Sameer V. Naik

Quantitative laser-saturated fluorescence measurements of nitric oxide in counter-flow diffusion flames under sooting oxy-fuel conditions

Abstract We report quantitative, spatially resolved, laser-saturated fluorescence measurements of nitric oxide ([NO]) concentration in sooting, high-temperature, oxygen-methane, counter-flow diffusion flames at atmospheric pressure. Six different flames containing 1%, 3%, and 10% N 2 in either the oxidizer or fuel streams are investigated at a global strain rate of 20 s −1 . Excitation of NO is obtained at 224.45 nm in the γ(0,0) band and detection is performed in a 2-nm region centered at 235.78 nm in the γ(0,1) band. Numerical computations for all counter-flow diffusion flames are conducted using OPPDIF with GRI Mech-3.0. The effect of gas-phase radiation is considered in the modeling. Excitation scans indicate no significant change in background for oxygen-rich as compared to air-rich flames and detection scans verify the absence of interferences from other species. Quantitative axial profiles of [NO] are presented for all six flames. Comparisons with modeling indicate good agreement in those regions of each flame having predicted temperatures below 2600 K. Enhanced radiative heat loss caused by soot formation leads to poorer agreement between predicted and measured NO concentrations in regions at higher flame temperatures (T > 2600 K), thus indicating the need for a combined soot formation and radiation model.

Quantitative Laser-Induced Fluorescence Measurements and Modeling of Nitric Oxide in High-Pressure (6-15 atm) Counterflow Diffusion Flames

Laser-induced fluorescence (LIF) measurements of NO concentration ([NO]) have been obtained along the centerline of methane–air counterflow diffusion flames at 6 to 15 atm. This study is an extension of our previous work involving measurements of [NO] in similar flames at two to five atm, wherein we had used a counterflow premixed flame for calibration. For the flames studied here, a method based on computed overlap fractions is developed to calibrate [NO] measurements at higher pressures. The linear LIF measurements of [NO], which are corrected for variations in the electronic quenching rate coefficient, are compared with numerical predictions from an opposed-flow flame code utilizing two Gas Research Institute (GRI) chemical kinetic mechanisms (versions 2.11 and 3.0). The effect of radiative heat loss on code predictions is accounted for by using an optically thin radiation model. The revised GRI mechanism (version 3.0) offers a significant improvement in prompt-NO predictions for these flames compared to the older version (2.11), especially at pressures below eight atm. However, a consistent discrepancy remains in the comparisons, particularly at peak NO locations for pressures lower than six atm. The measurements display a continuing trend of decreasing NO concentration with increasing pressure at 6–15 atm as expected for flames dominated by prompt NO. The discrepancy between measurements and predictions decreases with rising pressure so that the revised GRI mechanism predicts [NO] with reasonable accuracy at pressures above six atm.

LIF MEASUREMENTS AND CHEMICAL KINETIC ANALYSIS OF NITRIC OXIDE FORMATION IN HIGH-PRESSURE COUNTERFLOW PARTIALLY PREMIXED AND NONPREMIXED FLAMES

We report quantitative, spatially resolved, linear laser-induced fluorescence (LIF) measurements of nitric oxide concentration ([NO]) in laminar, methane/air counterflow partially premixed and nonpremixed flames at six pressures up to 15 atm using excitation near 226.03 nm in the γ(0,0) band of NO. For partially premixed flames, fuel-side equivalence ratios (φB) of 1.45, 1.6, and 2.0 are studied at a global strain rate of 20 s−1. For nonpremixed flames, a complete set of NO measurements at global strain rates of 20 s−1, 30 s−1, and 40 s−1 is presented to supplement previously reported data in such flames. The quantitative NO measurements are compared with predictions from an opposed-flow flame code utilizing two GRI chemical kinetic mechanisms (versions 2.11 and 3.0). The effect of radiative heat loss on NO predictions is assessed by using a modified version of the code that considers radiation in the optically thin limit. The linear LIF measurements of [NO] are corrected for variations in the electronic quenching rate coefficient by using major species and temperature profiles generated by the opposed-flow flame code plus quenching cross sections for NO available from the literature. A pathway analysis provides the relative contribution of different NO formation mechanisms to the total amount of NO produced at various pressures. Quantitative reaction path diagrams are used to investigate pictorially species interactions during NO formation. Finally, we identify key reactions controlling NO concentrations in counterflow partially premixed and nonpremixed flames by using a sensitivity analysis. For the nonpremixed flames, NO measurements at pressures of 2–5 atm disagree substantially with predictions from either GRI mechanism. On the other hand, both mechanisms predict observed NO concentrations reasonably well beyond 6 atm. In general, for our nonpremixed flames, NO formation is dominated by the prompt route at lower pressures with an increasing contribution from the N2O pathway at higher pressures. For partially premixed flames, GRI 3.0 mimics the basic quantitative trends found in our LIF measurements at 1–15 atm. The kinetic analysis indicates that the prompt mechanism dominates at lower pressures, whereas the thermal and N2O pathways become more important at higher pressures. At any given pressure, a reduction in partial premixing results in kinetic behavior approaching that of the nonpremixed flames.

