Brian Mcmillin

Temporally resolved, two-line fluorescence imaging of NO temperature in a transverse jet in a supersonic cross flow.

Temporally resolved temperature imaging of a fuel jet in a hot, nonoxidizing supersonic cross flow is described. The temperature measurements are obtained with two lasers and two intensified cameras, with a two-line ratio of planar laser-induced fluorescence from nitric oxide, seeded either in the jet or in both the jet and the cross flow. Diagnostic issues related to the application of the two-line technique in high-speed combustion flows are addressed and include temperature sensitivity, transition selection, measurement resolution, fluorescence lifetime, temporal resolution, and intensifier and camera dynamicrange limitations. Single-shot and frame-averaged side-view temperature images of the flow field are presented, and the measurement uncertainties, which are dominated by photon statistical noise and pulse-to-pulse laser fluctuations, are discussed.

Temperature measurements in gases by use of planar laser-induced fluorescence imaging of NO

Two techniques based on planar laser-induced fluorescence of NO are applied to the measurement of two-dimensional temperature fields in gaseous flows. In the single-line technique, the NO fluorescence signal, which is in general a function of temperature, pressure, and mole fraction, can be reduced to a function of temperature alone. In this limit, a single measurement of fluorescence can be directly related to temperature. In contrast, in the two-line thermometry technique the ratio of fluorescence signals resulting from excitation of two different rovibronic states is related to the fractional populations in the initial states, which are solely a function of temperature. The one-line method is applied to the study of a laminar heated jet, and the two-line technique is used to measure temperature in a supersonic underexpanded jet. In addition, energy transfer in NO laser-induced fluorescence is analyzed with multilevel rate equation models. Finally, an accurate model is developed for prediction of the temperature dependence of the NO fluorescence signal.

Planar fluorescence imaging of a transverse jet in a supersonic crossflow

Planar laser-induced fluorescence (PLIF) imaging has been used to examine the mixing and combustion of a sonic jet of gas injected transversely into a supersonic freestream flowing within a shock tube. Single-shot and frame-averaged fluorescence images have been acquired for nonreacting and reacting flows in side-view and end-view orientations. In the nonreacting experiments, nitric oxide seeded within the jet fluid was used to examine the penetration and mixing of the jet with the freestream, without the influence of chemical reaction and heat release. In the reacting experiments, the hydroxyl radical (OH), formed by the combustion of a hydrogen jet injected into an oxidizing freestream, was used to locate the reaction zones. The OH images indicate that combustion takes place primarily in the shear layer formed by the jet and the freestream, and in the boundary layer adjacent to the wall. For both the nonreacting and reacting results, the single-shot images show the presence of large-scale turbulent structures not apparent in the frame-averaged images. These results demonstrate the importance of examining and understanding the instantaneous flowfield, because it is the instantaneous, rather than mean state, of the flow that ultimately determines the extent to combustion.

Planar laser-induced fluorescence imaging of shock-tube flows with vibrational nonequilibrium

Single-shot planar laser-induced fluorescence images of nitric oxide in shock-heated flows with vibrational nonequilibrium are reported. The results demonstrate that planar laser-induced fluorescence imaging is a promising diagnostic technique for multidimensional high-speed flows because of its ability to examine shock structure and to visualize and measure vibrational nonequilibrium. The flows studied were generated within a shock tube and were composed of dilute mixtures of NO in argon. A narrow-band ArF laser tuned to the D +- X (0,1) R2 (28.5) transition of NO at 193.346 nm was used as the excitation source. The broadband fluorescence was collected at 90 deg to the path of excitation using an intensified, two-dimensional photodiode array. Images presented include a normal incident shock, a normal reflected shock, and a four-image sequence of the development of high-temperat ure supersonic flow over a two-dimensiona l blunt body. The vibrational relaxation in the downstream region of the normal shocks is analyzed and compared with calculations based on known relaxation rates.

Two-dimensional imaging of CF2 density by laser-induced fluorescence in CF4 etching plasmas in the gaseous electronics conference reference cell

Spatially resolved two-dimensional maps of the relative CF2 density in low-pressure radio-frequency Ar/CF4/O2 discharges generated within a parallel-plate Gaseous Electronics Conference reference cell have been obtained using planar laser-induced fluorescence imaging. The experiments cover a wide range of pressure, composition, flow rate, and power deposition conditions (13.3–133.3 Pa, 1%–100% CF4, 1%–10% O2, 5–100 sccm, 3–35 W). Typically, the centerline (r=0) axial CF2 distribution was symmetric with the local peak occurring near the center of the electrode gap, but, in all cases, significant radial variations in CF2 density were observed (14%–45% standard deviation from the mean) with the peak density occurring near the edge of the discharge region. Varying the pressure led to significant changes in both the magnitude and spatial distribution of CF2 density, while varying the composition, flow rate, and power primarily affected only the magnitude of the CF2 density, with only modest changes in the spat...

