Single-pulse two-dimensional temperature imaging in flames by degenerate four-wave mixing and polarization spectroscopy

We introduce a novel and simple experimental arrangement for single-pulse two-dimensional temperature mapping in flames by the coherent imaging techniques, degenerate four-wave mixing and polarization spectroscopy, utilizing a dual-wavelength dye laser and a diffraction grating. Each pulse of this dye laser consists of two wavelengths which were tuned to resonance with two different rotational transitions in theQ ₁ branch of theA ² –X ² (0, 0) band of the OH radical. A typical coherent imaging geometry where a sheet-shaped pump beam crossed an unfocused probe beam, was used. The two generated images of OH signal distributions were spatially separated by a diffraction grating and simultaneously detected on a single CCD chip. The two-dimensional single-pulse temperature map was extracted from these images. The precision of the methods is examined and a comparison between degenerate four-wave mixing and polarization spectroscopy is made.     

OH rotational temperature and number density measurements in high-pressure laminar flames using double phase-conjugate four-wave mixing

OH number densities and rotational temperatures up to 9 bar have been measured using double-phase-conjugate four-wave mixing (DPCFWM) in flat laminar premixed methane/air flames. By phase conjugating the backward pump to the forward pump with a phase-conjugate mirror conventional degenerate four-wave mixing (DFWM) becomes DPCFWM and quantitative measurements of OH radicals and OH rotational temperatures in flames at high pressure are possible. Our results show that conventional DFWM with a standard mirror is a low-biased measurement at high pressure primarly due to a fluctuating interaction volume which results from large fluctuating density gradients at the edge of the flame and from flame movement. A comparison is made between a laser-saturated fluorescence technique, conventional DFWM, and DPCFWM at 1, 5 and 9 bar.     

Multi-species detection in spray flames with tunable excimer lasers

Planar imaging with tunable excimer-laser sheet illumination is used to determine spatial distributions of different species in liquid-fuelled spray flames of commercial oil burning furnaces. Two burner configurations, which differ only in the fuel/air mixing devices, are investigated to understand why one configuration yields 30% less NO_x emission. Iso-octane and n-heptane fuels are used. To understand the origin for NO_x reduction spatial distributions of reactants (fuel, O₂), the reaction intermediate OH and the pollutant NO are recorded. OH and O₂ are measured by LIPF, NO by LIF. Fuel distributions are determined by another broad-band emission, whose origin is not yet identified. Both single shot and averaged distributions are recorded. The averaged distributions are extremely reproducible and depend sensitively on details of the burner geometry and the fuel/air mixing device. They can clearly be used to distinguish fine details in different injection systems. The spatial distribution of different species relative to each other yield considerable insight in the differences between the two combustion processes. On the basis of purely qualitative visualization it is possible to understand the origin for NO_x reduction: it results from faster injection of air in the one fuel/air mixing device.     

C2 creation,emission, and laser-induced fluorescence in flames and cold gases

We describe photochemical production of C₂ in the upper (d ³_g) and the lower (a ³_u) levels of the Swan-band transitions by 266 and 292-nm laser irradiation of flames and room-temperature flows of acetylene and ethylene. Topics treated include the spectroscopy of the Swan bands, lifetimes and quenching of the Swan-band emission, intensity dependences of the Swan-band emission in several environments, profiles of C₂ in low-pressure hydrocarbon flames, and the affect of Swan-band emission on three-photon-excited fluorescence detection of atomic hydrogen in hydrocarbon flames.This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences     

Degenerate four-wave mixing diagnostics on OH and NH radicals in flames

Degenerate four wave mixing (DFWM) is applied as a diagnostic to study OH and NH radicals in flames. DFWM is a coherent technique which offers the advantages of a highly collimated signal beam permitting efficient rejection of interfering radiation and requiring minimal optical access. Rotational temperatures have been determined from the DFWM spectra and are in close agreement with the temperatures measured using coherent anti-Stokes Raman scattering of nitrogen.Work performed at the Combustion Research Facility, Sandia National Laboratories, supported by the U.S. Department of Energy, Energy Conversion and Utilization Technologies Program and Office of Basic Energy Sciences, Division of Chemical Sciences     

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.     

Calibration of imaging laser-induced fluorescence measurements in highly absorbing flames

It has been described earlier that imaging measurements of laser-induced fluorescence (LIF) in flames can be calibrated to number densities with an integrated absorption measurement provided the integrated absorption is small. In this paper a method is presented that extends the technique to flames with substantial absorption, improves the number density determination and allows the experimental parameters to be chosen more freely. The method is based on an iterative computer procedure that reconstructs the 1-D spatially resolved absorption profile from laser measurements of the 1-D spatially resolved LIF and the integrated absorption of the laser beam. The technique is experimentally demonstrated by measurements of OH number densities in atmospheric flames. It is potentially a single-pulse method. Other applications of the iterative procedure are mentioned.     

