Photofragmentation Laser-induced Fluorescence Research Articles

Quantifying the thermal effect and methyl radical production in nanosecond repetitively pulsed glow discharges applied to a methane-air flame

In this work, we investigated non-equilibrium plasma produced by nanosecond repetitively pulsed glow discharges applied across a lean premixed methane-air flame. The flame is stationary, axisymmetric, and laminar. The discharges are applied on the symmetry axis crossing the reactant gases, flame front, and product gases, allowing phase-locked averaged measurements and comparisons with axisymmetric numerical simulations. The thermal effect and methyl radical production are quantified in the discharge in the reactant gas region. One-dimensional, two-beam, hybrid, femtosecond-picosecond, coherent anti-Stokes Raman scattering is used to acquire spatial and temporal profiles of temperature and oxygen-to-nitrogen concentration ratio. Photo-fragmentation laser-induced fluorescence is used to acquire quantitative two-dimensional profiles of methyl radicals in the discharge providing the first quantitative imaging of methyl produced ahead of a flame by plasma-induced methane dissociation. The spatial profiles of temperature and oxygen-to-nitrogen concentration ratio are in steady state, indicating that individual discharges have an insignificant heating effect. Upper and lower bounds of the produced mole fraction of methyl radicals in the plasma are obtained due to uncertainties in the collisional quenching rates of excited state methylidyne radicals in the plasma. The discharges produce a maximum of 600–1100 ppm of methyl radicals upstream of the flame front within 25 ns. This amount is similar to the predicted methyl mole fraction for the flame without plasma and thus represents a significant chemical perturbation to the reactants upstream of the flame front. The produced methyl follows an exponential decay in the first microsecond after the discharge with a decay constant of 8 µs close to the flame, and 0.8 µs further from the flame. The decay then deviates from the exponential curve and the methyl persists for tens of microseconds. The results suggest that for the tested configuration, the thermal effect of individual discharges through fast gas heating is negligible, while active chemical species are produced in large quantities in the reactant gases, upstream of the flame front.

2D-imaging of absolute OH and H2O2 profiles in a He–H2O nanosecond pulsed dielectric barrier discharge by photo-fragmentation laser-induced fluorescence

Pulsed dielectric barrier discharges (DBD) in He–H2O and He–H2O–O2 mixtures are studied in near atmospheric conditions using temporally and spatially resolved quantitative 2D imaging of the hydroxyl radical (OH) and hydrogen peroxide (H2O2). The primary goal was to detect and quantify the production of these strongly oxidative species in water-laden helium discharges in a DBD jet configuration, which is of interest for biomedical applications such as disinfection of surfaces and treatment of biological samples. Hydroxyl profiles are obtained by laser-induced fluorescence (LIF) measurements using 282 nm laser excitation. Hydrogen peroxide profiles are measured by photo-fragmentation LIF (PF-LIF), which involves photo-dissociating H2O2 into OH with a 212.8 nm laser sheet and detecting the OH fragments by LIF. The H2O2 profiles are calibrated by measuring PF-LIF profiles in a reference mixture of He seeded with a known amount of H2O2. OH profiles are calibrated by measuring OH-radical decay times and comparing these with predictions from a chemical kinetics model. Two different burst discharge modes with five and ten pulses per burst are studied, both with a burst repetition rate of 50 Hz. In both cases, dynamics of OH and H2O2 distributions in the afterglow of the discharge are investigated. Gas temperatures determined from the OH-LIF spectra indicate that gas heating due to the plasma is insignificant. The addition of 5% O2 in the He admixture decreases the OH densities and increases the H2O2 densities. The increased coupled energy in the ten-pulse discharge increases OH and H2O2 mole fractions, except for the H2O2 in the He–H2O–O2 mixture which is relatively insensitive to the additional pulses.

Flame front visualization in highly turbulent jet flames using CH3 photofragmentation laser-induced fluorescence

Flame front visualization using methyl photofragmentation laser-induced fluorescence (CH3-PF-LIF) was demonstrated in turbulent premixed methane/air flames. A pump–probe method was used to detect CH3, where CH3 was first photolyzed to CH (X2П) fragments using a pump laser (212.8 nm), and the fragments were subsequently excited to CH in the C2Σ+ state by a probe laser at 314.4 nm. By detecting fluorescence from CH (C-X), instantaneous two-dimensional flame front imaging with a high signal-to-noise (SNR) ratio was achieved in fuel-lean and turbulent flames. A laser excitation scan was conducted to ensure that the detection of CH3 did not interfere with the other emissions. Visualization using CH3-PF-LIF was compared with visualization with conventional CH planar laser-induced fluorescence (CH-PLIF). The SNR of CH3-PF-LIF imaging is almost ten times higher than those of CH-PLIF, and flame front visualization using CH3-PF-LIF shows more topological structures than CH-PLIF in fuel-lean and highly turbulent flames.

