Robert W. Pitz

Superequilibrium and thermal nitric oxide formation in turbulent diffusion flames

Abstract Measurements and modeling of the formation of superequilibrium radicals and nitric oxide in atmospheric pressure turbulent jet diffusion flames are presented which quantify the influence of superequilibrium on thermal NO x formation. Variation of fuel gas compositions (CO/H 2 /N 2 , CO/H 2 /CO 2 , and CO/H 2 /Ar) permits partial separation of chemical and fluid mechanical effects. Superequilibrium OH radical concentrations are measured by single-pulse laser saturated fluorescence and NO and NO 2 concentrations by probe sampling and chemiluminescent detection. Four different types of probes were used to quantify probe sampling effects. In turbulent reaction zones, virtually all of the NO x in the flame occurred in the form of NO but far downstream of the flame nearly half of the NO x occurred as NO 2 . Thermal NO x maximized near stoichiometric flame zones; the rich shift observed by others may be a probe sampling artifact. In turbulent CO/H 2 /N 2 jet diffusion flames, both measurements and a nonequilibrium turbulent combustion model show that superequilibrium decreases average temperatures by 250K, increases average OH concentrations by a factor of 4–6, and increases thermal NO x formation principally by broadening the range of mixture fraction (both rich and lean) where thermal NO x is formed. Calculated increases in thermal NO x due to superequilibrium in turbulent CO/H 2 /N 2 jet diffusion flames are factors of 2.5 at 1 atm and 1.4 at 10 atm. The two-scalar pdf model predicts that thermal NO x yield is independent of Reynolds number in disagreement with previous experimental reports.

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.

Unseeded velocity measurement by ozone tagging velocimetry

Ozone tagging velocimetry is developed for unseeded velocity measurement of air flows. An ozone line is photochemically created by an ArF excimer laser. After a fixed time delay the ozone line is imaged with a KrF excimer laser sheet (248 nm) that photodissociates the ozone and produces vibrationally excited O(2). The O(2) is excited by the same 248-nm light through the Schumann-Runge band, B (3)Sigma(u)(-)(upsilon =0, 2) ? X (3)Sigma(g)(-)(upsilon = 6, 7). An intensified CCD camera records the O(2) fluorescence from the initial and the final line positions to permit the velocity profile along the line to be determined.

Measurements of scalar dissipation in the reaction zones of turbulent nonpremixed H2-air flames

Abstract A new diagnostic system has been developed to obtain spatially and temporally resolved simultaneous multiple point measurements of species concentrations and temperatures in turbulent flames. The diagnostic approach is based on pulsed Raman scattering induced by a KrF excimer laser. The system has been applied in the reaction zones of turbulent nonpremixed H 2 -air flames to obtain statistics of species concentrations, temperature, and mixture fraction (ξ). Results of means, and fluctuations of the measured quantities do not differ markedly from the published point measurements in similar flames, validating the diagnostic approach. Instantaneous, one-dimensional (radial) scalar dissipation rates (χ) are measured in the reaction zones of the nonpremixed H 2 -air flames using the multipoint, multispecies detectivity of the diagnostic system. Radial profiles and probability density functions (pdfs) of scalar dissipation are reported. The pdfs of scalar dissipation are log-normally distributed and negatively skewed. The radial profiles of mean scalar dissipation are similar to ξ rms 3 , consistent with the k-e models of turbulent combustion. Measurements of χ are compared to laminar flamelet model calculations. Laminar flame strain rate parameters (a) estimated from the scalar dissipation measurements range from a = 50–1000s −1 , much lower than the extinction strain rates ( a ≈ 12,000s −1 ) of H 2 -air flames.

Laser Measurements on Nonpremixed & Air Flames for Assessment of Turbulent Combustion Models

L Introduction M practical combustion devices are turbulent nonpremixed flames. The development of appropriate computer models for practical combustors holds the potential for their computer-aided design with lower development costs, higher fuel efficiencies, lower emissions, and wider fuel specifications. However, to be reliable over a wide range of conditions, such models should be based upon a fundamental, quantitative understanding of fuel/air mixing and chemical reactions. Considerable progress has been made in modeling fuel/air mixing processes (particularly in larboratory-scale turbulent nonpremixed flames) using probability density functions (pdf) of a conserved scalar variable to account for turbulent fluctuations and Favre averaging to account for density variations. However, the empirical correlations used in the model still rely on comparisons with experiments in nonreacting flows. A better understanding of the analogies between reacting and nonreacting flows and their deviations can result from detailed experiment/modeling comparisons in turbulent nonpremixed flames. Measurements of at least some of the key model variables (such as pdfs of conserved and reactive scalars, Favre and conventional averages and fluctuations, inter mitt ency, extent of reaction, scalar fluxes, gradients, and correlations) are useful to directly test the model assumptions. This level of understanding is required to model turbulent chemistry interactions realistically or to predict in a fundamental way the stability limits (blowoff and relight) or the formation and emission of pollutants such as carbon monoxide and nitric oxide. Because of the problems in making detailed measurements in practical combustors and the difficulties in modeling their complex geometries and flow patterns, the development and testing of quantitative combustion models is based upon detailed comparisons with quantitative experimental data in well-controlled, laboratory-scale, nonpremixed flames operated over a range of known initial conditions. However, few comprehensive measurements have been attempted even in laboratory-scale nonpremixed flames because of the difficulties in obtaining data having sufficient temporal and spatial resolution, without significantly perturbing the flame under study. Fortunately, recent advances in laser measurement techniques and, in particular, in pulsed Raman scattering, laser velocimetry, and fluorescence imaging have greatly expanded the experimental capabilities to make nonperturbing quantitative measurements in nonpremixed reacting flows. This paper summarizes detailed experiments reported previously and presents new experimental results characterizing four H2-air jet diffusion flames. Admittedly, H2 jet diffusion flames do not possess some elements of practical combustor systems (e.g., swirl, recirculation, corn-

Single-pulse, simultaneous multipoint multispecies Raman measurements in turbulent nonpremixed jet flames

An imaging technique based on Raman scattering induced by a KrF excimer laser has been developed for quantitative simultaneous multiple-point measurements of absolute multispecies concentrations (O(2), N(2), H(2), and H(2)O) and temperature in turbulent flames. These multiparameter measurements along the laser line have a temporal resolution of 17 ns, a spatial resolution of 0.7 mm for each point, and single-shot standard deviations of ~8%. The UV Raman imaging system has been applied in turbulent nonpremixed hydrogen-air flames, providing what are to our knowledge the first reported single-pulse, simultaneous multiple-point quantitative images of multiple species and temperature.

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.

Visualization of Turbulent Flame Fronts with Planar Laser-Induced Fluorescence

This report concerns the quantitative time-resolved visualization of reaction zones in laminar, transitional, and turbulent nonpremixed flames. Two-dimensional OH molecular concentrations were measured with planar laser-induced fluorescence excited by a sheet of light (formed from a single tunable ultraviolet laser pulse) and detected with a two-dimensional, image-intensified photodiode array camera. From the resulting data details of instantaneous flame front structures (including positions, shapes, and widths) were obtained.

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

Single-pulse, laser-saturated fluorescence measurements of OH in turbulent nonpremixed flames

A single-pulse, laser-saturated fluorescence technique has been developed for absolute OH concentration measurements with a temporal resolution of 2 nsec, a spatial resolution of <0.1 mm(3), and an estimated accuracy of +/-30%. It has been applied in laminar, transitional, and turbulent hydrogen-air diffusion flames, providing the first reported quantitative measurements of average values, rms fluctuations, and probability density functions of OH radical concentration in nonpremixed flames.