Detection of trace nitric oxide concentrations using 1-D laser-induced fluorescence imaging

Spectrally resolved laser-induced fluorescence (LIF) with one-dimensional spatial imaging was investigated as a technique for detection of trace concentrations of nitric oxide (NO) in high-pressure flames. Experiments were performed in the burnt gases of premixed methane/argon/oxygen flames with seeded NO (15 to 50 ppm), pressures of 10 to 60 bar, and an equivalence ratio of 0.9. LIF signals were dispersed with a spectrometer and recorded on a 2-D intensified CCD array yielding both spectral resolution and 1-D spatial resolution. This method allows isolation of NO-LIF from interference signals due to alternative species (mainly hot O₂ and CO₂) while providing spatial resolution along the line of the excitation laser. A fast data analysis strategy was developed to enable pulse-by-pulse NO concentration measurements from these images. Statistical analyses as a function of laser energy of these single-shot data were used to determine the detection limits for NO concentration as well as the measurement precision. Extrapolating these results to pulse energies of ～?16 mJ/pulse yielded a predicted detection limit of ～?10 ppm for pressures up to 60 bar. Quantitative 1-D LIF measurements were performed in CH₄/air flames to validate capability for detection of nascent NO in flames at 10–60 bar.     

Cavity ring-down absorption and laser-induced fluorescence for quantitative measurements of CH radicals in low-pressure flames

The absolute, quantitative spatially resolved distribution of CH radicals was measured in the reaction zone of a low-pressure methane/air flame (25 Torr) using a combination of laser-induced fluorescence (LIF) and cavity ring-down (CRD) absorption spectroscopy operating on the A²–X²(0,0) transition. The spatially resolved 1-D image of LIF provides a direct measure of the CH distribution along the path of the laser beam in the CRD cavity. The temperature distribution was determined from measurements on a pair of rotational transitions. A series of LIF line images and CRD absorption measurements taken at various burner heights are combined to form a quantitative 2-D image of the CH distribution. This is used to interpret the CRD measurements along this inhomogeneous path. The 10 ppm peak CH concentration measured here on the centerline of the flame is in good agreement (within 15%) with earlier CH A–X LIF measurements calibrated by Rayleigh and Raman scattering. A 1-D LIF image collected simultaneously with CRD absorption was also used to quantify and optimize the spatial resolution of the CRD measurement. PACS 42.62.Fi; 82.33.Vx; 33.80.Gj     

Quantitative measurement of hydroxyl radical(OH) concentration in premixed flat flame by combining laser-induced fluorescence and direct absorption spectroscopy

An accurate and reasonable technique combining direct absorption spectroscopy and laser-induced fluorescence(LIF)methods is developed to quantitatively measure the concentrations of hydroxyl in CH_4/air flat laminar flame. In our approach, particular attention is paid to the linear laser-induced fluorescence and absorption processes, and experimental details as well. Through measuring the temperature, LIF signal distribution and integrated absorption, spatially absolute OH concentrations profiles are successfully resolved. These experimental results are then compared with the numerical simulation. It is proved that the good quality of the results implies that this method is suitable for calibrating the OH-PLIF measurement in a practical combustor.     

Attenuation corrections for in-cylinder NO LIF measurements in a heavy-duty Diesel engine

Quantification of the nitric oxide (NO) concentration inside the cylinder of a Diesel engine by means of laser-induced fluorescence (LIF) measurements requires, amongst others, knowledge of the attenuation of the ultraviolet radiation involved. We present a number of laser diagnostic techniques to assess this attenuation, enabling a correction for laser intensity and detection efficiency of the raw NO LIF data. Methods discussed include overall laser beam transmission, bidirectional laser scattering (bidirectional LIF), spectrally resolved fluorescence imaging, and Raman scattering by N₂. A combination of techniques is necessary to obtain the complete attenuation of laser beam and NO fluorescence. The overall laser beam transmission measurements and bidirectional LIF measurements (the latter yielding spatially resolved transmission) provide evidence of a non-uniform attenuation distribution, with predominant attenuation within or near the piston bowl. Fluorescence imaging of multiple vibrational bands through a spectrograph is shown to be a powerful method for obtaining spatially resolved data on the transmission losses of fluorescence. Special attention is paid to the role of CO₂ and O₂ as UV light absorbers, and the consequences to different excitation-detection schemes for NO. PACS 82.33.Vx; 42.62.Fi; 33.20.t     

