Measurement of temperature, fuel concentration and equivalence ratio fields using tracer LIF in IC engine combustion

A technique based on planar laser-induced fluorescence of 3-pentanone, for measurements of absolute concentration, temperature and fuel/air equivalence ratios in turbulent, high-pressure combustion systems such as an internal combustion engine is presented. Quasi-simultaneous excitation with 248 nm and 308 nm of 3-pentanone that is used as a fluorescence tracer doped to iso-octane, yields pairs of strongly temperature-dependent fluorescence images. Previous investigations have resulted in information on temperature and pressure dependence of absorption cross-sections and fluorescence quantum yields. Using these data the ratio of corresponding fluorescence images can be converted to temperature images. Instantaneous temperature distribution fields in the compression stroke and in the unburned end-gas of an SI engine were measured. The temperature fields obtained from the two-line technique are used to correct the original tracer-LIF images in order to evaluate quantitative fuel distributions in terms of number densities and fuel/air equivalence ratio. Received: 10 March 2000 / Revised version: 19 April 2000 / Published online: 16 August 2000     

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     

Opto-acoustic study of tinuvin-P and rhodamine 6G in solid polymeric matrices

Laser-induced opto-acoustic calorimetry has been used to examine the thermo-elastic properties of two polymer matrices doped with tinuvin-P and the radiation-less decay processes of rhodamine 6G (Rh6G) in them. The matrices assayed were methyl methacrylate (MMA) homopolymers P(MMA), and methyl methacrylate and 2-hydroxyethyl methacrylate (HEMA) copolymers P(MMA:HEMA). A slight difference in the thermo-elastic properties of the polymers, namely their adiabatic expansion coefficients, has been detected and found to correlate with the long-term stability of the laser material. This suggests a key role of the heat-dissipation processes in the photostability of these polymeric materials. On the other hand, the fluorescence quantum yield of Rh6G is shown to vary with the polymer-matrix composition in a way consistent with its lasing efficiency. The Rh6G absorption and fluorescence quantum yield also change dramatically upon increasing its concentration, which suggests the co-existence of different Rh6G forms with different photophysical properties. Received: 10 February 2000 / Revised version: 12 June 2000 / Published online: 10 January 2001     

Experimentally based correction procedures for planar laser-induced fluorescence measurements of mean species concentration

Experimentally-based correction procedures are demonstrated which enhance the quantitative nature of planar laser-induced fluorescence (PLIF) images for mean species concentration by correcting for the influence of the electronic quenching rate coefficient. Implementation of these methods requires only the ability to make PLIF and laser-saturated fluorescence (LSF) measurements. Though applied herein to NO, these procedures are broadly applicable both in terms of species and users. Moreover, they are generally effective regardless of the error gradients associated with spatial variations in the electronic quenching rate coefficient. In such general environments, these methods produce quenching-corrected, spatially resolved PLIF images of mean species concentration with a total uncertainty equivalent to that of a single LSF measurement. Received: 22 November 1999 / Revised version: 3 March 2000 / Published online: 16 June 2000     

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     

Laser-induced fluorescence detection of acetylene in low-pressure propane and methane flames

Laser-induced fluorescence is used to detect and record profiles of acetylene formed as an intermediate species in 10-Torr premixed propane and methane flames. In low-temperature regions of the flames, excitation spectra confirm acetylene as the spectral carrier. The spectra of acetylene overlap those of O₂ and NO in terms of both excitation and detection wavelengths, however, acetylene can be detected with relatively little interference in the vicinity of 228 nm, using a detection wavelength of 260 nm. The fluorescence lifetime of acetylene in the flame conditions studied is approximately 20 ns, much shorter than the radiative lifetime, due to a high quenching rate for all the colliders investigated. This can be exploited in low-pressure flames to avoid interference from acetylene in monitoring nitric oxide. The acetylene mole fraction in propane flames reaches its peak value at nearly the same location as that of HCO, slightly closer to the burner than the peak CH mole fraction. The acetylene fluorescence signal is easily detected in propane flames over equivalence ratios from 0.6 to 1.2, although it increases under fuel-rich conditions. In methane flames, the acetylene signal is much weaker and is undetectable for fuel-lean conditions. Received: 5 August 2002 / Revised version: 30 September 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. Fax: +1-202/767-1716, E-mail: brad@code6185.nrl.navy.mil     

