Joseph A. Wehrmeyer

Simultaneous temperature and multispecies measurement in a lifted hydrogen diffusion flame

This paper reports that ultraviolet (UV) spontaneous vibrational Raman scattering and laser-induced predissociative fluorescence (LIPF) techniques are combined and applied to a lifted hydrogen jet diffusion flame. Simultaneous, temporally and spatially resolved point measurements of temperature, major species concentrations (H[sub 2], O[sub 2] N[sub 2], H[sub 2]O), and absolute hydroxyl radical concentration (OH) are obtained with a single excimer laser for the first time. For OH measurements, the use of LIPF makes quenching corrections unnecessary. Results demonstrate that fuel and oxidizer are in a rich, premixed, and unignited condition in the center core of the lifted flame base. In the lifted zone, combustion occurs in an intermittent annular turbulent flame brush and strong finite-rate chemistry effects result in nonequilibrium values of temperature, major species, and OH radicals. Downstream in the slow three-body recombination zone, the major species concentrations are in partial equilibrium, the OH concentrations are in superequilibrium, and the temperatures are in subequilibrium. Far downstream in the flame, equilibrium values of temperature, OH radical, and major species are found.

Raman measurement of mixing and finite-rate chemistry in a supersonic hydrogen-air diffusion flame

Abstract Uhraviolet (UV) spontaneous vibrational Raman scattering and laser-induced predissociative fluorescence (LIPF) from a KrF excimer laser are combined to simultaneously measure temperature, major species concentrations (H2, O2, N2, H2O), and OH radical concentration in a supersonic lifted co-flowing hydrogen-air diffusion flame. The axisymmetric flame is formed when a sonic jet of hydrogen mixes with a Mach 2 annular jet of vitiated air. Mean and rms profiles of temperature, species concentrations, and mixture fraction are obtained throughout the supersonic flame. Simultaneous measurements of the chemical species and temperature are compared with frozen chemistry and equilibrium chemistry limits to assess the local state of the mixing and chemistry. Upstream of the lifted flame base, a very small amount of reaction occurs from mixing with hot vitiated air. Downstream of the lifted flame base, strong turbulent mixing leads to subequilibrium values of temperature and OH concentration. Due to the interaction of velocity and temperature in supersonic compressible flames, the fluctuations of temperature and species concentrations are found to be higher than subsonic flames. Farther downstream, slow three-body recombination reactions result in superequilibrium OH concentrations that depress temperatures below their equilibrium values.

Design and energetic characterization of a liquid-propellant-powered actuator for self-powered robots

This paper describes the design of a power supply and actuation system appropriate for position or force controlled human-scale robots. The proposed approach utilizes a liquid monopropellant to generate hot gas, which is utilized to power a pneumatic-type actuation system. A prototype of the actuation system is described, and closed-loop tracking data are shown, which demonstrate good motion control. Experiments to characterize the energetic performance of a six-degree-of-freedom actuation system indicate that the proposed system with a diluted propellant offers an energetic figure of merit five times greater than battery-powered DC motors. Projections based on these experiments indicate that the same system powered by undiluted propellant would offer an energetic figure of merit in an order of magnitude greater than a comparable battery-powered DC motor actuated system.

Flame flow tagging velocimetry with 193-nm H 2 O photodissociation

In a new nonintrusive, instantaneous flow tagging method called hydroxyl tagging velocimetry (HTV), a molecular grid of hydroxyl (OH) radicals is written into a flame and the displaced grid is imaged at a later time to give the flames velocity profile. Single-photon photodissociation of vibrationally excited H(2)O, when a 193-nm ArF excimer laser is used, produces a tag line of superequilibrium OH and H photoproducts in a high-temperature flow field that itself may contain ambient OH. The tag line OH concentration is composed mostly of direct OH photoproducts, but OH is also indirectly produced through H photoproduct reactions with oxygen-bearing species. For lean and modestly rich flames the OH tag lifetime is of the order of 1 ms. For very rich H(2)-air flames (equivalence ratio of 4.4) the lifetime drops to 200 ns. After displacement the position of the OH tag line is revealed through fluorescence caused by OH (A-X) (3 <-- 0) excitation by using a 248-nm tunable KrF excimer laser. A HTV grid of multiple tag lines, providing multipoint velocity information, is experimentally demonstrated in a turbulent H(2)/N(2)-air diffusion flame.

