Multiplex CARS measurements in supersonic H2/air combustion

2 and O₂ multiplex coherent anti-stokes Raman spectroscopy (CARS) employing a single dye laser has been explored to simultaneously determine the temperature and concentrations of H₂ and O₂ in a hydrogen-fueled supersonic combustor. Systematic calibrations were performed through a well-characterized H₂/air premixed flat-flame burner. In particular, temperature measurement was accomplished using the intensity ratio of the H₂ S(5) and S(6) rotational lines, whereas extraction of the H₂ and O₂ concentrations was obtained from the H₂ S(6) and O₂ Q-branch, respectively. Details of the calibration procedure and data reduction are discussed. Quantification of the supersonic mixing and combustion characteristics applying the present technique has been demonstrated to be feasible. The associated detection limits as well as possible improvements are also identified. Received: 1 July 1997/Revised version: 29 September 1998     

The application of single-pulse CARS for temperature measurements in a turbulent stagnation flame

N₂ Q-branch CARS spectra have been recorded and evaluated for temperature determination in a turbulent, premixed CH₄/air stagnation flame with a burner of 40 mm diameter and 22 kW thermal load. Temperature histograms on the flame axis at different distances from the stagnation plate have been measured. Problems of practical applicability are addressed, including those arising from the limited spatial resolution of the BOXCARS geometry, from an insufficient dynamic range of the diode array detector, and from a memory effect of the detector in the case of measurements in highly turbulent flame areas with strong intermittency. Some information is given on the computerized acquisition and on the evaluation of the large amounts of data that are necessary for extensive investigations in large combustion systems.     

Polarization spectroscopy applied to C2 detection in a flame

The detection of C₂ radicals in a premixed acetylene-oxygen flame by using polarization spectroscopy is reported. The signal was recorded in the Swan system,d ³ II _g–a ₃ II _u (0, 0), using a pulsed dye laser. The spectrum shows a very good signal-to-noise ratio with clearly resolved rotational structures of theP andR triplets. The dependence of the signal on the pump-beam polarization was also studied. The spatial distribution of the signal from C₂ radicals in the flame was measured as a demonstration of the use of polarization spectroscopy in combustion diagnostics.     

A computational analysis of strained laminar flame propagation in a stratified CH4/H2/air mixture

Propagation of a H₂-added strained laminar CH₄/air flame in a rich-to-lean stratified mixture is numerically studied. The back-support effect, which is known to enhance the consumption speed of a flame propagating into a leaner mixture compared to that into a homogeneous mixture, is evaluated. A new method is devised to characterize unsteady reactant-to-reactant counterflow flames under transiently decreasing equivalence ratio, in order to elucidate the influence of flow strain on the back-support effect. In contrast to the conventional reactant-to-product configurations, the current configuration is more relevant to unsteady stratified flames back-supported by their own combustion products. Moreover, since H₂ distribution downstream of the flame is known to play a crucial role in back-supported CH₄/air flames, the influence of H₂ addition in the upstream mixture is examined. The results suggest that a larger strain rate leads to a larger equivalence ratio gradient at the reaction zone through increased flow divergence, which amplifies the back-support. Meanwhile, since H₂ addition in the upstream mixture does not affect the downstream H₂ content, the relative increase in the consumption speed, i.e. the back-support, is suppressed with larger H₂ addition. Especially, when the upstream H₂ content decreases with the equivalence ratio, the H₂ preferentially diffuses toward the unburned gas, which mitigates H₂ accumulation in the preheat zone and further weakens the back-support.     

Application of spontaneous Raman and Rayleigh scattering and 2D LIF for the characterization of a turbulent CH4/H2/N2 jet diffusion flame

4 /H₂/N₂ diffusion flame. Important aspects of the measuring technique, such as accuracy, cross talk between different Raman bands, and the correction procedure for background from laser-induced fluorescence are discussed. In addition, a 2D LIF and Rayleigh imaging system were used to study the structures of OH, CH, NO, and temperature distributions in the flame. A comparison between two different CH detection schemes is presented. A main goal of the investigations was a detailed and accurate characterization of the investigated flame as well as the study of experimental techniques. Joint pdfs of the temperature and major species concentrations were determined at nearly 100 measuring locations covering the complete flame. Parts of the results are presented in the paper in order to discuss effects of differential diffusion, flame extinction, and interaction between flow field and chemistry. The measured data sets which are available on the Internet are well suited for testing and validating mathematical flame models. Received: 8 July 1997/Revised version: 23 October 1997     