Single-shot,spatially-resolved stand-off detection of atomic hydrogen via backward lasing in flames

We report on an experimental demonstration of spatially-resolved detection of atomic hydrogen in flames using a single-ended configuration yielding 656-nm lasing in the backward direction upon 2-photon pumping with 205-nm femtosecond laser pulses. Spatial resolution is achieved by temporally-resolved detection of the backward lasing using a streak camera. The method is demonstrated in CH₄/O₂ flames; both in a setup consisting of two flames, with variable spacing between the flames, and in a single flame. Results from the two-flame experiment show that the backward lasing technique is able to determine changes in the separation between the flames as this distance was altered. By maximizing the temporal resolution of the streak camera, obtaining a highest spatial resolution of 1.65 mm, it is possible to resolve the hydrogen signal present in the two reaction zones in the single flame, where the separation between the reaction zones is ∼2 mm. The lasing signal is strong enough to allow single-shot measurements. Results obtained by backward lasing are compared with 2-photon planar laser-induced fluorescence (LIF) images recorded with detection perpendicular to the laser beam path and the results from the two methods qualitatively agree. Although further studies are needed in order to extract quantitative hydrogen concentrations, the present results indicate great potential for spatially resolved single-ended measurements, which would constitute a very valuable asset for combustion diagnostics in intractable geometries with limited optical access. It appears feasible to extend the technique to detection of any species for which resonant two-photon-excited lasing effect has been observed, such as O, N, C, CO and NH₃.     

Soot-visualization strategies using laser techniques

Strategies for spatially resolved soot volume-fraction measurements have been investigated in sooting laboratory flames with known soot characteristics. Two techniques were compared: Laser-Induced Fluorescence in C₂ from Laser-Vaporized Soot (LIF(C₂)LVS), and Laser-Induced Incandescence of soot (LII). The LII signal is the increased temperature radiation from soot particles which have been heated to temperatures of several thousand degrees as a consequence of absorption of laser radiation. The LIF(C₂)LVS technique is based on the production of C₂ radicals from laser-vaporized soot which occurs for laser intensities ≥10⁷ W/cm². A laser wavelength is chosen such that besides vaporizizng the soot, it also excites the C₂ radicals, and the subsequent C₂ fluorescence signal is detected. The signals from both techniques showed good correlation with soot volume fractions in the studied flame. The dependence of the signals on experimental parameters was studied, and the influence of interfering radiation, such as background flame luminosity and fluorescence from polyaromatic hydrocarbons, on studied signals was established. The potential of the two techniques for imaging of soot volume fractions in laboratory flames was demonstrated. Advantages and disadvantages of the studied techniques are discussed.     

Two-photon polarization spectroscopy applied for quantitative measurements of atomic hydrogen in atmospheric pressure flames

Two-photon – polarization spectroscopy has been applied for the first time to determine absolute number densities and kinetic temperatures of atomic hydrogen via the 1 S–2 Stransition in flames at atmospheric pressure. This sensitive laser spectroscopic technique, so far mainly developed and applied for quantitative plasma diagnostics, is also well suited for real combustion processes, because the signal is directly related to the two-photon absorption itself and therefore not limited by quenching or photo-ionization. It can be applied with high spatial and temporal resolution in a wide range of pressure and temperature. Furthermore, a well-established calibration procedure allows for a precise determination of the absolute ground-state density of atomic hydrogen. The measurements also demonstrate that a certain potential of this method comes especially into its own in conjunction with laser radiation of highest spectral quality, i.e. pulsed single-longitudinal mode UV radiation.     

Correction of imaging errors in spatially resolved laser scattering experiments in flames

Imaging errors in spatially resolved measurements using laser scattering techniques in flames are discussed. An experimental method based on Fourier deconvolution to correct the measurements for the imaging errors is presented. The method is especially important when large gradients are recorded with lowf-number lenses in order to obtain a sufficient signal-to-noise ratio. The technique is demonstrated experimentally on OH profiles measured by laser-induced fluorescence in an atmospheric acetylen-oxygen flame.     

