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Plasma-Assisted Ignition in Scramjets

This study assesses the prospect of main-fuel ignition with plasma-generating devices in a supersonic flow. Progress from this study has established baseline conditions for operation, such as the required operational time of a device to initiate a combustion shock train as predicted by computational fluid dynamics computations. Two plasma torches were investigated: a direct current constricted-arc design and an alternating current unconstricted-arc design based on a modified spark plug. Both plasma torches are realistic in size and operate within the same current and voltage constraints, although differing substantially in orifice geometry. To compare the potential of each concept, the flow physics of each part of the igniter/fuel-injector/combustor system was studied. To understand the constraints involved with the ignition process of a hydrocarbon fuel jet, an experimental effort to study gaseous and liquid hydrocarbons was conducted, involving the testing of ethylene and JP-7 fuels with nitrogen and air plasmas. Results from individual igniter studies have shown plasma igniters to produce hot pockets of highly excited gas with peak temperatures up to 5000 K at only 2 kW total input power. In addition, ethylene and JP-7 flames with a significant level of the hydroxyl radical, as determined by planar laser-induced fluorescence, were also produced in a Mach 2 supersonic flow with a total temperature and pressure of 590 K and 5.4 atm. Information from these experiments is being applied to the generation of constraints and the development of a configuration with perceived high ignition potential in full scramjet combustor testing.

Passive optical diagnostic of Xe-propelled Hall thrusters. I. Emission cross sections

This paper presents a set of xenon apparent emission excitation cross sections for emission lines that have diagnostic value in the analysis of Xe-propelled Hall thruster plasmas. Emission cross sections are presented for three excitation processes involving ground-state xenon atoms: e−+Xe, Xe++Xe, and Xe2++Xe. The cross sections are derived from luminescence spectra produced at single-collision conditions. Apparent emission excitation cross sections are tabulated for 12 visible and 8 near-infrared lines for electron energies ranging from 10to70eV. In case of the near-infrared lines, radiation trapping effects are accounted for by measuring the detailed pressure dependence of the apparent emission cross sections and extrapolating to zero pressure. A semiempirical expression for the pressure dependence is derived that allows zero-pressure extrapolation from threshold to 70eV. Ion-induced cross sections are reported for the same emission lines at an energy per unit charge E∕q of 300eV, chosen for typical Ha...

Diode Laser Diagnostics for High Speed Flows

Tunable diode laser absorption spectroscopy (TDLAS) using single mode diode lasers that are temperature stabilized and current tuned were used to measure the air velocity in the isolator section of a research scramjet flowpath. Tests were conducted in the Research Cell 18 direct connect wind tunnel facility at WPAFB. TDLAS was used to detect water and oxygen at concentrations of 2.2x10 17 cm -3 and 1.8x10 18 cm -3 , respectively, in the isolator operated at simulated flight Mach numbers of 3.5 and 4.0 and a dynamic pressure of 1000 psf. To achieve these simulated flight conditions, a natural gas fueled vitiator was used to elevate the air temperature, and a Mach-1.8 facility nozzle was installed to expand the flow. The velocity was determined at these conditions, and above and below, by varying the vitiator temperature in 100oR increments from 798oR to 1593oR in order to provide calculated flow velocities from 600 m/s to 850 m/s based on isentropic equations. The TDLAS velocity data agree to within 22 m/s and 32 m/s (1 σ) for data obtained at 100 millisecond and 10 millisecond sampling intervals, respectively.

Quantitative detection of singlet O2 by cavity-enhanced absorption

A method for the practical determination of the absolute concentration of single (a1delta(g)) oxygen is discussed. The method is based on sensitive off-axis integrated-cavity-output spectroscopy (ICOS). Off-axis ICOS allows narrowband, continuous-wave lasers to be used in conjunction with optical cavities to record sensitive absorption measurements. The details of the method as well as spectroscopic data confirming the first observation of the (1, 0) band of the b1sigma(g)(+) - a1delta(g) Noxon system are presented. The absolute transition probabilities for the b1sigma(g)(+) - a1delta(g) Noxon system, which are not known precisely from experiments, are determined by quantum chemistry theory.

Rate constants and branching ratios for the reactions of various positive ions with naphthalene from 300 to 1400 K

Abstract Temperature dependent rate constants and branching ratios are reported for the reactions of a variety of ions with recombination energies ranging from 9.26 eV (NO + ) to 21.56 eV (Ne + ) with naphthalene. For most ions, the measurements are made between 300 and 370 K in a variable temperature-selected ion flow tube (VT-SIFT). For the reactions of Ar + and N 2 + , data have also been measured between 300 and 500 K in the selected ion flow tube. In addition, for the reactions of O 2 + and N 2 + , data have been obtained between 500 and 1400 K in a high temperature flowing afterglow (HTFA). These are among the first determinations of branching ratios for ion–molecule reactions measured over 700 K. All reactions are found to proceed at the Langevin collision rate for all temperatures studied. The reactions proceed by nondissociative and dissociative charge transfer except for the reaction involving F + where some of the reactivity is attributed to chemical channels. No dissociative charge transfer is observed for ions with recombination energies equal to or less than that for N 2 + at room temperature. At higher temperatures in the N 2 + reaction and for ions with higher recombination energies (F + , Ne + ), naphthalene cation dissociation is observed, implying a threshold over 16 eV. This value is substantially higher than the known thermodynamic threshold because of kinetic shifts and quenching of the excited state of C 10 H 8 + by collisions with the helium buffer gas. The observed product thresholds and branching ratios are presented within the context of previous work and the implications for combustion chemistry are discussed.

