Laser-induced incandescence for soot particle size and volume fraction measurements using on-line extinction calibration

A novel technique for two-dimensional measurements of soot volume fraction and particle size has been developed. It is based on a combined measurement of extinction and laser-induced incandescence using Nd:YAG laser wavelengths of 532 nm and 1064 nm. A low-energy laser pulse at 532 nm was used for extinction measurements and was followed by a more intense pulse at 1064 nm, delayed by 15 ns, for LII measurements. The 532-nm beam was split into a signal beam passing the flame and a reference beam, both of which were directed to a dye cell. The resulting fluorescence signals, from which the extinction was deduced, together with the LII signal, were registered on a single CCD detector. Thus the two-dimensional LII image could be converted to a soot volume fraction map through a calibration procedure during the same laser shot. The soot particle sizes were evaluated from the ratio of the temporal LII signals at two gate time positions. The uncertainty in the particle sizing arose mainly from the low signal for small particles at long gate times and the uncertainty in the flame temperature. The technique was applied to a well-characterized premixed flat flame, the soot properties of which had been previously thoroughly investigated. Received: 21 June 2000 / Revised version: 11 September 2000 / Published online: 7 February 2001     

Negligibility of temporal laser wing effects in saturated LIF diagnostics

The temporal influence of a laser pulse on saturated laser-induced fluorescence is investigated by means of numerical solutions of rate equations describing dynamical models of two- and four-level molecular systems. It turns out that temporal wings of the laser become negligible if the saturated fluorescence power from the entire pulse is monitored instead of the total energy obtained from the integrated fluorescence. The negligibility is evaluated relative to the saturation status obtained with temporal sampling of the fluorescence signal as is commonly adopted in the time-resolved technique. The advantages and the limitations of power measurements are discussed with reference to the experimental feasibility of pulse acquisitions for the determination of the flawless saturation degree. Received: 4 December 2002 / Revised version: 14 February 2003 / Published online: 5 May 2003 RID="*" ID="*"Corresponding author. Fax: +39-06/3048-4811, E-mail: michele.marrocco@casaccia.enea.it     

Laser-induced incandescence: excitation and detection conditions,material transformations and calibration

Successful implementation of laser-induced incandescence (LII) relies upon judicious choice of excitation and detection conditions. Excitation conditions encompass choice of excitation wavelength and laser fluence. Detection conditions include choice of detection wavelength, spectral band pass about the central wavelength, detection delay and duration relative to the excitation laser pulse usually corresponding to the peak of the signal intensity. Examples of applying these parameters to LII are illustrated by way of examples: soot/polycyclic aromatic hydrocarbon and metal aerosol systems. Tradeoffs must be recognized. Laser-induced chemical and structural changes of the aerosol must be considered, particularly in light of heterogeneous aerosols. Diagnostics of such changes are outlined as they will affect interpretation of the LII signal. Finally, calibration (for LII) must be chosen to be appropriate for aerosols from practical sources as they may be mixed organic and inorganic composition. A research paper in response to the call for papers from the Third International Discussion Meeting and Workshop on Laser-Induced Incandescence July 30th–August 1st, Ottawa, Canada (2008).     

Derivation of a temperature-dependent accommodation coefficient for use in modeling laser-induced incandescence of soot

This paper presents a derivation of an expression to estimate the accommodation coefficient for gas collisions with a graphite surface, which is meant for use in models of laser-induced incandescence (LII) of soot. Energy transfer between gas molecules and solid surfaces has been studied extensively, and a considerable amount is known about the physical mechanisms important in thermal accommodation. Values of accommodation coefficients currently used in LII models are temperature independent and are based on a small subset of information available in the literature. The expression derived in this study is based on published data from state-to-state gas-surface scattering experiments. The present study compiles data on the temperature dependence of translational, rotational, and vibrational energy transfer for diatomic molecules (predominantly NO) colliding with graphite surfaces. The data were used to infer partial accommodation coefficients for translational, rotational, and vibrational degrees of freedom, which were consolidated to derive an overall accommodation coefficient that accounts for accommodation of all degrees of freedom of the scattered gas distributions. This accommodation coefficient can be used to calculate conductive cooling rates following laser heating of soot particles.     