2D imaging of absolute methyl concentrations in nanosecond pulsed plasma by photo-fragmentation laser-induced fluorescence

The methyl radical plays a central role in plasma-assisted hydrocarbon chemistry but is challenging to detect due to its high reactivity and strongly pre-dissociative electronically excited states. We report the development of a photo-fragmentation laser-induced fluorescence (PF-LIF) diagnostic for quantitative 2D imaging of methyl profiles in a plasma. This technique provides temporally and spatially resolved measurements of local methyl distributions, including in near-surface regions that are important for plasma-surface interactions such as plasma-assisted catalysis. The technique relies on photo-dissociation of methyl by the fifth harmonic of a Nd:YAG laser at 212.8 nm to produce CH fragments. These photofragments are then detected with LIF imaging by exciting a transition in the B-X(0, 0) band of CH with a second laser at 390 nm. Fluorescence from the overlapping A-X(0, 0), A-X(1, 1), and B-X(0, 1) bands of CH is detected near 430 nm with the A-state populated by collisional B-A electronic energy transfer. This non-resonant detection scheme enables interrogation close to a surface. The PF-LIF diagnostic is calibrated by producing a known amount of methyl through photo-dissociation of acetone vapor in a calibration gas mixture. We demonstrate PF-LIF imaging of methyl production in methane-containing nanosecond pulsed plasmas impinging on dielectric surfaces. Absolute calibration of the diagnostic is demonstrated in a diffuse, plane-to-plane discharge. Measured profiles show a relatively uniform distribution of up to 30 ppm of methyl. Relative methyl measurements in a filamentary plane-to-plane discharge and a plasma jet reveal highly localized intense production of methyl. The utility of the PF-LIF technique is further demonstrated by combining methyl measurements with formaldehyde LIF imaging to capture spatiotemporal correlations between methyl and formaldehyde, which is an important intermediate species in plasma-assisted oxidative coupling of methane.

Investigation of the chemical effect of pilot injection on main combustion in a gasoline controlled auto-ignition engine by in-cylinder measurements and numerical simulation of H2O2, HO2, and OH radicals

Compared to conventional combustion in gasoline engines, Gasoline Controlled Auto-Ignition (GCAI) has the potential to significantly reduce both fuel consumption and nitrogen oxides (NOX) raw emissions. However, GCAI combustion still has a limited operating range due to limitations in combustion stability at low loads and high rates of cylinder pressure rise at high engine loads. Previous research identified hydrogen peroxide (H2O2) decomposition as an effective means to control GCAI, which is formed among other species with pilot fuel injection during the negative valve overlap (NVO) phase. Since the chemical effect of H2O2 on the main combustion has not been experimentally confirmed so far, the aim of the experimental study described in this paper is to detect H2O2 during early compression using optical methods. This is approached by conducting Photofragmentation Laser-Induced Fluorescence (PFLIF) of H2O2 by a pump laser and simultaneous hydroxyl radical Laser-Induced Fluorescence (OH-LIF) detection in the combustion chamber of a dedicated GCAI single-cylinder research engine at low loads after NVO with pilot injection (PI). Thereto, the pilot injection strategy as well as the global air/fuel ratio (AFR) are varied. Interestingly, no H2O2, hydroxyl (OH) or hydroperoxyl (HO2) constituents were detected under the operating conditions studied, despite relatively high pilot injection fuel masses and a highly accurate H2O2 detection limit of about 1 ppm. Numerical three-dimensional computational fluid dynamics (3D-CFD) combustion simulations using a semi-detailed reaction kinetic mechanism predicted 5–8 times higher H2O2 concentrations than the detection limit, yielding an uncertainty estimate of the numerical methods. Furthermore, the 3D-CFD simulations showed a significant mixture stratification which is a possible reason for the lack of H2O2 detection under these operating conditions.