Raman scattering measurements in flames using a tunable KrF excimer laser.

Using a narrow-band tunable KrF excimer laser as a spontaneous vibrational Raman scattering source, we demonstrate that single-pulse concentration and temperature measurements, with only minimal fluorescence interference, are possible for all major species (O(2), N(2), H(2)O, and H(2)) at all stoichiometries (fuel-lean to fuel-rich) of H(2)-air flames. Photon-statistics-limited precisions in these instantaneous and spatially resolved single-pulse measurements are typically 5%, which are based on the relative standard deviations of single-pulse probability distributions. Optimal tuning of the narrow-band KrF excimer laser (248.623 nm) for the minimization of OH A(2)Sigma-X(2)II and O(2)B(3)Sigma(u)(-)-X(3)Sigma(g)(-) fluorescence interference is determined from fluorescence excitation spectra. In addition to the single-pulse N(2) Stokes/anti-Stokes ratio temperature measurement technique, a time-averaged temperature measurement technique ispresented that matches the N(2) Stokes Raman spectrum to theoretical spectra by using a single intermediate sta frequency to account for near-resonance enhancement. Raman flame spectra in CH(4)-air flames are presented that have good signal-to-noise characteristics and show promise for single-pulse UV Raman measurements in hydrocarbon flames.

Hydroxyl tagging velocimetry in a supersonic flow over a cavity

Hydroxyl tagging velocimetry (HTV) measurements of velocity were made in a Mach 2 (M 2) flow with a wall cavity. In the HTV method, ArF excimer laser (193 nm) beams pass through a humid gas and dissociate H2O into H + OH to form a tagging grid of OH molecules. In this study, a 7 x 7 grid of hydroxyl (OH) molecules is tracked by planar laser-induced fluorescence. The grid motion over a fixed time delay yields about 50 velocity vectors of the two-dimensional flow in the plane of the laser sheets. Velocity precision is limited by the error in finding the crossing location of the OH lines written by the excimer tag laser. With a signal-to-noise ratio of about 10 for the OH lines, the determination of the crossing location is expected to be accurate within +/- 0.1 pixels. Velocity precision within the freestream, where the turbulence is low, is consistent with this error. Instantaneous, single-shot measurements of two-dimensional flow patterns were made in the nonreacting M 2 flow with a wall cavity under low- and high-pressure conditions. The single-shot profiles were analyzed to yield mean and rms velocity profiles in the M 2 nonreacting flow.

EXPERIMENTAL AND NUMERICAL INVESTIGATION OF PREMIXED TUBULAR FLAMES

Fundamental flame response in a stretched and curved flow field is investigated in a unique opticallyaccessible tubular burner. Time-averaged, one-dimensional spatially resolved temperature and major species measurements are obtained in a set of stretched, φ =0.175 premixed H 2 /air tubular flames using visible laser-induced Raman spectroscopy. The very lean H 2 /air flames are formed under relatively high stretch rates, 90≤ κ ≤215 s −1 , with 227 s −1 being the extinction condition. In tubular flames, both stretch and thermal-diffusive effects are dependent on both aerodynamic straining and flame curvature. Thermal-diffusive effects in highly curved (∼2 mm flame radius) tubular flames significantly influence the flame structure, leading to flame temperature increases of ∼120 K over the planar unstretched flame temperature (∼1180 K). The standard program for modeling stretched planar flames (Oppdif) is modified for the cylindrical geometry of the tubular flame. Comparisons of the Raman measurements with numerical simulations for tubular premixed flames, using complex chemistry and detailed transport properties, show excellent agreement at low rates of stretch (i.e., κ ≤127 s −1 ). At higher flame stretch, hence increased curvature, numerical simulations using the currently available transport data and chemical kinetic mechanisms incorrectly predict the flame structure. The experimental observations show extinction occurring (∼227 s −1 ), while numerical simulations overpredict the extinction limit (∼750 s −1 ). Simulations using four different H 2 /air chemical kinetic mechanisms show that the flame structure is very sensitive to the particular mechanism and the molecular diffusion coefficients. Evaluation of molecular diffusion coefficients indicates that the thermodiffusive properties of the deficient reactant species, H 2 , strongly affect the tubular flame structure. Thus, the products in the flame zone ( φ =0.34) are enriched from the initial reactant mixture ( φ =0.175) by flame curvature and the rapid diffusion of H 2 .