CARS temperature measurement in a LOX/CH4 spray flame

Coherent anti‐Stokes Raman scattering (CARS) spectroscopy has been used to investigate cryogenic liquid oxygen/gaseous methane (LOX/CH₄) flames on a medium‐size test facility at a pressure of 0.24 MPa and mass flow of 0.025 kg/s. Single‐shot, broadband CARS spectra with simultaneous detection of the Q‐branches of hydrogen and water molecules were recorded with good signal‐to‐noise ratio. Temperature was deduced from the H₂ and H₂O CARS profiles. The spatial temperature distribution in a comparatively harsh environment has been measured successfully. The measurements took place in the windowed combustion chamber of the DLR M3 test facility, aiming to provide data for validation of rocket combustor modeling. Copyright © 2010 John Wiley & Sons, Ltd.     

Analysis of degenerate four-wave mixing spectra of NO in a CH4/N2/O2 flame

4 /N₂/O₂ flame to spectral simulations based on a two-level theory for stationary, saturable absorbers by Abrams et al. Temperatures determined from least-squares fits of simulations to experimental spectra in the A²Σ⁺?X²Π⁺(0,0) band are compared to temperatures obtained from OH absorption spectroscopy and a radiation-corrected thermocouple. We find that DFWM rotational temperatures derived from Q-branch spectra agree with thermocouple and are independent of pump laser intensity for low to moderate saturation (I≈I_sat). However, the temperatures are systematically low and depend on pump intensity if the analysis neglects saturation effects. We demonstrate a method for obtaining an effective pump saturation intensity for use with the two-level model. This approach for analyzing saturated DFWM line intensities differs from previous work in that the use of the theory of Abrams et al. rather than a transition-dipole-moment power law allows treatment of a much wider range of saturation. Based on the observed signal-to-noise ratio an NO detection sensitivity of 25 ppm is projected, limited by a DFWM background interference specific to hydrocarbon flames. Received: 15 September 1998 / Revised version: 18 November 1998 / Published online: 24 February 1999     

Quantitative one-dimensional single-pulse multi-species concentration and temperature measurement in the lift-off region of a turbulent H2/air diffusion flame

In this article, we report on one-dimensional single-pulse measurements of temperature and major-species concentration (O₂, N₂, H₂O and H₂) in a turbulent H₂/air jet diffusion flame using Raman and Rayleigh scattering of KrF* excimer-laser radiation. Spatial resolution of 0.5 mm along a 6mm long line has been obtained, with reasonable error limits for mole fraction ( = 5 % for N₂ detection) and temperature (T = 8 %) determination at flame temperatures. We present various profiles showing the composition and temperature along a line at different heights in the flame with particular emphasis on the lift-off region (i.e. lowx/D). In this zone, temperature and mixture fraction can be determined simultaneously — from a single laser pulse — in a spatial region extending from unburnt gas in the center of the jet across the flame front into the cool air of the surrounding atmosphere. This allows for the first time the systematic study of the shape and width of the high-temperature region and the corresponding concentration and temperature gradients. The comparison of averaged data and scatter plots with previous pointwise measurements shows good agreement.     

Multiplex rotational CARS of N2, O2, and CO with excimer pumped dye lasers: Species identification and thermometry in the intermediate temperature range with high temporal and spatial resolution

Pure rotational CARS spectra of N₂, O₂, air, and CO have been obtained using excimer laser pumped dye-lasers. The combination of the folded BOXCARS phase matching geometry with the broad-band laser multiplex method allowed high spatial and temporal resolution. Species and concentration analysis as well as thermometry up to 700 K is demonstrated, and possible applications are discussed.     

