NO Laser-induced Fluorescence Research Articles

How well do we know thermal-NO?An investigation of NO formation in flames over a wide temperature range

This study investigates the large variability and uncertainty in the thermal-initiation reaction rates found in the literature. An experimental study is conducted at atmospheric pressure in lean, premixed, laminar methane-air stagnation flames. Flame temperatures ranging from 1900K to 2500K are achieved by varying the argon concentration, in 21% and 40% oxygen mixtures balanced with nitrogen. The conditions are selected to promote the thermal route in the overall post-flame NO formation. One-dimensional velocity, temperature, and NO concentration profiles are measured with Particle Tracking Velocimetry (PTV), NO multi-line thermometry, and NO Laser-Induced Fluorescence (NO-LIF), respectively. While velocity and temperature measurements are accurately predicted by different thermochemical models, the simulated NO-LIF signal profiles show significant disagreement and large variability, relative to the measurements. Results show that, across all conditions, none of the studied mechanisms are able to capture accurately both the NO concentration, and formation rate in the post-flame region, suggesting that the fundamental chemistry remains inaccurate. The discrepancy in the predictions appears to be linked to the chosen parameters of the Arrhenius rate, specifically the pre-exponential factor, and the activation energy. Sensitivity and Reaction Pathway Analyses suggest that the differences in the Arrhenius parameters could originate from different consideration of the base radical chemistry, as well as different relative contributions of the four NO-formation routes. As a result, some models can predict similar NO concentrations but using significantly different underlying base and NOx chemistry. This implies that the models could diverge significantly in conditions where other non-thermal routes are more important. This study demonstrates the need for spatially-resolved experimental data across a broad range of experimental conditions, promoting and considering a variety of routes, in order to optimize NOx chemistry models with reduced uncertainty.

Experimental and modeling study of the high-temperature combustion chemistry of tetrahydrofurfuryl alcohol

Lignocellulosic tetrahydrofuranic (THF) biofuels have been identified as promising fuel candidates for spark-ignition (SI) engines. To support the potential use as transportation biofuels, fundamental studies of their combustion and emission behavior are highly important. In the present study, the high-temperature (HT) combustion chemistry of tetrahydrofurfuryl alcohol (THFA), a THF based biofuel, was investigated using a comprehensive experimental and numerical approach.Representative chemical species profiles in a stoichiometric premixed methane flame doped with ∼20% (molar) THFA at 5.3 kPa were measured using online gas chromatography. The flame temperature was obtained by NO laser-induced fluorescence (LIF) thermometry. More than 40 chemical products were identified and quantified. Many of them such as ethylene, formaldehyde, acrolein, allyl alcohol, 2,3-dihydrofuran, 3,4-dihydropyran, 4-pentenal, and tetrahydrofuran-2-carbaldehyde are fuel-specific decomposition products formed in rather high concentrations. In the numerical part, as a complement to kinetic modeling, high-level theoretical calculations were performed to identify plausible reaction pathways that lead to the observed products. Furthermore, the rate coefficients of important reactions and the thermochemical properties of the related species were calculated. A detailed kinetic model for high-temperature combustion of THFA was developed, which reasonably predicts the experimental data. Subsequent rate analysis showed that THFA is mainly consumed by H-abstraction reactions yielding several fuel radicals that in turn undergo either β-scission reactions or intramolecular radical addition that effectively leads to ring enlargement. The importance of specific reaction channels generally correlates with bond dissociation energies. Along THFA reaction routes, the derived species with cis configuration were found to be thermodynamically more stable than their corresponding trans configuration, which differs from usual observations for hydrocarbons.

SiO multi-line laser-induced fluorescence for quantitative temperature imaging in flame-synthesis of nanoparticles