Laser-diagnostic multi-species imaging in strongly swirling natural gas flames

Within the TECFLAM group a standard swirl burner is investigated, both experimentally using optical and probe measurements and by simulations using different modeling attempts. The present study is focused on the laser-based investigation of the NO distribution within the reacting flow field of a strongly swirling, confined 150-kW natural gas flame. Simultaneous quantitative measurements of NO- and OH-concentration fields by laser-induced fluorescence imaging (LIF) and temperature distribution (Rayleigh scattering) are performed. Mixing properties of the unburned gases are investigated for the isothermal and the combusting flow using tetrahydrothiophene (THT) as a new fluorescing tracer. These measurements show which areas are sufficiently mixed allowing for the application of planar Rayleigh thermometry. Areas where THT-LIF interferes with OH-LIF detection are localized and omitted from data evaluation. The data is analyzed yielding global scalar fields for comparison with model simulations and correlations between the different measured scalars are investigated showing an almost linear correlation of NO concentration and temperature within the swirl flame whereas no apparent correlation between NO and OH concentration was found. Received: 20 April 2000 / Revised version: 16 May 2000 / Published online: 20 September 2000     

Fluorescence decay times of indigo-derivatives measured by means of a synchroscan streak camera

Employing a synchronously pumped, modelocked dye laser for excitation in connection with a commercial, continuously operated streak camera the solvent dependent fluorescence decaytimes of several indigo-derivatives exhibiting a low fluorescence quantum efficiency were determined with a temporal resolution of about 5 ps in order to further elucidate their energy relaxation mechanisms, which are the object of continuous controversy.     

ps-LIF measurements of minor species concentration in a counterflow diffusion flame interacting with a vortex

Interactions of vortices and flame fronts may be considered as the basic structural elements of turbulent combustion. Additionally, they play an important role in flame instabilities as well as extinction and ignition processes. An ideal geometry to study these interactions is the counterflow diffusion burner with an additional actuator-driven nozzle for the generation of a vortex ring. This burner has already been well-characterized by other methods including CARS, LDA and PLIF. We present first quantitative measurements of minor species concentration in this flame using a short-pulse laser and time- and spatially resolved fluorescence detection with a streak camera. Quench-free OH concentrations are obtained by analysis of the time-resolved profiles. The high power density of the laser pulses allowed linewise detection of hydrogen using a three-photon excitation scheme. Simultaneously, shape and position of the vortex was monitored using two-dimensional detection of flame emissions. Spatially resolved concentration profiles of H and OH are presented for different interaction heights and times in the vortex. For steady flames, comparisons with model calculations are shown. Received: 19 July 2000 / Revised version: 13 December 2000 / Published online: 21 February 2001     

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     

Effective A-state fluorescence lifetime of formaldehyde in atmospheric pressure CH4/air flames

Using excitation pulses of ∼30-ps duration and a fast photomultiplier detector, effective fluorescence lifetimes of the A-stateof formaldehyde after excitation at 355 and 339 nm have been measured in the preheating zone of an atmospheric pressure, premixed methane/air flame. The fluorescence lifetimes were determined as a function of height above the exit of a slot burner and were thus probed in regions of varying gas temperature and composition. The fluorescence lifetimes were independent of the intensity of the excitation pulse and decreased as a function of height in the burner from ∼18±8 ns at 1.2 mm down to 7±1 ns at 3.8 mm. This trend of the effective fluorescence lifetime with composition and temperature in the flame can qualitatively be reproduced using calculated major species mole fractions and species-specific quenching cross sections for CH from the literature. Received: 13 June 2001 / Revised version: 27 September 2001 / Published online: 29 November 2001     

Pressure and composition dependences of acetone laser-induced fluorescence with excitation at 248, 266, and 308 nm

In previous studies, acetone has been successfully applied as a tracer for planar laser-induced fluorescence (PLIF) measurements of concentration and temperature. The desire to extend acetone PLIF capability to conditions of varying pressure and composition has motivated studies of the effects of these quantities on fluorescence yield. The present work explores pressure and composition effects over a 0.5 to 16 atm range for the three excitation wavelengths of greatest interest for diagnostics: 248, 266, and 308 nm. In accord with previous studies, fluorescence per acetone molecule is seen to increase with pressure, apparently towards a high-pressure limit for each wavelength, with the most significant effect observed at short wavelengths. Bath gas composition is also seen to affect fluorescence intensity, with an impact related to the effectiveness of the bath gas species at vibrationally relaxing excited acetone. A model of fluorescence yield considering the relative rates of intersystem crossing and vibrational relaxation for excited singlet acetone describes the measured pressure and composition dependences well. To explain an oxygen fluorescence quenching effect that is observed experimentally, a term is added to the model to represent oxygen-assisted intersystem crossing. The data and model results provide useful guidance for diagnostic applications. A key conclusion is that long excitation wavelengths are preferable from the standpoint of minimizing pressure and composition dependences. Received: 17 December 1998 / Revised version: 21 May 1999 / Published online: 30 July 1999     