Simultaneous [formula omitted] and [formula omitted] imaging in premixed turbulent flames for instantaneous flame structure visualization

Photofragmentation laser-induced fluorescence (PF-LIF) of methyl radical (CH3) and planar laser-induced fluorescence (PLIF) of formaldehyde (CH2O) were simultaneously carried out in premixed methane-air turbulent flames to visualize flame structure and extract quantitative features of CH3 and CH2O layers. A CH3 layer is located along the edge of a CH2O layer, and they partially overlap with each other. CH3 is closer to the reaction zone of a premixed flame compared with CH2O that is regarded as a typical marker of the preheating zone. The results show that the CH3 can be served as a flame marker for the transitional zone between the preheating zone and the reaction zone. The thicknesses of CH3 and CH2O layers and their distances were then quantitatively estimated to reduce the influence of layer merging and strong curvature. It was found that the broadening of CH2O layers against turbulence is more sensitive than that of CH3 layers. In stoichiometric and rich flames, layer broadening was clearly observed even with a slightly increased Karlovitz number.

Production of HONO from NO<sub>2</sub> uptake on illuminated TiO<sub>2</sub> aerosol particles and following the illumination of mixed TiO<sub>2</sub>∕ammonium nitrate particles

Abstract. The rate of production of HONO from illuminated TiO2 aerosols in the presence of NO2 was measured using an aerosol flow tube system coupled to a photo-fragmentation laser-induced fluorescence detection apparatus. The reactive uptake coefficient of NO2 to form HONO, γNO2→HONO, was determined for NO2 mixing ratios in the range 34–400 ppb, with γNO2→HONO spanning the range (9.97 ± 3.52) × 10−6 to (1.26 ± 0.17) × 10−4 at a relative humidity of 15 ± 1 % and for a lamp photon flux of (1.63 ± 0.09) ×1016 photons cm−2 s−1 (integrated between 290 and 400 nm), which is similar to midday ambient actinic flux values. γNO2→HONO increased as a function of NO2 mixing ratio at low NO2 before peaking at (1.26 ± 0.17) ×10-4 at ∼ 51 ppb NO2 and then sharply decreasing at higher NO2 mixing ratios rather than levelling off, which would be indicative of surface saturation. The dependence of HONO production on relative humidity was also investigated, with a peak in production of HONO from TiO2 aerosol surfaces found at ∼ 25 % RH. Possible mechanisms consistent with the observed trends in both the HONO production and reactive uptake coefficient were investigated using a zero-dimensional kinetic box model. The modelling studies supported a mechanism for HONO production on the aerosol surface involving two molecules of NO2, as well as a surface HONO loss mechanism which is dependent upon NO2. In a separate experiment, significant production of HONO was observed from illumination of mixed nitrate/TiO2 aerosols in the absence of NO2. However, no production of HONO was seen from the illumination of nitrate aerosols alone. The rate of production of HONO observed from mixed nitrate/TiO2 aerosols was scaled to ambient conditions found at the Cape Verde Atmospheric Observatory (CVAO) in the remote tropical marine boundary layer. The rate of HONO production from aerosol particulate nitrate photolysis containing a photocatalyst was found to be similar to the missing HONO production rate necessary to reproduce observed concentrations of HONO at CVAO. These results provide evidence that particulate nitrate photolysis may have a significant impact on the production of HONO and hence NOx in the marine boundary layer where mixed aerosols containing nitrate and a photocatalytic species such as TiO2, as found in dust, are present.

Quantitative feature extraction of turbulent premixed flames by photofragmentation laser-induced fluorescence

Planar laser-induced fluorescence (PLIF) techniques have been widely used for flame structure visualization in recent years. Nevertheless, most of the studies were concerned with qualitative analysis of the obtained images and the quantitative analysis were much fewer. This work presents a quantitative feature extraction method to process the ribbon structures that are captured in images and represent the radical layers in flames. The extracted features include ridge lines, curvatures, and thicknesses. This method was demonstrated by processing the images of methyl (CH3) photofragmentation laser-induced fluorescence (PF-LIF) and formaldehyde (CH2O) PLIF measurements in stoichiometric, premixed methane/air flames. The flames were generated by a McKenna burner under the Reynolds numbers of 1930, 3870, and 5800. The ridge lines followed well with the wrinkled layers of CH3 and CH2O. The CH2O layers were thickened, but the CH3 layers had minor change with increased Reynold numbers.