Spectroscopic,calibration and RET issues for linear laser-induced fluorescence measurements of nitric oxide in high-pressure diffusion flames

Two different strategies are compared for linear laser-induced fluorescence (LIF) measurements of nitric oxide concentration ([NO]) in counter-flow diffusion flames at high pressures via the A-X(0,0) system. Excitation of NO via a rovibronic transition at 226.03 nm is found to be slightly better compared to a previously utilized excitation wavelength of 225.58 nm. An indirect approach based on the computed spectral overlap fraction is verified and applied to calibrate [NO] measurements in counter-flow diffusion flames at high pressures. A five-level model for NO molecular dynamics is presented and utilized to investigate the effects of rotational energy transfer (RET) on linear LIF measurements of [NO] at pressures up to 15 atm. The results indicate that rotational relaxation effects are essentially negligible under high-pressure conditions at low laser fluences, and thus they need not be accounted for when measuring [NO] using linear LIF. The calibration technique is validated by direct comparisons to [NO] measurements made at pressures up to 5 atm via another calibration method, based on doping NO in counter-flow premixed flames at the same pressure. Using this calibration technique, LIF measurements of [NO] are obtained in a series of counter-flow diffusion flames at pressures up to 15 atm. These measurements are found to be in excellent agreement with previously reported measurements of [NO] in similar flames. PACS 07.35.+k; 33.20.Sn; 42.62.Fi     

Simultaneous measurement of localized heat release with OH/CH2O-LIF imaging and spatially integrated OH∗ chemiluminescence in turbulent swirl flames

The in-situ and localized observation of heat release in turbulent flames is important for the validation of computational modeling of turbulent flows with combustion. In the present work we obtain localized information on heat release rate (HRR) by the commonly accepted technique of the simultaneous and single-shot planar imaging of OH and CH₂O concentrations by laser-induced fluorescence (LIF). Additionally, we combine this with the simultaneous line-of-sight and temporally resolved chemiluminescence detection of OH^?, spatially integrated within the flame volume, interrogated by the laser sheets used for the HRR imaging technique. The combined diagnostic methods are demonstrated for a swirl-stabilized, premixed turbulent methane/air flame of 30-kW thermal power, and they show the existence of correlations between both HRR-sensitive diagnostic techniques.     

Flame turbulences recorded at 1 kHz using planar laser induced fluorescence upon hot band excitation of OH radicals

Laser-induced fluorescence (LIF) in flames is excited by a diode-pumped all-solid-state disk laser system which operates at a pulse repetition rate of 1 kHz and a tunable wavelength around 1030 nm. The laser fundamental is converted to 343 nm and used to excite into the hot band transition of OH radicals in H₂/O₂ diffusion flames of an industrial burner and in the premixed flame of a microburner. The OH radical emission around 308 nm is resolved spectrally and spatially using a light-sheet technique. Imaging of the planar LIF (PLIF) by a gated camera visualizes the turbulent flame behavior on the millisecond time scale without averaging. To our knowledge this is the first time that an all-solid-state laser providing at the same time a kHz repetition rate as well as pulse energies of up to 5.5 mJ is available for PLIF observation of OH radicals. PACS 33.50.-j; 42.55.Xi; 82.33.Vx     

Experimental evaluation of strategies for quantitative laser-induced-fluorescence imaging of nitric oxide in high-pressure flames (1-60 bar)

Nitric oxide laser-induced-fluorescence (NO-LIF) 2-D imaging measurements using a new multi-spectral detection strategy are reported for high-pressure flames (1-60 bar). This work builds on previous research that identified interference LIF from O₂ and CO₂ in high-pressure flames and optimized the choice of excitation strategies as a function of application conditions. In this study, design rules are presented to optimize the LIF detection wavelengths for quantitative 2-D NO-LIF measurements over a wide range of pressures (1-60 bar) and temperatures. Simultaneous detection of LIF in multiple wavelength regions enables correction of the NO signal for interference from O₂ and CO₂ and allows simultaneous imaging of all three species. New experiments of wavelength-resolved 1-D LIF in slightly lean (? = 0.9) and slightly rich (? = 1.1) methane/air flames are used to evaluate the design rules and estimate the NO detection limits for a wide range of flame conditions. The quantitative 2-D measurements of NO in the burnt gas are compared with model calculations (using GRI-Mech 3.0) versus pressure for slightly lean and slightly rich flames. The discussions and demonstrations reported in this study provide a practical guideline for application of instantaneous 1-D or 2-D NO-LIF imaging strategies in high-pressure combustion systems.     