Effects of quenching on electronic-resonance-enhanced coherent anti-Stokes Raman scattering of nitric oxide

We investigate the effects of gas-mixture composition on the electronic-resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS) signals of nitric oxide (NO). From previous laser-induced fluorescence (LIF) studies, quenching rates are known to change drastically, by factors of 400–800, in mixtures of CO2∕O2∕N2. The observed ERE-CARS signal remains constant to within 30% whereas LIF signals from NO are predicted to decrease by more than two orders of magnitudes in the same environments. This is very significant for using NO ERE-CARS in high-pressure combustion environments where the electronic quenching rate can vary rapidly as a function of both space and time.

Pressure, Temperature and Velocity Measurements in Underexpanded Free Jets using Laser-Induced Fluorescence Imaging

We report measurements of pressure, temperature, and velocity in an underexpanded jet using planar laser-induced fluorescence of nitric oxide. Ultraviolet transitions near 44, 097.5 cm -1 in the A-X (0,0) system of nitric oxide were excited using narrow linewidth laser radiation generated from an optical parametric system, injection seeded using a distributed feedback diode laser. Planar laser-induced fluorescence images with excellent spatial resolution and signal-to-noise ratio were acquired by tuning the frequency of the laser radiation over the nitric oxide absorption line. The images were corrected on a shot-to-shot basis for fluctuations in the laser spatial profile. Line shapes constructed from the corrected planar laser-induced fluorescence images were used to determine pressure and temperature values along the centerline of the jet. Good agreement between laser-induced fluorescence and previously reported N 2 coherent anti-Stokes Raman scattering measurements was observed. The laser-induced fluorescence measurements also compared well with calculations of pressure and temperature using computational fluid dynamics codes. Velocity was measured in supersonic regions of the flowfield on the basis of the Doppler shift in the nitric oxide absorption lines. Planar laser-induced fluorescence images were acquired using laser sheets propagating at 90 and 45 degrees with respect to the flow direction; velocity was determined from the frequency shift of the absorption lines for these two shifts. The laser-induced fluorescence technique can potentially be applied to obtain instantaneous measurements of thermodynamic properties and multiple velocity components in high-speed turbulent flows.

Perturbative theory and modeling of electronic-resonance-enhanced coherent anti-Stokes Raman scattering spectroscopy of nitric oxide.

A theory is developed for three-laser electronic-resonance-enhanced (ERE) coherent anti-Stokes Raman scattering (CARS) spectroscopy of nitric oxide (NO). A vibrational Q-branch Raman polarization is excited in the NO molecule by the frequency difference between visible Raman pump and Stokes beams. An ultraviolet probe beam is scattered from the induced Raman polarization to produce an ultraviolet ERE-CARS signal. The frequency of the ultraviolet probe beam is selected to be in electronic resonance with rotational transitions in the A (2)Sigma(+)<--X (2)Pi (1,0) band of NO. This choice results in a resonance between the frequency of the ERE-CARS signal and transitions in the (0,0) band. The theoretical model for ERE-CARS NO spectra has been developed in the perturbative limit. Comparisons to experimental spectra are presented where either the probe laser was scanned with fixed Stokes frequency or the Stokes laser was scanned with fixed probe frequency. At atmospheric pressure and an NO concentration of 100 ppm, good agreement is found between theoretical and experimental spectral peak locations and relative intensities for both types of spectra. Factors relating to saturation in the experiments are discussed, including implications for the theoretical predictions.