Temperature measurements in flames using water molecule overtone . spectra detected by intracavity laser absorption spectroscopy

Received: 14 June 1996 / Revised version: 6 September 1996     

H2 CARS thermometry in a fuel-rich,premixed, laminar CH4/air flame in the pressure range between 5 and 40 bar

To establish H₂ CARS thermometry at high pressure, accumulated H₂ Q-branch CARS spectra were recorded in the exhaust of a fuel-rich CH₄/air flame at pressures between 5 and 40 bar. Temperatures were deduced by fitting theoretical spectra to experimental data points. The Energy-Corrected Sudden (ECS) scaling law was employed to set up an empirical model for the calculation of H₂ linewidths in high-pressure hydrocarbon flames with H₂ as a minority species. Experimental H₂ CARS spectra could be simulated very accurately with this model. The evaluated temperatures agreed well with reference temperatures obtained by spontaneous rotational Raman scattering of N₂.     

Two-photon degenerate four-wave mixing (DFWM) for the detection of ammonia: Applications to flames

Two-photon Degenerate four-wave mixing (DFWM) has been used for the detection of ammonia in a cell and in an atmospheric pressure flame. The NH₃ molecules were excited from the ground state X to either the C' or the B state and the DFWM signal was recorded simultaneously with the laser-induced fluorescence signal, originating from the C-A and B-A transitions, respectively. During the flame experiments sequential measurements of NH₃ and OH in an NH₃/O₂ flame were also performed.     

Wide- and narrow-band saturated fluorescence measurements of hydroxyl concentration in premixed flames from 1 bar to 10 bar

We have studied the use of wide-band detection in conjunction with saturation of a rovibronic transition of OH within itsA ^{2 +}–X ²(0,0) band. For wide-band detection, in which fluorescence is detected from the entire excited rotational manifold, the fluorescence yield is sensitive to collisions in two ways. First, it is sensitive to the ratio of rate coefficients describing rotational energy transfer and electronic quenching; this ratio determines the number of neighboring rotational levels that are populated during the laser pulse. Second, the fluorescence yield can vary with the total collisional rate coefficient; only after a sufficient number of collisions, corresponding to 2.5 ns in an atmospheric flame, does the rotational manifold reach steady state. We also compare measurements employing wide-band (detecting theR ₁ andR ₂ branches) and narrow-band (detecting a single transition) saturated fluorescence of OH. Over a wide range of conditions — obtained by varying the equivalence ratio, temperature, N₂ dilution, and pressure — the wide- and narrow-band fluorescence techniques compare well. Given this good agreement, wide-band saturated fluorescence could be especially useful for analyzing atmospheric flames with XeCl-excimer lasers; one can potentially obtain 2—D images of OH which have a high signal-to-noise ratio and a reduced sensitivity to laser irradiance and quenching.     

Simultaneous CH planar laser-induced fluorescence and particle imaging velocimetry in turbulent nonpremixed flames

jet =18600). Here, PLIF images reveal a CH layer of thickness typically <1 mm from flame base to tip. Furthermore, in these permanently blue flames, we observe instantaneous flamefront strain rates – derived from the PIV data – in excess of ±10⁴ s^-1 without flame extinction. Received: 16 October 1997/Revised version: 30 October 1997     

Ultrafast optical switching by instantaneous laser-induced grating formation and self-diffraction in barium fluoride

Received: 13 January 1999     

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     

Laser-induced fluorescence detection of singlet CH2 in low-pressure methane/oxygen flames

Methylene, CH₂, is a chemically important intermediate in hydrocarbon combustion but has previously eluded optical detection in a combustion environment. The CH₂ signal as a function of height above the burner surface in a premixed, laminar, methane/oxygen flame (5.6 Torr and fuel equivalence ratio 1.05) is measured by laser-induced fluorescence (LIF) in the B ₁ – ã¹ A ₁ electronic system. The ã state which lies 3165 cm^–1 above the ground state is populated at the high temperatures of the flame (800–1800 K). Although less than one photon for each laser pulse is detected, we can unambiguously attribute the LIF features in the region 450 to 650 nm to CH₂ by both scanning the excitation laser and dispersing fluorescence. LIF temperatures and CH and OH LIF concentration profiles are also obtained for the flame. The CH₂ radical concentration maximum occurs closer to the burner than that of either OH or CH, as expected from models of methane combustion chemistry.     

Two-dimensional imaging of glyoxal (C2H2O2) in acetylene flames using laser-induced fluorescence

2 H₂O₂). Laser-induced fluorescence spectra from glyoxal vapor using the same excitation wavelength of 428 nm showed the same strongest lines as the signal from the flame. Glyoxal was visualized in two different modes; two-dimensional imaging and a spatial-spectral mode where spectra were obtained at different spatial positions in the flame simultaneously. For the premixed laminar rich flame it is shown that glyoxal is produced early in the flame, before the signals for C₂ and CH appear. For the turbulent non-premixed flames it is shown that glyoxal is produced in a layer on the fuel rich side of the flames. Here the fuel is premixed with ambient air. This layer is thin and has a high spatial resolution. The general trend was that the glyoxal signal appeared in regions with a lower temperature compared with the emission from C₂ and CH. The imaging of glyoxal in turbulent acetylene flames is a promising tool for achieving new insight into flame phenomena, as it gives very good structural information on the flame front. Tests so far do not indicate that the detected glyoxal is a result of photo-production. To our knowledge, this is the first detection of glyoxal in flames using laser-induced fluorescence. Received: 19 December 1996/Revised version: 26 May 1997     

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.     

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.