Optical and microscopy investigations of soot structure alterations by laser-induced incandescence

2 at 1064 nm, vaporization/fragmentation of soot primary particles and aggregates occurs. Optical measurements are performed using a second laser pulse to probe the effects of these changes upon the LII signal. With the exception of very low fluences, the structural changes induced in the soot lead to a decreased LII intensity produced by the second laser pulse. These two-pulse experiments also show that these changes do not alter the LII signal on timescales less than 1 μs for fluences below the vaporization threshold. Received: 20 October 1997/Revised version: 16 February 1998     

Radiative lifetime measurements in neutral zirconium using time-resolved laser induced fluorescence in supersonic free-jet

Radiative lifetimes of 20 odd-parity excited energy levels in Zr I, out of which 10 for the first time, have been investigated in the energy range 17 400-29 300 cm⁻¹. The levels belong to the 4d²5s5p and 4d³5p electronic configurations. Time-resolved laser-induced fluorescence spectroscopy technique in supersonic free-jet has been employed in the present work to circumvent the effects of collision in lifetime measurements.     

Soot volume fraction measurements in aero-engine exhausts using extinction-calibrated backward laser-induced incandescence

Control and reduction of soot particle emissions from aeronautic turbines requires a monitoring system suitable for quantification of these emissions. Currently, such emissions are estimated using the technique of smoke number. This is an extractive method, which is not sensitive enough for the low emission levels of modern gas turbines. Within a recent European project, AEROTEST, part of the project aimed at investigating an alternative soot monitoring technique, laser-induced incandescence (LII) as an in-situ optical diagnostic for quantification of soot emissions. For aero-engine applications, especially those involving large-scale turbines, it is necessary to perform the measurements at long distance from the turbine. The LII technique is favourable in this respect as it provides for non-intrusive measurements and, by detecting the isotropic LII signal along the same axis as the incoming laser beam (so called backward LII), both the laser and the detector can be built inside one system located several meters from the turbine. The concept was initiated in the previous European projects, AEROJET I and II. This paper describes the modified version of the system and the procedure developed to achieve reliable and quantitative soot volume fraction measurements in the exhausts of aero-engines. Application of the backward LII technique is demonstrated in the exhaust of a military turbojet engine for different engine speeds.     

The influence of wavelength in extinction measurements and beam steering in laser-induced incandescence measurements in sooting flames

The accuracy of laser-induced incandescence (LII) measurements is significantly influenced by the calibration process and the laser profile degradation due to beam steering. Additionally, the wavelength used for extinction measurements, needed for LII calibration, is critical and should be kept as high as possible in order to avoid light absorption by molecular species in the flame. The influence of beam steering on the LII measurement was studied in turbulent sooting C₂H₄/air flames at different pressures. While inhomogeneities in the laser profile become smoothed out in time-averaged measurements, especially at higher pressure, the corresponding single-shot beam profiles reveal an increasing effect of beam steering. In the current configuration it was observed that the resulting local laser fluence remains within certain limits (30% to 200%) of the original value. A sufficiently high incident laser fluence can thus prevent the local fluence from dropping below the LII threshold value of approximately 0.3 J/cm² at the cost of increased soot surface vaporization. A spatial resolution in the dimension of the sheet thickness of below 1 mm cannot be guaranteed at increased pressure of 9 bars due to beam steering. A feasibility study in a combustor at technical conditions demonstrates the influence of both effects beam steering and choice of calibration wavelength and led to the conclusion that, however, a shot-to-shot calibration of LII with simultaneously measured extinction can be realized.     