Analysis of the local conformation of proteins with two-dimensional fluorescence techniques

Two 2D fluorescence techniques are described which allow the study of conformational changes in proteins in their native form in μM solutions using aromatic amino acids (tryptophan, tyrosine) as intrinsic fluorescence markers. Simultaneous time- and wavelength-resolved fluorescence spectra are measured using a 80 ps laser source in conjunction with streak detection in the exit plane of an astigmatism-corrected spectrometer. This approach allows identification of different photophysical processes by their associated lifetime and spectral intensity distribution; errors due to the more common integration over a wider spectral range are avoided. Time-resolved spectra are sensitive to changes in the collisional environment (dynamic quenching) and can thus be used to monitor local conformation changes close to the respective fluorophors. This is demonstrated for the Ras protein which undergoes a drastic conformation change while binding to different nucleotides. Excitation-emission spectra are two-dimensional fluorescence images with one axis corresponding to the excitation and the other to the emission wavelength. Thus, they contain all conventional excitation and fluorescence spectra of a given substance. The 2D structure facilitates the interpretation of these spectra and allows the direct identification of resonance effects, scattering and the isolation of the contribution of different fluorophors to the complete spectrum. This is demonstrated for mixtures of tyrosine and tryptophan. In this case, both wavelength-resolved spectra and temporal decays are affected by energy transfer processes between the two amino acids. In a last example, both static and time-resolved spectral methods are combined to determine the respective contribution of static and dynamic quenching in calsequestrin. Evaluation of the fluorescence data is in good agreement with a recent crystallographic analysis which shows that all tryptophans are located in a conserved domain of the protein. Addition of Ca²⁺ ions leads to a more compact form of calsequestrin and to polymers. This information would not be obtainable from either of the two techniques alone. Received: 10 February 2000 / Published online: 13 September 2000     

Temperature and excitation wavelength dependencies of 3-pentanone absorption and fluorescence for PLIF applications

The temperature and excitation wavelength dependencies of 3-pentanone absorption and fluorescence were studied in support of planar laser-induced fluorescence (PLIF) imaging of temperature and mixture fraction in flows of practical interest. The temperature dependencies (300–875 K) of absorption and fluorescence were measured for gaseous 3-pentanoneat atmospheric pressure in a nitrogen bath gas using 248, 266, and 308 nm excitation. The results indicate that the fluorescence signal per unit mole fraction using 248 nm excitation is highly temperature-sensitive below 600 K, while the signal from 308 nm excitation is not temperature sensitive below 500 K. For quantitative measurements over a broad range of temperatures, one must choose excitation schemes carefully to balance the trade-off between measurement sensitivity and the amount of signal at the expected conditions. As an example of such a choice and to show the capabilities of ketone PLIF techniques, we include temperature and mixture fraction images of a 300–650 K heated air jet using near-simultaneous 308 and 266 nm excitation. Received: 29 May 2002 / Revised version: 5 November 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +1-650/723-1748, E-mail: jkoch@stanford.edu RID="**" ID="**"E-mail: hanson@me.stanford.edu     

Electromagnetic interaction of fluorophores with designed two-dimensional silver nanoparticle arrays

We study the fluorescence enhancement of dye molecules adsorbed on regular two-dimensional arrays of designed silver nanoparticles. The silver particles show two orthogonal optical resonances at different wavelengths because of their elongated shape. The short-wavelength resonance was designed to fit to the absorption maximum of the fluorophore. When the excitation light drives the short-wavelength resonance, the measured fluorescence intensity is strongly enhanced compared to that for the orthogonal particle orientation. This shows directly a strong electromagnetic coupling between the nanoparticles and the fluorophore. Additionally enhanced photochemical bleaching is observed due to the interaction of fluorophores with the particles. Using a rate model describing the fluorescence enhancement and the bleaching enhancement, an average value for the particle-induced increase in the radiative fluorescence rate is obtained, together with a lower limit for the averaged particle-induced field intensity enhancement factor. Received: 3 July 2001 / Revised version: 3 September 2001 / Published online: 15 October 2001     