Diode laser absorption tomography using data compression techniques

Tunable diode laser absorption spectroscopy (TDLAS) shows promise for in situ monitoring in high-speed flows. However, the dynamic nature of typical flows of supersonic combustors, gas turbine engines and augmenters can also lead to inhomogenities that cannot be captured by a single line-of-sight TDLAS measurement. Instead, multiple measurements varied over several spatial locations need to be made. In the current study, shock train structure in the isolator section of the Research Cell 18 supersonic combustion facility at Wright-Patterson AFB is measured. Although only two view angles are available for measurement, multiple absorption features along with a priori computational fluid dynamics (CFD) simulations enable estimates of two dimensional flow features to be formed. Vector quantization/kmeans data clustering is used to identify key flow features from the temporal history of the raw sinograms. Through the use of multiple absorption features that are measured nearly simultaneously, an approximate two-dimensional image can be formed. This image can be further refined through the use of an optimal set of basis functions that can be derived from a set of CFD simulations that describes the flow shapes.

Development of an In Flight Non-intrusive Mass Capture System

The demonstration of an in-flight Tunable Diode Laser Absorption Spectroscopy (TDLAS) system for the measurement of mass capture is being developed in the Hypersonic International Flight Research Experimentation (HIFiRE) Flight 1 (see Kimmel et al AIAA 2007-534 for full description). The key to integration into a flight payload is to make a system that will both fit into the flight system meaning weight, size and power requirements as well as being able to survive in the much harsher flight environment as compared to the laboratory. This document contains the design consideration and overview of the system as it progressed from bench type hardware to being a fully integrated flight payload.

Determination of photofragment ion translational energy and angular distributions in an octopole ion guide: A case study of the Ar2+ and (N2O⋅H2O)+ cluster ions

The first measurement of ion photodissociation product recoil velocity and angular anisotropy in an octopole ion guide are presented. The experimental and numerical procedures required to obtain photofragment ion translational energy and angular distributions are discussed. Cluster ions are photodissociated in an octopole ion guide, and photofragment ion velocity distributions are measured using time-of-flight (TOF). The instrumental discrimination function is determined using guiding field variation (VAR). A validation study using the 2Σg+←2Σu+ transition of Ar2+ probed at 300 nm and a photodissociation dynamics study of (N2O⋅H2O)+ to form N2OH++OH, N2O++H2O, and H2O++N2O in the 458–657 nm range are presented. The H2O+ and N2O+ photofragment translational energy and angular distributions are derived, and new information regarding the photodissociation of the (N2O⋅H2O)+ cluster ion is obtained.

Lifted Flame Speed Enhancement by Plasma Excitation of Oxygen

O2(a 1 Δg) were observed for each flame by comparing flame stabilization locations with and without the plasma generated species. Atmospheric pressures were utilized to investigate the effects of O3 and showed up to a 10% enhancement in the flame speed for 1300 ppm of O3 addition to the O2/N2 oxidizer of lifted C3H8 flames. Numerical simulations showed that the O3 decomposition early in the preheat zone of the flame produced O which rapidly reacted with C3H8 to abstract an H, which led to OH production. The subsequent reaction of the OH with fuel fragments produced H2O and other stable species, yielding chemical heat release to enhance the flame speed. The effect of O2(a 1 Δg) was studied at low pressure (27 Torr) and was isolated by adding NO to the plasma afterglow to eliminate O3. For transport times on the order of one second in the presence of NO, the only remaining oxygen species were O2(X 3 Δg) and O2(a 1 Δg). Under these conditions, the enhancement of O2(a 1 Δg) could be studied in isolation, becoming an ideal source for combustion experiments. It was found that O2(a 1 Δg) was a better oxidizer than O2 by significantly enhancing the propagation speed of C2H4 flames. The present experimental results will have a direct impact on the development of elementary reaction rates with O2(a 1 Δg) and O3 at flame conditions to establish detailed plasma-flame kinetic mechanisms.

A microwave-augmented plasma torch module

A new plasma torch device which combines arc and microwave discharges to enhance the size and enthalpy of the plasma torch is described. A cylindrical-shaped plasma torch module is integrated into a tapered rectangular cavity to form a microwave adaptor at one end, which couples the microwave power injected into the cavity from the other end to the arc plasma generated by the torch module. A theoretical study of the microwave coupling from the cavity to the plasma torch, as the load, is presented. The numerical results indicate that the microwave power coupling efficiency exceeds 80%. Operational tests of the device indicate that the microwave power is coupled to the plasma torch and that the arc discharge power is increased. The addition of microwave energy enhances the height, volume and enthalpy of the plasma torch when the torch operates at a low airflow rate, and even when the flow speed is supersonic, a noticeable microwave effect on the plasma torch is observed. In addition, the present design allows the torch to be operated as both a fuel injector and igniter. Ignition of ethylene fuel injected through the centre of a tungsten carbide tube acting as the central electrode is demonstrated.