Multi-line fluorescence imaging of the rotational temperature field in a shock-tunnel free jet

A quasi-steady, highly underexpanded free jet of argon seeded with nitric oxide (NO) was generated at the exit of a converging, axisymmetric nozzle supplied by a shock-tunnel reservoir at 4200 K and 3.0 atm. During each run of the facility, an isolated transition in theA2Σ ←X2∏ (0, 0) band of NO at ∼ 226 nm was pumped with a pulse of frequency-doubled dye laser light formed into a thin sheet and directed perpendicularly through the axis of the jet. The red-shifted components of the resulting fluorescence at 90° with respect to the laser were imaged onto an intensified, charge-coupled device array. A ratio of images obtained by exciting lines originating from two different rotational states could be used to infer the mean rotational temperature field. However, because of the extreme variations in temperature and density present in the free jet, no single pair of lines simultaneously provided adequate signal levels and temperature sensitivity over the flows entire temperature range (i.e., ∼ 100–3100 K). Instead, a combination of images obtained with four different transitions was used. Excellent agreement was observed between multi-line temperature evaluations from single-shot and frame-averaged images and a numerical simulation of the flow performed by the method of characteristics.

Planar laser-induced fluorescence imaging of shock-induced ignition

Planar laser-induced fluorescence (PLIF) imaging has been developed for use in studies of shock-induced ignition. PLIF measurements of OH have been made in stoichiometric mixtures of H2 and O2 diluted in 90% Ar using two shock tube endwall configurations. These OH images, which serve as indicators of ignition, were first obtained in the weak and strong ignition limits in the gas behind a shock reflected from a planar endwall. In both limits the images exhibited planar combustion waves. The absence of observable ignition kernels in the weak ignition limit presumably results from lack of optical access to the corners of the shock tube where kernel formation is expected to occur. A grooved endwall was used to introduce thermal nonuniformities and thereby promote isolated regions of ignition within the imaged region. Sequences of PLIF images defining the temporal evolution of the ignition process were acquired in the weak and strong ignition limits for this endwall configuration. These images exhibit highly nonuniform structure and serve to illustrate the utility of 2-D imaging techniques for ignition studies.

Two-dimensional images of CF/sub 2/ density in CF/sub 4//Ar plasmas by laser-induced fluorescence in a GEC RF reference cell

Spatially resolved two-dimensional (2-D) maps of the relative CF/sub 2/ density in low-pressure CF/sub 4//Ar RF discharges, generated within a parallel-plate gaseous electronics conference (GEC) reference reactor, have been obtained using planar laser-induced fluorescence imaging (PLIF). The results illustrate the changes in CF/sub 2/ density and distribution in the central plane of the discharge region as flowrate is varied over 10-100 sccm at constant power and pressure, and as pressure is varied over 13.3-133.3 Pa at constant flowrate and power.

Near-infrared diode laser hydrogen fluoride monitor for dielectric etch

A hydrogen fluoride (HF) monitor, using a tunable diode laser, is designed and used to detect the etch endpoints for dielectric film etching in a commercial plasma reactor. The reactor plasma contains HF, a reaction product of feedstock gas CF4 and the hydrogen-containing films (photoresist, SiOCH) on the substrate. A near-infrared diode laser is used to scan the P(3) transition in the first overtone of HF near 1.31μm to monitor changes in the level of HF concentration in the plasma. Using 200ms averaging and a signal modulation technique, we estimate a minimum detectable HF absorbance of 6×10−5 in the etch plasma, corresponding to an HF partial pressure of 0.03mTorr. The sensor could indicate, in situ, the SiOCH over tetraethoxysilane oxide (TEOS) trench endpoint, which was not readily discerned by optical emission. These measurements demonstrate the feasibility of a real-time diode laser-based sensor for etch endpoint monitoring and a potential for process control.

Dual-Laser PLIF Imaging Techniques for Shock Tube Studies of Mixing and Combustion

Recent advances in the development of planar laser-induced fluorescence diagnostics for imaging of shock-tube-generated combustion flow fields are described, including measurement strategies for (effectively) instantaneous temperature measurements, or alternatively, multiple species imaging. Both techniques are based on the use of two independent, tunable dye laser sources and two time-gated, intensified CCD cameras. In both cases, the lasers illuminate the same planar region but are sequentially pulsed to temporally separate the respective fluorescence signals, and each camera records signal induced by only one of the lasers. Results from nonreacting and reacting jet-in-cross flow experiments are presented, including temperature measurements of fuel-seeded nitric oxide (NO), and simultaneous species imaging of fuel-seeded NO and the combustion-generated hydroxyl radical (OH). The visualizations are used to provide spatially correlated information on the fuel distribution and the location of reaction zones, and the temperature measurements are used to examine the fuel/freestream mixing.