Single-shot laser-induced fluorescence imaging of formaldehyde with XeF excimer excitation

Single-shot formaldehyde laser-induced fluorescence (LIF) imaging measurements in a technical scale turbulent flame have been obtained using XeF excimer laser excitation in the ?¹A₂-˜X¹A₁ transition at 353.2 nm. Measurements have been carried out in a 150 kW natural gas swirl burner where formaldehyde distribution fields have the potential, in combination with OH concentration fields, to visualize the heat release distribution and therefore give an optimal visualization of flame-front positions. The extended areas where formaldehyde was detected in the swirl flame indicates the presence of low temperature chemistry in preheated gas pockets before ignition. Received: 31 January 2000 / Revised version: 2 March 2000 / Published online: 5 April 2000     

New laser-induced fluorescence scheme for simultaneous OH and NO detection by a single laser set-up

In this communication, we propose a new laser-induced fluorescence (LIF) scheme that allows the simultaneous detection of OH and NO by a single laser set-up. OH is detected by the second harmonic (SH) of a dye laser tuned to the (0,0)-band of the 3064 ? system, while its third harmonic (TH) is used to detect NO through excitation of the (2,0)-band of the γ system. This scheme is presented and discussed within the framework of its potential use in field instruments for the measurement of tropospheric OH concentration. Received: 8 March 2002 / Revised version: 22 April 2002 / Published online: 8 August 2002     

Droplet velocity and acceleration measurements in dense sprays by laser flow tagging

The feasibility of liquid-phase velocity measurements in dense sprays by 2D laser-based flow tagging is demonstrated. Velocity measurements in dense sprays are difficult with conventional techniques because of the high number densities of droplets, the optical thickness of the medium, and multiple light-scattering effects. The present flow-tagging technique is based on phosphorescent tracer molecules, which are excited by a grid of pulsed ‘write’ laser beams. The motion of the tagged droplet groups can be observed by a CCD camera in this way. In addition, multiple consecutive velocity measurements are performed by ‘droplet-group tracking’. This yields the acceleration along the trajectory of individual groups of droplets in unsteady sprays. Received: 5 June 2000 / Revised version: 28 July 2000 / Published online: 6 September 2000     

Compositional analysis of ferroelectric films coated with carbon layer using laser-induced plasmas spectroscopy

An experimental investigation of the effect of different thickness of carbon layer coated on ferroelectric films on the atomic emission intensity using laser-induced plasmas spectroscopy technique with charge-coupled device (CCD) experimental system has been conducted. The experimental results show that the intensity of the spectra emitted with the carbon layer thickness of 15 μm is much higher than that of pure ferroelectric films. By using this best experimental condition, the atomic concentration ratios of ferroelectric films are evaluated by rationing the integrated intensities of selected spectral emission lines of the plasma produced from the films. And the experimental results show that N_La/N_Co and N_Sr/N_Co atomic concentration ratios are almost in agreement with the corresponding values obtained by traditional compositional analysis techniques of inductively coupled plasma (ICP).     

Exhaust-gas imaging via planar laser- induced fluorescence of sulfur dioxide

The potential use of planar laser-induced fluorescence (PLIF) of sulfur dioxide (SO₂) for visualization of exhaust-gas distributions is outlined and demonstrated. Strong absorption features in the UV spectral range allow excitation of SO₂ with the fourth harmonic of a Nd:YAG laser at 266 nm. Fluorescence emissions are mostly red-shifted and can be easily detected in single-shot imaging arrangements with a good signal-to-noise ratio. This study uses a premixed methane/air flame that is doped with SO₂ to demonstrate the technique. The signal strength has a pronounced temperature dependence for excitation at 266 nm. Received: 14 January 2002 / Revised version: 30 January 2002 / Published online: 14 March 2002     

Modeling laser-induced incandescence of soot: enthalpy changes during sublimation, conduction, and oxidation

This paper presents an analysis of several equations used to model laser-induced incandescence (LII) of soot. The analysis focuses on sub-models of the change in particle enthalpy during sublimation, conduction, and oxidation. Assuming that pressure is constant, expressing the conductive cooling rate in terms of enthalpy instead of energy, thereby accounting for expansion work, increases the signal decay rate and has an effect comparable to increasing the thermal accommodation coefficient from 0.30 to 0.38. Accounting for oxidative heating decreases the signal decay rate and has an effect comparable to decreasing the accommodation coefficient from 0.30 to 0.25. As an estimate of magnitude of these effects, primary particle sizes inferred from signal decay rates measured at low fluences may be over-predicted by as much as 17% if oxidation is neglected in the model at O₂ partial pressures of ～0.2 bar, under-predicted by 24% if expansion work is neglected, and under-predicted by only 9% if both are neglected. This paper also provides updated parameterizations for average enthalpies of formation, molecular weights, and total pressures of sublimed carbon clusters for use in LII models.     