Multiplex H_2 CARS thermometry in a microwave assisted diamond CVD plasma

Rapid temperature measurements in a low pressure, microwave assisted, diamond CVD plasma are reported. By using a “modeless” laser as the Stokes source for H CARS, accurate single-shot and averaged temperatures were obtained which agreed with values obtained from laser induced fluorescence measurements. The speed of data acquisition afforded by multiplex CARS allowed variations of temperature to be monitored with changing plasma conditions induced by variations of pressure, gas composition and microwave power. The application of the technique for “on-line” monitoring of plasma processes is briefly discussed. Received: 13 February 1996 / Accepted: 12 June 1996     

Cavity ring-down measurements of seeded NO in premixed atmospheric-pressure H2/air and CH4/air flames

Quantitative aspects of using cavity ring-down absorption spectroscopy near 226 nm for measurements of NO mole fractions in premixed atmospheric-pressure flames are discussed. Measurements in methane–air flames showed strong broadband absorption near 226 nm by hot CO₂ molecules, precluding using the cavity ring-down method in these flames at atmospheric pressure. In hydrogen–air flames, the broadband absorption at this wavelength was substantially lower. Absorption cross sections derived from non-seeded cavity ring-down spectra suggest that absorption by water is the major contribution to the background in these flames. The detectability limit for NO by cavity ring-down measurements in hydrogen–air flames using the current setup is estimated to be 10 ppm. Effects of the cold boundary layer on the measured NO mole fraction were accounted for by measuring the radial distributions of temperature and NO mole fraction using coherent anti-Stokes Raman scattering and laser-induced fluorescence (LIF), respectively. Measurements performed in seeded stoichiometric and lean hydrogen–air flames showed no reburning at temperatures above 1750 K, demonstrating the adequacy of using these flames for calibration of LIF measurements. At lower temperatures, the mole fraction of NO in the hot gases was up to 30% lower than that expected from the degree of seeding in the cold gases. PACS 42.62.Fi; 42.68.Ca; 82.33.Vx     

Parametric four-wave mixing in the H2Σ+,H′2Π(v′=0) states of NO

2 Σ⁺,H^′2Π(v^′=0)←X²Π(v^′′=0) two-photon transition of NO, both near-infrared and vacuum ultraviolet radiation were emitted along the laser propagation direction. The analyses of emission and excitation spectra revealed that the parametric four-wave mixing (PFWM) process coexisted with amplified spontaneous emission. Polarization properties of the IR radiation are found to be dependent on the rotational levels. Pressure and laser power behaviors of the generated waves were reported. The mechanism of PFWM was discussed in terms of selection rules of the relevant ro-vibronic transitions. Received: 19 September 1996/Revised version: 27 January 1997     

Imaging of flames and cold flows in air by diffraction from a laser-induced grating

Nonresonant laser-induced gratings are created in gases employing the second-harmonic output of a Nd: YAG laser in a degenerate four-wave mixing beam geometry. The diffraction efficiency of the gratings has been investigated as a function of laser intensity and gas pressure. Single-shot images of a helium flow in ambient air illustrate that diffraction of light from a laser-induced grating has the potential for remote, two-dimensional diagnostics of gas mixing processes. In addition, this coherent technique is used to image a sooty flame.     

Stabilization of a premixed CH4/O2/N2 flame using femtosecond laser-induced plasma

The influence of femtosecond laser-induced plasma (FLIP) on the stability of a premixed CH(4)/O(2)/N(2) flame is investigated at atmospheric pressure. The laser energy, laser repetition rate, the equivalence ratios, and the volume percentage of oxygen in O(2)/N(2) blends are varied. Our findings indicate that the flame blow-off velocity is a function of these parameters. It has been experimentally found that the flame blow-off velocity increases by a factor of two with FLIP than without FLIP. A high-repetition-rate and a great energy laser-induced plasma flameholding, as a non-intrusive optical flameholding, may be a feasible alternative for any combustor.     

Determination of spatial temperature distributions in a laminar premixed ethylene/air flame by laser-induced atomic fluorescence

A quick and simple detection system for spatially resolved temperature measurements in flames based on laser-induced thermally assisted atomic line fluorescence of seeded rubidium atoms is described. The fluorescence light from two atomic states is dispersed and simultaneously recorded by a CCD camera. The fluorescence ratio distributions lead directly to absolute temperature distributions. The practical use, the spatial and temperature resolution and error limits of the method are discussed and compared with other procedures for temperature measurements.     