Photofragmentation laser-induced fluorescence of ozone: an in situ tool for precise mapping of ozone concentration in non-thermal plasmas

The absolute spatially resolved concentration of ozone has been determined in the near afterglow of a novel type of atmospheric multi-hollow barrier discharge using photofragmentation laser-induced fluorescence. The method consists of two steps: (i) photodissociation of O3 to O(3P) and O2(X3Σg−) with considerable vibrational excitation; (ii) predissociation laser-induced fluorescence of the molecular fragment via the O2 (B 3Σu−, v′ = 0 ← X 3Σg−, v″ = 6) transition. Both processes happen during the same laser pulse, requiring the same excitation wavelength (≈248 nm). The fluorescence signal has been calibrated by infrared absorption. The method allows in situ observation before the afterglow chemistry significantly affects the O3 concentration, and is capable of revealing inhomogeneous ozone distributions in the reactor.

Photofragmentation laser-induced fluorescence of ozone using narrowband and broadband KrF excimer lasers

Photofragmentation laser-induced fluorescence (PF-LIF) is used for the high-sensitivity measurement of ozone density; however, its theoretical basis is not well established. In this paper, O3 PF-LIF, using a tunable KrF excimer laser (248–249 nm), was examined using both measurements and calculations to provide a theoretical basis for this measurement method. The KrF laser photolyzed O3 to produce O, which was then measured using LIF with the same laser pulse that had photolyzed the ozone. A calculation including photochemical reactions and a LIF model of O reproduced the measured results well which demonstrated the reliability of the calculation. A recommended laser energy density of 100–400 mJ cm−2 was obtained by considering the depletion of ozone by the laser photolysis and the saturation of the LIF. It was also demonstrated that a broadband KrF excimer laser, which is not line narrowed and therefore not tunable, could also be used for O3 PF-LIF. The signal intensity of the broadband PF-LIF was only reduced to 1/2–1/3 of the narrowband PF-LIF.

Instantaneous imaging of ozone in a gliding arc discharge using photofragmentation laser-induced fluorescence

Ozone vapor, O3, is here visualized in a gliding arc discharge using photofragmentation laser-induced fluorescence. Ozone is imaged by first photodissociating the O3 molecule into an O radical and a vibrationally hot O2 fragment by a pump photon. Thereafter, the vibrationally excited O2 molecule absorbs a second (probe) photon that further transits the O2-molecule to an excited electronic state, and hence, fluorescence from the deexcitation process in the molecule can be detected. Both the photodissociation and excitation processes are achieved within one 248 nm KrF excimer laser pulse that is formed into a laser sheet and the fluorescence is imaged using an intensified CCD camera. The laser-induced signal in the vicinity of the plasma column formed by the gliding arc is confirmed to stem from O3 rather than plasma produced vibrationally hot O2. While both these products can be produced in plasmas a second laser pulse at 266 nm was utilized to separate the pump- from the probe-processes. Such arrangement allowed lifetime studies of vibrationally hot O2, which under these conditions were several orders of magnitude shorter than the lifetime of plasma-produced ozone.

Simultaneous Visualization of Hydrogen Peroxide and Water Concentrations Using Photofragmentation Laser-Induced Fluorescence

A concept based on photofragmentation laser-induced fluorescence (PFLIF) is for the first time demonstrated for simultaneous detection of hydrogen peroxide (H2O2) and water (H2O) vapor in various mixtures containing the two constituents in a bath of argon gas. A photolysis laser pulse at 248 nm dissociates H2O2 into OH fragments, whereupon a probe pulse, delayed 100 ns and tuned to an absorption line in the A2Σ+ (v = 1) ← X2Π(v = 0) band of OH near 282 nm, induces fluorescence. The total OH fluorescence reflects the H2O2 concentration, while its spectral shape is utilized to determine the H2O concentration via a model predicting the ratio between the fluorescence intensities of the A2Σ+ (v = 1) → X2Π(v = 1) and the A2Σ+ (v = 0) → X2Π(v = 0) bands. The H2O detection scheme requires that the bath gas has a collisional cross-section with OH(A) that is significantly lower than that of H2O, which is the case for argon. Spectrally dispersed OH fluorescence spectra were recorded for five different H2O2/H2O/Ar mixtures; the H2O2 concentration in the range of 30-500 ppm and the H2O concentration in the range of 0-3%. Fluorescence intensity ratios predicted by the model for these mixtures agree very well with corresponding experimental data, which thus validates the model. The concept was also demonstrated for two-dimensional imaging, using two intensified charge-coupled device (CCD) cameras for signal detection. Water content was here sensed through the different temporal characteristics of the two fluorescence bands by triggering the two cameras so that one captures the total OH fluorescence while the other one captures only the early part, which mainly stems from A2Σ+ (v = 1) → X2Π(v = 1) fluorescence. Hence, the H2O2 concentration is reflected by the image of the camera recording the total OH fluorescence, whereas H2O concentration is extracted from the ratio between the two camera images. Quantification of the concentrations was carried out based on calibration measurements performed in known mixtures of H2O2 (30-500 ppm) and H2O (0-3%) in bulk argon. The detection limits for single-shot imaging are estimated to be 20 ppm for H2O2 and 0.05% for H2O. The authors believe that the concept provides a valuable asset in, for example, pharmaceutical or aseptic food packaging applications, where H2O2/H2O vapor is routinely used for sterilization.