Silicon monoxide (SiO) is an intermediate in the gas-phase synthesis of SiO2 nanoparticles and coatings. We demonstrate a method for in situ imaging the gas-phase temperature via multi-line laser-induced fluorescence (LIF) using excitation in the A 1Π–X 1Σ+(0,0) band near 235 nm. A low-pressure lean (3 kPa, φ = 0.39) premixed hydrogen/oxygen flame was seeded with 210 ppm hexamethyldisiloxane (HMDSO) to produce SiO2 nanoparticles. Spectral regions with no interference from other species in the flame were located, and the excitation-spectral range that provides the best temperature sensitivity was determined from numerical experiments. Quenching rates of the selected transitions were also determined from fluorescence lifetime measurements, and found to be independent of the excited rotational state. Upon laser light-sheet excitation, images of fluorescence were recorded for a sequence of excitation wavelengths and pixel-wise multi-line fitting of the spectra yields temperature images. The results were compared against multi-line NO-LIF temperature imaging measurements using the A 2Σ+–X 2Π(0,0) band near 225 nm from 500 ppm NO added to the gas flow as a thermometry target. Both methods show good qualitative agreement with each other and demonstrate that temperature can be evaluated from the zone in the reactor where SiO is naturally present without adding tracers. SiO LIF exhibited high signal-to-noise ratios of the order of ten times that of NO LIF.

Laser-induced fluorescence of NO isolated in solid p-H2

The p-H2 matrix has emerged as a novel host for matrix isolation partly because of its diminished cage effect. So far, most spectral investigations were performed with infrared absorption. We investigated the fluorescence of NO isolated in solid p-H2 upon irradiation at 193nm and observed emission from the A2Σ+(v=1), A2Σ+(v=0), B2Π, and a4Π states, which is distinct from preceding reports of NO in noble-gas matrices. The unique properties of p-H2 for electronic transitions, such as a small matrix shift on T00 and distinct phonon interactions, are discussed.

Effect of fluctuations on time-averaged multi-line NO-LIF thermometry measurements of the gas-phase temperature

Multi-line NO laser-induced fluorescence (LIF) thermometry enables accurate gas-phase temperature imaging in combustion systems through least-squares fitting of excitation spectra. The required excitation wavelength scan takes several minutes which systematic biases the results in case of temperature fluctuations. In this work, the effect of various types (linear, Gaussian and bimodal) and amplitudes of temperature fluctuations is quantified based on simulated NO-LIF excitation spectra. Temperature fluctuations of less than ±5 % result in a negligible error of less than ±1 % in temperature for all cases. Bimodal temperature distributions have the largest effect on the determined temperature. Symmetric temperature fluctuations around 900 K have a negligible effect. At lower mean temperatures, fluctuations cause a positive bias leading to over-predicted mean temperatures, while at higher temperatures the bias is negative. The results of the theoretical analysis were applied as a guide for interpreting experimental multi-line NO-LIF temperature measurements in a mildly turbulent pilot-plant scale flame reactor dedicated for nanoparticle synthesis.

Nitric oxide kinetics in the afterglow of a diffuse plasma filament

A suite of laser diagnostics is used to study kinetics of vibrational energy transfer and plasma chemical reactions in a nanosecond pulse, diffuse filament electric discharge and afterglow in N2 and dry air at 100 Torr. Laser-induced fluorescence of NO and two-photon absorption laser-induced fluorescence of O and N atoms are used to measure absolute, time-resolved number densities of these species after the discharge pulse, and picosecond coherent anti-Stokes Raman spectroscopy is used to measure time-resolved rotational temperature and ground electronic state N2(v = 0–4) vibrational level populations. The plasma filament diameter, determined from plasma emission and NO planar laser-induced fluorescence images, remains nearly constant after the discharge pulse, over a few hundred microseconds, and does not exhibit expansion on microsecond time scale. Peak temperature in the discharge and the afterglow is low, T ≈ 370 K, in spite of significant vibrational nonequilibrium, with peak N2 vibrational temperature of Tv ≈ 2000 K. Significant vibrational temperature rise in the afterglow is likely caused by the downward N2–N2 vibration–vibration (V–V) energy transfer. Simple kinetic modeling of time-resolved N, O, and NO number densities in the afterglow, on the time scale longer compared to relaxation and quenching time of excited species generated in the plasma, is in good agreement with the data. In nitrogen, the N atom density after the discharge pulse is controlled by three-body recombination and radial diffusion. In air, N, NO and O concentrations are dominated by the reverse Zel'dovich reaction, N + NO → N2 + O, and ozone formation reaction, O + O2 + M → O3 + M, respectively. The effect of vibrationally excited nitrogen molecules and excited N atoms on NO formation kinetics is estimated to be negligible. The results suggest that NO formation in the nanosecond pulse discharge is dominated by reactions of excited electronic states of nitrogen, occurring on microsecond time scale.