CO2 addition and pressure effects on laminar and turbulent lean premixed CH4 air flames

An experimental study on CH₄–CO₂–air flames at various pressures is conducted by using both laminar and turbulent Bunsen flame configurations. The aim of this research is to contribute to the characterization of fuel lean methane/carbon dioxide/air premixed laminar and turbulent flames at different pressures, by studying laminar and turbulent flame propagation velocities, the flame surface density and the instantaneous flame front wrinkling parameters. PREMIX computations and experimental results indicate a decrease of the laminar flame propagation velocities with increasing CO₂ dilution rate. Instantaneous flame images are obtained by Mie scattering tomography. The image analysis shows that although the height of the turbulent flame increases with the CO₂ addition rate, the flame structure is quite similar. This implies that the flame wrinkling parameters and flame surface density are indifferent to the CO₂ addition. However, the pressure increase has a drastic effect on both parameters. This is also confirmed by a fractal analysis of instantaneous images. It is also observed that the combustion intensity S_T/S_L increases both with pressure and the CO₂ rate. Finally, the mean fuel consumption rate decreases with the CO₂ addition rate but increases with the pressure.     

Parametric four-wave mixing in the 3d and 4d Rydberg states of NO

2 Σ⁺,H^′ 2Π^±(v^′=2), and 4dσ,πO²Σ⁺,O^′ 2Π^±(v^′=0) Rydberg states of NO molecules are described. The analyses of the two-photon excitation functions and infrared emission spectra revealed that only the ²Π^- component was involved in PFWM. This is in good accordance with the absence of amplified spontaneous emission (ASE) in the ²Σ⁺ and ²Π⁺ components due to their predissociative character. Results provide some support for the mechanism that the generation of the vacuum ultraviolet radiation was brought by PFWM in which ASE served as a third driving wave. Received: 6 January 1998     

Low pressure premixed CH4/air flames with forced periodic mixture fraction oscillations: experimental approach

An experimental setup for the generation and investigation of periodic equivalence ratio oscillations in laminar premixed flames is presented. A special low-pressure burner was developed which generates stable flames in a wide pressure range down to 20 mbar and provides the possibility of rapid mixture fraction variations. The technical realization of the mixture fraction variations and the characteristics of the burner are described. 1D laser Raman scattering was applied to determine the temperature and concentration profiles of the major species through the flame front in correlation to the phase-angle of the periodic oscillation. OH* chemiluminescence was detected to qualitatively analyze the response of the flame to mixture fraction variations by changing shape and position. Exemplary results from a flame at p=69 mbar, forced at a frequency of 10 Hz, are shown and discussed. The experiments are part of a cooperative research project including the development of kinetic models and numerical simulation tools with the aim of a better understanding and prediction of periodic combustion instabilities in gas turbines. The focus of the current paper lies on the presentation of the experimental realization and the measuring techniques.     

Addition of NO2 to a laminar premixed ethylene-air flame: Effect on soot formation

Experiments were conducted on a laminar premixed ethylene-air flame at equivalence ratios of 2.34 and 2.64. Comparisons were made between flames with 5% NO₂ added by volume. Soot volume fraction was measured using light extinction and light scattering and fluorescence measurements were also obtained to provide added insight into the soot formation process. The flame temperature profiles in these flames were measured using a spectral line reversal technique in the non-sooting region, while two-color pyrometry was used in the sooting region. Chemical kinetics modeling using the PREMIX 1-D laminar flame code was used to understand the chemical role of the NO₂ in the soot formation process. The modeling used kinetic mechanisms available in the literature. Experimental results indicated a reduction in the soot volume fraction in the flame with NO₂ added and a delay in the onset of soot as a function of height above the burner. In addition, fluorescence signals—often argued to be an indicator of PAH—were observed to be lower near the burner surface for the flames with NO₂ added as compared to the baseline flames. These trends were captured using a chemical kinetics model that was used to simulate the flame prior to soot inception. The reduction in soot is attributed to a decrease in the H-atom concentration induced by the reaction with NO₂ and a subsequent reduction in acetylene in the pre-soot inception region.