Quantitative Imaging of Ozone Vapor Using Photofragmentation Laser-Induced Fluorescence (LIF).

In the present work, the spectral properties of gaseous ozone (O3) have been investigated aiming to perform quantitative concentration imaging of ozone by using a single laser pulse at 248 nm from a KrF excimer laser. The O3 molecule is first photodissociated by the laser pulse into two fragments, O and O2. Then the same laser pulse electronically excites the O2 fragment, which is vibrationally hot, whereupon fluorescence is emitted. The fluorescence intensity is found to be proportional to the concentration of ozone. Both emission and absorption characteristics have been investigated, as well as how the laser fluence affects the fluorescence signal. Quantitative ozone imaging data have been achieved based on calibration measurements in known mixtures of O3. In addition, a simultaneous study of the emission intensity captured by an intensified charge-coupled device (ICCD) camera and a spectrograph has been performed. The results show that any signal contribution not stemming from ozone is negligible compared to the strong fluorescence induced by the O2 fragment, thus proving interference-free ozone imaging. The single-shot detection limit has been estimated to ∼400 ppm. The authors believe that the presented technique offers a valuable tool applicable in various research fields, such as plasma sterilization, water and soil remediation, and plasma-assisted combustion.

Strategy for single-shot CH3 imaging in premixed methane/air flames using photofragmentation laser-induced fluorescence

Single-shot imaging of methyl radical (CH3) in premixed methane/air flames is demonstrated using photofragmentation laser-induced fluorescence (PF-LIF) technique. A pump-probe strategy was adopted with the pump laser at 212.8 nm photolyzing CH3, and with the probe laser at 426.8 nm detecting the photolyzed CH (X2Π) fragments. Spatially resolved spectrograph of the PF-LIF signal from a stable laminar flame was recorded across the reaction zone to investigate potential interferences. The results indicate that the single-photon channel, CH3 + 212.8 nm → CH (X2Π) + H2, dominates the photofragmentation process. The CH2 radical was excluded from being an interfering precursor of the CH (X2Π) fragments owing to its relatively low concentration and small absorption cross section. Naturally present CH in the flame was identified as the main interference, but was conservatively estimated to account for only less than 4% of the total PF-LIF signal. Signal-to-noise ratio of around 10 was realized for single-shot imaging of natural CH3 in turbulent jet flames.

Investigation of photochemical effects in flame diagnostics with picosecond photofragmentation laser-induced fluorescence

Photofragmentation laser-induced fluorescence (PFLIF) is for the first time performed based on picosecond laser pulses for detection of hydroperoxyl radicals (HO2) in a stoichiometric laminar methane/air flame. Photofragmentation is performed with a pump laser pulse of 80ps duration and a wavelength of 266nm, whereupon the produced OH photofragments are detected by a second picosecond probe laser pulse, inducing fluorescence via excitation in the A2Σ+(v=1) ← X2Π(v=0) band of OH near 283nm. Excitation spectra of the OH photofragments formed in the reaction zone were recorded for pump-probe delays ranging from 0 to 5ns. The spectra suggest that the population distribution of the nascent OH fragments is rotationally cold and that it takes on the order of 5ns for the nascent non-equilibrium rotational distribution to relax into a thermal distribution. The radial OH-fragment distribution was extracted from spectral images (radial position versus emission wavelength) recorded at six different pump-probe delays. Photochemical OH production was observed both in the reaction zone and the product zone. Comparison with a kinetic model for OH production suggests that more than 20% of the oxygen fragments produced by photolysis in the reaction zone are formed in the excited 1D state, explaining a very rapid initial signal growth. The OH-production model was also compared with previous reaction-zone data, acquired with nanosecond laser pulses in the same flame, indicating that no O(1D), but only O(3P), is formed. A plausible explanation of the discrepancy between the two results is that the picosecond pulses, having more than two-orders of magnitude higher irradiance than the nanosecond pulses used in the previous study, might cause 2-photon photodissociation, allowing production of O(1D). In terms of flame diagnostics with PFLIF, it is concluded that a setup based on nanosecond laser pulses, rather than picosecond pulses, appears preferable since photochemical OH production in the reaction zone can be avoided while for short delay times the ratio between the photofragment signal and the photochemical interference in the product zone, stemming from CO2 photolysis, is sufficiently large to clearly visualize the photofragments.