Simultaneous high-speed measurement of temperature and lifetime-corrected OH laser-induced fluorescence in unsteady flames

A means of performing simultaneous, high-speed measurements of temperature and OH lifetime-corrected laser-induced fluorescence (LIF) for tracking unsteady flames has been developed and demonstrated. The system uses the frequency-doubled and frequency-tripled output beams of an 80 MHz mode-locked Ti:sapphire laser to achieve ultrashort laser pulses (order 2 ps) for Rayleigh-scattering thermometry at 460 nm and lifetime-corrected OH LIF at 306.5 nm, respectively. Simultaneous, high-speed measurements of temperature and OH number density enable studies of flame chemistry, heat release, and flame extinction in unsteady, strained flames where the local fluorescence-quenching environment is unknown.     

Spatially resolved absolute concentration and fluorescence-lifetime determination of H2CO in atmospheric-pressure CH4/air flames

Using laser-induced fluorescence (LIF), spatially resolved concentration profiles of formaldehyde (H₂CO) were obtained in the preheating zone of atmospheric-pressure premixed CH₄/air flames stabilized on the central slot of a multiple-slot burner similar in construction to domestic boilers. The isolated pQ₁(6) rotational line (339.23 nm) in the 2¹ ₀4¹ ₀ vibronic combination transition in the ?¹A₂- ¹A₁ electronic band system around 339 nm was excited in the linear LIF intensity regime. For a quantification of quenching effects on the measured LIF signal intensities, relative fluorescence quantum yields were determined from direct fluorescence lifetime as a function of height above the slot exit. Absolute H₂CO number densities in the flames were evaluated from a calibration of measured LIF signal intensities versus those obtained in a low-pressure sample with a known H₂CO vapor pressure. Peak concentrations in the slightly lean and rich flames reached (994±298) and (174±52) ppm, respectively. Received: 25 September 2000 / Published online: 30 November 2000     

Two-color laser-induced incandescence and cavity ring-down spectroscopy for sensitive and quantitative imaging of soot and PAHs in flames

Laser-induced incandescence is a technique which enables the measurement of soot volume fractions. However, the laser-induced soot emission might be affected by a fluorescence background generally ascribed to the polycyclic aromatic hydrocarbon compounds (PAHs) present at the soot location. In this paper, spatially resolved distributions of PAH absorbance and soot are obtained in sooting diffusion flames. The original method developed here consists in comparing the emission distributions induced by two different laser wavelengths: (1) at 1064 nm emission signals are exempt from PAH fluorescence and (2) at 532 nm both soot incandescence and PAH emission contribute to the total signal. In addition, the absolute absorption coefficient of the PAH mixture is determined by comparing absorption measurements obtained by cavity ring-down spectroscopy (CRDS) at 1064 nm and 532 nm. The proposed method can provide highly sensitive 2D imaging of PAHs and soot using the fundamental and the second-harmonic frequencies of a single YAG laser. Finally, 2D distributions of PAH absorbance and soot volume fraction calibrated by CRDS are obtained in two diffusion flames, particularly in a very low-sooting flame exhibiting a maximum PAH absorbance of 6×10^-4 cm^-1 and a maximum soot volume fraction of 3 ppb only. The respective spatial distributions of PAHs and soot are shown to vary with the initial C/O ratio. PACS 33.20.Lg; 42.62.Fi; 44.40.+a     

Combined soot optical characterization using 2-D multi-angle light scattering and spectrally resolved line-of-sight attenuation and its implication on soot color-ratio pyrometry

Soot characterization using multiple techniques has been performed in a series of nitrogen-diluted ethylene coflow laminar diffusion flames. Soot aggregate sizes have been measured in two dimensions, as opposed to traditional point measurements, by a newly developed two-dimensional multi-angle light scattering technique where image processing was applied to align images for Guinier analysis. Extinction measurements have also been performed using spectrally resolved line-of-sight attenuation with an imaging spectrometer. Spectrally and spatially resolved extinction measurements have been obtained as well. Combined with previously obtained time-resolved laser-induced incandescence measurements of primary particle diameters, the scattering and absorption components of extinction can be estimated. The so-called dispersion exponent that describes the wavelength dependence of spectral emissivity was determined in two dimensions and found to improve the accuracy of soot color-ratio pyrometry measurements.     