Single pulse vibrational Raman scattering by a broadband KrF excimer laser in a hydrogen–air flame

Spontaneous vibrational Raman scattering (VRS) is produced by a broadband excimer laser at 248 nm (KrF) in a H(2)-air flame and VRS spectra are recorded for lean, stoichiometric, and rich flames. Except at very lean flame conditions, laser-induced fluorescence (LIF) processes interfere with VRS Stokes lines from H(2), H(2)O, and O(2). No interference is found for the N(2) Stokes and N(2) anti-Stokes lines. In a stoichiometric H(2)/air flame, single-pulse measurements of N(2) concentration and temperature (by the VRS Stokes to anti-Stokes ratio) have relative standard deviation of 7.7 and 10%, respectively. These single pulse measurement errors compare well with photon statistics calculations using measured Raman cross sections.

Ozone Tagging Velocimetry Using Narrowband Excimer Lasers

Ozone tagging velocimetry (OTV), a nonintrusive, unseeded, time-of-flight velocity measurement technique, consists of a write step, where a 193-nm pulsed excimer laser creates an O 3 line via O 2 uv absorption, and a subsequent read step, where a 248-nm excimer laser photodissociates the O 3 and fluoresces the vibrationally exdted O 2 product, revealing the tag line displacement. For the flrst time, instantaneous OTV images and velocity measurements are reported in airflows at room temperature. The narrowband lasers are tuned to the O 2 Schumann-Runge transitions improving the OTV signal strength by a factor of six over that obtained using two broadband lasers. This improvement is less than expected from absorption ratio estimates, due in part to incomplete laser locking efficiency and possibly to laser bleaching. Diffusion of the O 3 tag line is shown to be important only for write-read delay times of the order of milliseconds or greater Modeling of O 3 concentration vs time shows O 3 is long lived at room temperature and relatively insensitive to water vapor, but O 3 peak concentration and lifetime greatly decrease at high temperature, though high pressure increases peak O 3 concentration

Opposed jet flames of lean or rich premixed propane-air reactants versus hot products

Several opposed jet flames, produced by a lean H2-air jet opposing a rich or lean C3H8-air jet, are investigated. Spontaneous Raman spectroscopy is used for major species concentration and temperature measurements along the opposed jet centerline. The hot products of the H2-air flame simulate the burnt gases of strong-burning near-stoichiometric reactants as they impinge upon a weak-burning lean or rich hydrocarbon-fueled reactant mix, a situation encountered in stratified charge operation of direct injection spark ignition engines. In addition the H2-air flame hot products facilitate experimental data interpretation through the absence of carbon-bearing species. Good agreement between numerical and experimental data are obtained for a rich (equivalence ratio, φ = 1.25) C3H8-air jet versus a lean (φ = 0.4) H2-air jet. Two lean C3H8-air jets (φ = 0.64 or 0.60), versus the φ = 0.4 H2-air jet, are also investigated. For both of these flames, the amount of CO2 production strongly depends upon φ, with the φ = 0.64 flame having a peak CO2 mole fraction an order of magnitude higher than for the φ = 0.60 flame, and the C3H8 flames burning either as a normal flame (high CO2) or as a “negative flame speed” flame producing little CO2 and then only through diffusion of C3H8 into the hot products jet. The numerically predicted and experimental CO2 profiles agree well for the positive flame speed flame, but the large discrepancy between predicted and measured peak CO2 in the negative flame speed flame suggests modeling improvements are needed for this type of flame.