Numerical investigation of the effect of signal trapping on soot measurements using LII in laminar coflow diffusion flames

Laser-induced incandescence has been rapidly developed into a powerful diagnostic technique for measurements of soot in many applications. The incandescence intensity generated by laser-heated soot particles at the measurement location suffers the signal trapping effect caused by absorption and scattering by soot particles present between the measurement location and the detector. The signal trapping effect was numerically investigated in soot measurements using both a 2D LII setup and the corresponding point LII setup at detection wavelengths of 400 and 780 nm in a laminar coflow ethylene/air flame. The radiative properties of aggregated soot particles were calculated using the Rayleigh–Debye–Gans polydisperse fractal aggregate theory. The radiative transfer equation in emitting, absorbing, and scattering media was solved using the discrete-ordinates method. The radiation intensity along an arbitrary direction was obtained using the infinitely small weight technique. The contribution of scattering to signal trapping was found to be negligible in atmospheric laminar diffusion flames. When uncorrected LII intensities are used to determine soot particle temperature and the soot volume fraction, the errors are smaller in 2D LII setup where soot particles are excited by a laser sheet. The simple Beer–Lambert exponential attenuation relationship holds in LII applications to axisymmetric flames as long as the effective extinction coefficient is adequately defined.     

Picosecond laser-induced fluorescence from gas-phase polycyclic aromatic hydrocarbons at elevated temperatures. II. Flame-seeding measurements

Picosecond laser-induced radiative emission from flames injected with aromatic substances has been measured spectrally and temporally resolved. The measurements were performed in various seeded regions and for different stoichiometric ratios of the surrounding gas. The wavelength of the excitation radiation was 266 nm. Changes in the lifetime and the spectral composition of the emission were observed with changes in the equivalence ratio and the position in the flame. Considerable agreement with previously reported cell measurements was obtained for those regions close to the injection zone. Temperatures were determined from spectrally and temporally resolved measurements. The comparison with elastic scattering gave reasonable results at low seeding rates for naphthalene, and is hoped to be improved even further in future experiments by increasing the time resolution and the signal-to-noise ratio of the measurements. Downstream and towards the surrounding gas, the lifetimes increased and the spectral profiles shifted and broadened towards the red. This effect increased when the equivalence ratio for the surrounding gas decreased and the oxygen concentration increased. The study was also directed towards characterizing features in the emission that could be indicative of a transition from the seeded aromatic substance to the formation of soot. An indicator for molecular or particle growth was the composition of the spectral emission in terms of UV, blue and green–yellow bands and the ratio between elastic-scattering signal and total emission signal. Spatially resolved measurements across the seeding region using a gated intensified CCD camera allowed a closer study of the molecular-growth region from the parent aromatic substance seeded to the soot formed. The fluorescence properties of dimers and their cyclodehydrogenated compounds and polymers containing aryl units are also discussed. Received: 11 July 2000 / Revised version: 30 October 2000 / Published online: 21 February 2001     

Real-time measurements of trace gases using a compact difference-frequency-based sensor operating at 3.5 μm

3 for on-line absorption measurements of H₂CO, CH₄, and H₂O near 3.5 μm is reported. Formaldehyde levels of 30 ppb, corresponding to absorptions of 2×10^-4 have been measured using absorption spectroscopy. In this paper we report specifically the performance of this sensor as part of the 1997 Lunar–Mars Life Support Test program at the NASA Johnson Space Center. Received: 1 April 1998     

Oxygen-distribution imaging with a novel two-tracer laser-induced fluorescence technique

We introduce a new technique for imaging oxygen concentrations in fuel/air mixtures that takes advantage of the different responses of toluene and 3-pentanone to collisional quenching by molecular oxygen. Since laser-induced fluorescence signals from both tracers upon excitation at 248 nm are spectrally well separated, simultaneous detection is possible. The technique is first applied to instantaneous imaging in turbulent mixing processes of interacting seeded air and nitrogen flows. Received: 1 August 2001 / Revised version: 29 October 2001 / Published online: 29 November 2001