Dynamics of photon-induced degradation and fluorescence in riboflavin microparticles

An unexpected blue-fluorescence band (around 420 nm) from both micrometer-sized dried particles and aqueous droplets of riboflavin [7,8-dimethyl-10-(D-1^′-ribityl)-isoalloxazine] is observed when the microparticles are irradiated with a pulsed UV (355- or 351-nm) laser. The intensity of the band increases quadratically with input laser energy density (fluence) and is attributable to a one-photon-excited fluorescence of lumichrome (7,8-dimethyl-alloxazine) that is produced by photo-degradation of riboflavin. The well-known greenish-yellow fluorescence band (at 560 nm for dried particles and 535 nm for aqueous droplets) from riboflavin increases sublinearly with UV-laser fluence. With a laser input fluence above 5 J/cm² the riboflavin fluorescence decays earlier and the lumichrome fluorescence reaches a maximum later than the peak of the input laser pulse. The temporal dynamics of the 420- and 535-nm fluorescence peaks are consistent with a rate-equation simulation of photon-induced conversion of riboflavin to lumichrome and the subsequent fluorescence of lumichrome. Received: 28 September 2000 / Revised version: 11 December 2000 / Published online: 21 February 2001     

Quantitative measurements of the CH radical in sooting diffusion flames at atmospheric pressure

The potential of Laser Induced Fluorescence detection of the CH radical using C–X (0–0) excitation is investigated in a sooting methane/air diffusion flame at atmospheric pressure. Fluorescence is detected using the very narrow (<0.4 nm) Q-branch of the C–X (0–0) band, which enables the measurement of CH in sooting flames without interference from PAH fluorescence and soot emissions. Absolute concentrations are obtained using Cavity Ring Down Spectroscopy. 1D CH profiles in the sooting zone are recorded using a CCD camera with an excellent signal-to-noise ratio. The C–X (0–0) excitation associated with Q-branch detection is shown to be three times more efficient than the B–X scheme. Received: 4 March 2002 / Revised version: 5 November 2002 / Published online: 5 May 2003 RID="*" ID="*"Corresponding author. Fax: +33-3/2033-6463, E-mail: eric.therssen@univ-lille1.fr     

Fluorescence-lifetime measurements in atmospheric-pressure flames using nanosecond-pulsed lasers

Measurements of fluorescence lifetimes are needed to quantify concentration measurements when using linear laser-induced fluorescence. However, lifetimes are only a few nanoseconds for many important species at atmospheric pressure. When using a typical Q-switched laser with a pulse width of about 10 ns, the fluorescence follows the shape of the laser pulse and the lifetime cannot be easily measured. In this paper, a technique is described for experimentally determining the fluorescence lifetime in atmospheric-pressure flames using a nanosecond-pulsed laser; that is, measurement of a lifetime an order-of-magnitude faster than the laser pulse itself. This technique relies on an observable temporal shift in the fluorescence signal as a function of the lifetime. Simulations show the efficacy of this approach, and data in liquid samples and in an atmospheric-pressure flame show excellent agreement with prior picosecond measurements. This technique is successful because only the temporal shift is examined and details of the fluorescence profile are ignored. Received: 23 August 2001 / Revised version: 19 November 2001 / Published online: 17 January 2002     

MeV Al+ and Al2+ ions implantation in Si(100): surface roughness and defects in the bulk

This paper deals with the implantation of high-energy (1.0–3.0 MeV) atomic and molecular Al⁺ ions in Si(100) to a fluence of 5×10¹⁴ Al atoms/cm² at room temperature. The molecular effect, i.e. the increase of the displacement yield compared with the sum of the atomic yields, and the damage formation as well as defect behaviour after annealing have been investigated. A detailed experimental study has been made of the evolution of extended secondary defects which form during thermal anneals of Al⁺ or Al₂ ⁺ irradiated silicon. The samples have been examined using combined Rutherford backscattering and channeling experiments together with transmission electron microscopy observations. The surface structure of the implanted wafers has been measured by atomic force microscopy. The results for the implantation-induced roughness at the Si surface, resulting from Al⁺ or Al₂ ⁺ irradiation at the same energy/atom, total atomic fluence, flux rate, and irradiation temperature, are presented and discussed. Received: 19 August 1999 / Accepted: 20 October 1999 / Published online: 23 February 2000