Spontaneous Raman scattering measurements of temperature and major species concentration in hydrocarbon-air flames require detailed knowledge of the Raman spectra of the hydrocarbons present when fuels more complex than methane are used. Although hydrocarbon spectra have been extensively studied at room temperature, there are no data available at higher temperatures. Quantum mechanical calculations, when available are not sufficiently accurate for combustion applications. This work presents experimental measurements of spontaneous Stokes-Raman scattering spectra of methane, ethylene, ethane, dimethyl ether, formaldehyde and propane in the temperature range 300-860 K. Raman spectra from heated hydrocarbons jets have been collected with a higher resolution than is generally employed for Raman measurements in combustion applications. A set of synthetic spectra have been generated for each hydrocarbon, providing the basis for extrapolation to higher temperatures. The spectra provided here will enable simultaneous measurements of multiple hydrocarbons in flames. This capability will greatly extend the range of applicability of Raman measurements in combustion applications. In addition, the experimental spectra provide a validation dataset for quantum mechanical models.

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Cavity enhanced absorption spectroscopy (CEAS) is a promising technique for studying chemical reactions due to its desirable characteristics of high sensitivity and fast time response by virtue of the increased path length and relatively short photon residence time inside the cavity. Offaxis CEAS (OA-CEAS) is particularly suited for the shock tube applications as it is insensitive to slight misalignments, and cavity noise is suppressed due to non-overlapping multiple reflections of the probe beam inside the cavity. Here, OA-CEAS is demonstrated in the mid-IR region at 1310.068 cm-1 to monitor trace concentrations of hydrogen peroxide (H2O2). This particular probe frequency was chosen to minimize interference from other species prevalent in combustion systems and in the atmosphere. The noise-equivalent detection limit is found to be 3.25 x 10 −5 cm −1 , and the gain factor of the cavity is 131. This corresponds to a detection limit of 74 ppm of H2O2 at typical high-temperature combustion conditions (1200 K and 1 atm) and 12 ppm of H2O2 at ambient conditions (296 K and 1 atm). To our knowledge, this is the first successful application of the OA-CEAS technique to detect H2O2 which is vital species in combustion and atmospheric science.

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An improved Raman gain spectrometer for flame measurements of gas temperature and species concentrations is described. This instrument uses a multiple-pass optical cell to enhance the incident light intensity in the measurement volume. The Raman signal is 83 times larger than from a single pass, and the Raman signal-to-noise ratio (SNR) in room-temperature air of 153 is an improvement over that from a single-pass cell by a factor of 9.3 when the cell is operated with 100 passes and the signal is integrated over 20 laser shots. The SNR improvement with the multipass cell is even higher for flame measurements at atmospheric pressure, because detector readout noise is more significant for single-pass measurements when the gas density is lower. Raman scattering is collected and dispersed in a spectrograph with a transmission grating and recorded with a fast gated CCD array detector to help eliminate flame interferences. The instrument is used to record spontaneous Raman spectra from N(2), CO(2), O(2), and CO in a methane-air flame. Curve fits of the recorded Raman spectra to detailed simulations of nitrogen spectra are used to determine the flame temperature from the shapes of the spectral signatures and from the ratio of the total intensities of the Stokes and anti-Stokes signals. The temperatures measured are in good agreement with radiation-corrected thermocouple measurements for a range of equivalence ratios.

Presented are the results of preliminary investigations of a direct-current microdischarge based miniaturized plasma thruster called a Microdischarge Plasma Thruster (MPT). The MPT has a triode con¦guration consisting of electrodes made of molybdenum separated by mica dielectric with argon propellant. Several variations of the triode were studied in order to determine a con¦guration that provided a relatively stable con¦guration. The thruster can produce an intense plume with a bluish outline. Currentvoltage characteristics of the thruster are reported. Preliminary spectral measurements reveal that the plume emission arises from excited ground states of argon, with no emission from argon ions detected.

A simple Microdischarge Plasma Thruster (MPT) for small satellite propulsion has been designed and studied by performing experiments and running simulations with a numerical model. The MPT comprises a tri-layer sandwich structure with a dielectric layer sandwiched between two electrode layers, and a contoured through-hole drilled into the structure. Each layer is a few hundred micron thick and the hole diameter is also of approximately this size. The device operates at Ar §ow rates of ∼ 1 sccm with moderate electrode voltages (∼ 1000 V). Spectral measurements of the plume are used to determine its composition and calculate the electron excitation temperature. A two-dimensional computational model has been developed to provide a detailed description of plasma dynamics inside the MPT including power deposition, ionization, coupling of plasma phenomena with the high-speed §ow, and propulsion system performance. Gas heating, primarily due to ion Joule heating, is found to have a strong in §uence on the overall discharge behavior.

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