Influence of ferrocene addition to a laminar premixed propene flame: Laser diagnostics, mass spectrometry and numerical simulations

The detailed influence of ferrocene in a low-pressure, fuel-rich, laminar, premixed propene/oxygen/argon flat flame was investigated experimentally using molecular beam sampling mass spectrometry (MBMS), laser-induced fluorescence (LIF), and compared to numerical simulations. MBMS was applied to analyze the species profiles of important intermediates in the flames with and without ferrocene doping. The concentration profile of iron atoms was measured with absorption sensitive LIF, which provides absolute number densities without additional calibrations. The flame temperature was obtained by two-line OH LIF measurements. One dimensional numerical simulations of the flames using detailed models from the literature were performed and the modeling results are compared with the experimental measurements. The iron measurements show reasonable agreement with the numerical simulation, while some discrepancies were found at larger heights. The MBMS measurements show a decrease in flame velocity when ferrocene was added, which was not provided by the model.     

Rapid wavelength scans over one octave and application to laser-induced fluorescence

Rapid excitation scans of laser-induced fluorescence (LIF) have been demonstrated. Broadband light was generated in a photonic crystal fiber and transmitted through a long fiber. Due to group-velocity dispersion in the long fiber, a wavelength scan emerged from the fiber in time. The wavelength was swept over approximately one octave in approximately 150 ns. The generated light was used to excite LD 700 Perchlorate diluted in methanol. The LIF excitation scan had a spectral resolution of approximately 15 nm, and the integrated fluorescence spectrum was found to be within 7% of the integrated absorption spectrum of the dye molecule. The method presented makes possible spatially and spectrally resolved LIF excitation scans with scanning speeds up to the limits set by the excited-state lifetime of the dye molecule.     

Correction of LIF temperature measurements for laser absorption and fluorescence trapping in a flame

A computational method is described in order to correct OH LIF temperature measurements for absorption of laser energy and trapping of fluorescence. Calculations are performed in a large range of flame conditions and can be used as a correction data base both in case of (0-0) and (1?0) excitations. Comparison of corrected temperatures profiles obtained in a 40 Torr methanol/air flame, for both kinds of Laser-Induced Fluorescence (LIF) excitations shows a very good agreement. This method is applied to measure the temperature profile of a methanol flame perturbed by a sampling probe. The LIF collection volume is located at the actual probe sampled volume using an experimental procedure already described. Spatial resolution and sensitivity of temperature measurements are sufficiently efficient to highlight, for the first time by LIF, an indubitable cooling effect due to the probe presence that induces important OH profile change. According to flame chemical modelling, it is shown that both effects are strongly correlated.     

An investigation of neutral and ion number densities within laser-produced titanium plasmas in vacuum and ambient environments

Optical absorption spectroscopy in combination with laser-induced fluorescence imaging is applied to determine spatially and temporally resolved number densities within laser-produced titanium plasmas, expanding into vacuum and low-pressure nitrogen. Contour mapping of species number density and subsequent volumetric integration to yield the total number of absorbing species in the plume are demonstrated for Ti I expanding into vacuum. The results obtained indicate that for an incident KrF energy density of ~4 J cm^-2 the total plume content is >10¹⁷ Ti neutrals and ions. The ground-state neutral and ground-state ion yields are both observed to increase linearly with laser fluence above thresholds of ~2.2 J cm^-2 and ~3.7 J cm^-2, respectively. Reduction in absorption linewidths and spatial widening of the corresponding LIF images, observed for plume expansion in the presence of low-pressure ambient gases, reflects the reduction in species velocities and randomisation of the velocity distributions of plume species with increasing ambient pressure.