrocarbon fuels are preferred because of their higher energy densities, one way to accelerate the ignition process is to add hydrogen. In applications such as power generation gas turbines, the fuel blends may already contain levels of hydrogen, particularly coal-derived fuels containing mostly CO and H2. Fundamental data on the effects of H2 addition are useful for obtaining a better understanding of the chemical kinetics of ignition behind shock waves and the ignition of hydrogen-containing fuel mixtures in general.