Two-dimensional OH-thermometry in reacting flows using photofragmentation laser-induced florescence

In the present work, the possibility of using photofragmentation laser-induced fluorescence (PFLIF) for thermometry in different reacting flows is investigated. Hydroxyl (OH) fragments are created by UV-laser (266nm) fragmentation of hydrogen peroxides (H2O2 and HO2), whereupon the fluorescence, induced while scanning the wavelength of a second laser across the A2Σ+(v = 1)–X2Π(v = 0) absorption band (around 282nm) of the generated OH fragments, is collected and detected. The temperature is determined by fitting simulated OH-excitation spectra of different temperatures to the experimentally recorded spectrum. In combustion, hydrogen peroxides are intermediate species formed during the low-temperature oxidation of the fuel, and hence they are present in a region covering a wide temperature span, ranging from unburnt to burnt gas temperatures. Thus, LIF of OH photofragments stemming from hydrogen peroxides allows for thermometry covering a wider temperature range than LIF of naturally present OH radicals. There is another important advantage of the concept in that the temperature sensitivity of OH excitation spectra is greater at lower temperatures. The method is demonstrated for two-dimensional (2-D) thermometry in three different measurement situations, namely a free flow of vaporized H2O2 at room temperature, a preheated mixture of CH4/N2/O2/O3 at intermediate temperatures (300–600K), where the OH fragments stem from photodissociation of O3 followed by chemical reactions, and in an optical homogeneous charge compression ignition (HCCI) engine prior to ignition, i.e. at elevated pressures and temperatures. It is found that the technique performs well in all three cases, with measured temperatures in good agreement with thermocouple readings, for the two first cases, and with temperatures calculated based on the ideal gas law using measured pressure traces as input for the engine measurements. The quantitative 2-D temperature images acquired in the engine experiments reveal inhomogeneous temperature distributions, clearly illustrating the capacity of the technique to yield crucial experimental input to engine modelers and designers. The accuracy of the technique in the temperature range 300–600K is lower than 23K. For the room temperature case the precision is 4.3%, corresponding to 12K.

Analysis of in-cylinder H2O2 and HO2 distributions in an HCCI engine – Comparison of laser-diagnostic results with CFD and SRM simulations

By applying a novel approach to evaluate photofragmentation laser-induced fluorescence (PFLIF) imaging, experimental quantitative information on the temporal in-cylinder distribution of hydrogen peroxide (H2O2) in a homogeneous charge compression ignition (HCCI) engine was extracted. The results from PFLIF were then compared to those obtained from chemical kinetics simulations using computational fluid dynamics (CFD) and a stochastic reactor model (SRM). For the CFD simulations, a sector mesh was applied using Reynolds-averaged Navier–Stokes (RANS) equations together with a reduced chemical kinetic model. These simulations provided detailed information on the spatial distribution of H2O2, HO2 as well as other important species and temperature. The SRM, which offers substantially reduced simulation times but no spatial information, was used with the same reduced kinetic model. Two-dimensional images from PFLIF and CFD show a fair temporal agreement, while details of the spatial distributions disagree. The CFD images show that the combustion chemistry is affected by the interaction with the cylinder walls with, for instance, a local delay of the formation and consumption of H2O2. By using probability density functions (PDFs) of H2O2 and HO2 mass fractions, comparisons could be made between experimental data and both the CFD and SRM simulations. In general the range of mass fractions show good agreement but the experimental distributions are wider. Possible reasons for this discrepancy are actual heterogeneities in the H2O2/HO2 concentration distributions not predicted by the model, spatial temperature variations, which will influence the strength of the PFLIF signal, spatial variations in the laser profiles, not accounted for in the data processing, and photon noise. The good agreement between the CFD and SRM shows the relevance of fast PDF based simulation tools.