Dual-pump coherent anti-Stokes Raman scattering system for temperature and species measurements in an optically accessible high-pressure gas turbine combustor facility

The development and implementation of a dual-pump coherent anti-Stokes Raman scattering (DP-CARS) system employing two optical sub-systems to measure temperature and major species concentrations at multiple locations in the flame zone of a high-pressure, liquid-fueled gas turbine combustor are discussed. An optically accessible gas turbine combustor facility (GTCF) was utilized to perform these experiments. A window assembly has been designed, fabricated, and assembled in the GTCF to allow optical access from three directions using a pair of thin and thick fused silica windows on each side. A lean direct injection (LDI) device consisting of an array of nine integrated air swirlers and fuel injectors was operated using Jet-A fuel at inlet air temperatures up to 725 K and combustor pressures up to 1.03 MPa. The DP-CARS system was used to measure temperature and CO2/N2 concentration ratio on single laser shots. An injection-seeded optical parametric oscillator (OPO) was used as a narrowband pump laser source in order to potentially reduce shot-to-shot fluctuations in the CARS data. Large prisms mounted on computer-controlled translation stages were used to direct the CARS beams either into the main leg optical system for measurements in the GTCF or to a reference leg optical system for measurements of the non-resonant spectrum and for alignment of the CARS system. The spatial maps of temperature and major species concentrations were obtained in high-pressure LDI flames by translating the CARS probe volume in the axial and vertical directions inside the combustor rig without loss of optical alignment.

Development of High-Spectral-Resolution Planar Laser-Induced Fluorescence Imaging Diagnostics for High-Speed Gas Flows

AEROSPACE LETTERS are brief communications (approximately 2000 words) that describe new and potentially important ideas or results, including critical analytical or experimental observations that justify rapid publication. They are stringently prescreened, and only a few are selected for rapid review by an Editor. They are published as soon as possible electronically and then appear in the print version of the journal.

Single-laser-shot detection of nitric oxide in reacting flows using electronic resonance enhanced coherent anti-Stokes Raman scattering

Single-laser-shot electronic resonance enhanced coherent anti-Stokes Raman scattering (ERE-CARS) spectra of nitric oxide (NO) were generated using the 532 nm output of an injection-seeded Nd:YAG (yttrium aluminum garnet) laser as the pump beam, a broadband dye laser at approximately 591 nm as the Stokes beam, and a 236 nm narrowband ultraviolet probe beam. Single-laser-shot ERE-CARS spectra of NO were acquired in an atmospheric-pressure hydrogen/air counterflow diffusion flame. The single-shot detection limit in this flame was found to be approximately 30 ppm, and the standard deviation of the measured NO concentration was found to be approximately 20% of the mean.

Laser-saturated and linear laser-induced fluorescence measurements of nitric oxide in counterflow diffusion flames under non-sooting oxygen-enriched conditions

We report quantitative, spatially resolved, laser-induced fluorescence (LIF) measurements of NO concentration ([NO]) in non-sooting, oxygen-enriched counterflow diffusion flames at atmospheric pressure. Three different flames containing 25%, 50%, and 100% CH 4 , respectively, in the fuel stream and 100%, 50%, and 35% O 2 , respectively, in the oxidizer stream are investigated at a global strain rate of 20 s m 1 . Excitation of NO is achieved at 224.45 nm in the n (0,0) band and detection is performed in a 2-nm region centered at 235.78 nm in the n (0,1) band. Excitation scans ensure no spectral background problems under the high-temperature conditions of oxygen enrichment. Detection scans obtained at the excitation wavelength verify no interferences from any other species, particularly O 2 . Quantitative axial profiles of [NO] are presented for all three flames. Linear LIF measurements are compared to laser-saturated fluorescence (LSF) measurements to assess the utility of a broadband LSF technique at temperatures approaching 3000 K. Numerical computations for all counterflow diffusion flames are conducted using OPPDIF with GRI Mech-3.0. The effect of gas-phase radiation is considered in the modeling. The results indicate excellent agreement between the measured and predicted NO concentrations for all three flames. A comparison of the linear LIF and LSF measurements also yields excellent agreement.