Simultaneous Visualization of Water and Hydrogen Peroxide Vapor Using Two-Photon Laser-Induced Fluorescence and Photofragmentation Laser-Induced Fluorescence

A concept based on a combination of photofragmentation laser-induced fluorescence (PF-LIF) and two-photon laser-induced fluorescence (LIF) is for the first time demonstrated for simultaneous detection of hydrogen peroxide (H2O2) and water (H2O) vapor. Water detection is based on two-photon excitation by an injection-locked krypton fluoride (KrF) excimer laser (248.28 nm), which induces broadband fluorescence (400-500 nm) from water. The same laser simultaneously photodissociates H2O2, whereupon the generated OH fragments are probed by LIF after a time delay of typically 50 ns, by a frequency-doubled dye laser (281.91 nm). Experiments in six different H2O2/H2O mixtures of known compositions show that both signals are linearly dependent on respective species concentration. For the H2O2 detection there is a minor interfering signal contribution from OH fragments created by two-photon photodissociation of H2O. Since the PF-LIF signal yield from H2O2 is found to be at least ∼24,000 times higher than the PF-LIF signal yield from H2O at room temperature, this interference is negligible for most H2O/H2O2 mixtures of practical interest. Simultaneous single-shot imaging of both species was demonstrated in a slightly turbulent flow. For single-shot imaging the minimum detectable H2O2 and H2O concentration is 10 ppm and 0.5%, respectively. The proposed measurement concept could be a valuable asset in several areas, for example, in atmospheric and combustion science and research on vapor-phase H2O2 sterilization in the pharmaceutical and aseptic food-packaging industries.

Quantitative detection of hydrogen peroxide in an HCCI engine using photofragmentation laser-induced fluorescence

A photofragmentation laser-induced fluorescence (PF-LIF) technique was applied to an HCCI engine for hydrogen peroxide (H2O2) measurements. A pump–probe strategy was adopted with the pump laser (266nm) photolyzing H2O2, and with the probe laser (283nm) detecting the generated OH photofragments. An on-line calibration process was performed by introducing a known amount of vaporized H2O2/water solution into the engine cylinder. Crank-angle resolved mass fractions of H2O2 were obtained, and for the first time, single-shot imaging of H2O2 was realized in the HCCI engine. It has also been verified that the PF-LIF signal originates mainly from H2O2 with a smaller interference from HO2. The crank-angle resolved experimental data were compared to mass fractions calculated with the software package Digital Analysis of Reaction Systems (DARS). The calculated H2O2 profile agrees well with the experimental results regarding mass fraction level, while the shapes of the profiles deviate slightly. Calculated mass fractions of HO2 indicate that interfering signal contributions from HO2 is the major reason for the deviation.

Photofragmentation laser-induced fluorescence imaging in premixed flames

Two-dimensional measurements of primarily hydroperoxyl radicals (HO 2) are, for the first time, demonstrated in flames. The measurements are performed in different Bunsen-type premixed flames (H 2/O 2, CH 4/O 2, and CH 4/air) using photofragmentation laser-induced fluorescence (PF-LIF). Photofragmentation is done by laser radiation at 266 nm, and the generated OH photofragments are probed through fluorescence induced by a laser tuned to the Q 1(5) transition at 282.75 nm. The signal due to naturally occurring OH radicals, recorded by having the photolysis laser blocked, is subtracted, providing an image that reflects the concentration of OH fragments generated by photolysis, and hence the presence of primarily HO 2, but also smaller contributions from H 2O 2 and, for the methane flames, CH 3O 2. For the methane flames the measured radial profiles of OH photofragments and natural OH agree well with corresponding profiles calculated for laminar, one-dimensional, premixed flames using CHEMKIN-II with the Konnov detailed C/H/N/O reaction mechanism. An interfering signal contribution is observed in the product zone of the methane flames. It is concluded that the major source for the interference is most likely hot CO 2, from which O atoms are produced by photolysis, and OH is rapidly formed as the O atoms react with H 2O and H 2. This conclusion is supported by the fact that the interference is absent for the hydrogen flame, but appears when CO 2 is seeded into the flame. Another strong indication is that the Konnov mechanism predicts a similar buildup of OH after photolysis.