Experimental and numerical study of the role of NCN in prompt-NO formation in low-pressure CH 4–O 2–N 2 and C 2H 2–O 2–N 2 flames

We report an experimental and modeling study on prompt-NO formation in low-pressure (5.3 kPa) premixed flames. Special emphasis is given to the quantitative detection (and prediction) of NCN, whose role in prompt-NO formation has recently been confirmed in alkane flames. Here a rich ( Φ = 1.25) CH 4–O 2–N 2 flame and rich ( Φ = 1.25) and stoichiometric C 2H 2–O 2–N 2 flames have been investigated. Absolute concentration profiles of CH and NCN radicals and NO species are obtained by combining laser-induced fluorescence (LIF) and cavity ring-down spectroscopy (CRDS). Temperature profile is determined in each flame using OH and NO–LIF thermometry. Flame modeling is performed to determine the role of NCN in prompt-NO formation and to test the capacity of the present chemical mechanisms to predict some intermediate species involved in prompt-NO formation. The methane flame is modeled using GDFkin®3.0_NCN mechanism [El Bakali et al., Fuel 85 (2006), 896–909]. The acetylene flames are modeled using the Lindstedt and Skevis C/H/O mechanism [Lindstedt and Skevis, Proc. Combust. Inst . 28 (2000), 1801–1807], completed by the submechanism issued from GDFkin®3.0_NCN for nitrogen chemistry. This submechanism includes the initiation reaction CH + N 2 = NCN + H. Rate constants of NO-sensitive reactions of the submechanism are modified by taking into account the recent literature. In particular, the C 2O route could be explored thanks to a significant presence of C 2O in acetylene flames. Globally, the modified submechanism of nitrogen chemistry coupled with the two hydrocarbon mechanisms leads to a satisfying prediction of NCN and NO mole fraction profiles, even though refinements of rate constant determination is still required. The role of NCN in prompt-NO formation in acetylene flames is demonstrated.

A paradigm shift in the interaction of experiments and computations in combustion research

A different approach to comparing experimental data and numerical simulation data is presented. Traditionally, when making comparisons with simulations, experimentalists have sought to measure the same fundamental quantities (e.g., mole fractions) that are output by numerical simulations. This approach often requires measurement of many variables to arrive at the desired quantity, and uncertainty may accumulate with each additional measurement. Because recent advances in computational resources have led to more detailed numerical models, more complete information is available within simulations. This allows for the possibility of using simulation results to derive predictions of measured signals (i.e., “computed signals”) rather than measuring many quantities to derive a single fundamental quantity. Three examples of comparing measured and computed signals are presented: NO laser-induced fluorescence (LIF) images in both non-sooting and sooting diffusion flames, and luminosity images of sooting diffusion flames. For illustration, the non-sooting LIF data is treated both by the traditional method of comparing fundamental quantities and by comparing measured and computed signals. In each example, the comparison of measured and computed signals yields quantitative information similar to that obtainable through comparison of traditional quantities, along with reduced noise in the experimental data.

Fluorescence quantum yield of carbon dioxide for quantitative UV laser-induced fluorescence in high-pressure flames

The fluorescence quantum yield for ultraviolet laser-induced fluorescence of CO2 is determined for selected excitation wavelengths in the range 215–250 nm. Wavelength-resolved laser-induced fluorescence (LIF) spectra of CO2, NO, and O2 are measured in the burned gases of a laminar CH4/air flame (φ=0.9 and 1.1) at 20 bar with additional NO seeded into the flow. The fluorescence spectra are fit to determine the relative contribution of the three species to infer an estimate of fluorescence quantum yield for CO2 that ranges from 2–8×10−6 depending on temperature and excitation wavelength with an estimated uncertainty of ±0.5×10−6. The CO2 fluorescence signal increases linearly with gas pressure for flames with constant CO2 mole fraction for the 10 to 60 bar range, indicating that collisional quenching is not an important contributor to the CO2 fluorescence quantum yield. Spectral simulation calculations are used to choose two wavelengths for excitation of CO2, 239.34 and 242.14 nm, which minimize interference from LIF of NO and O2. Quantitative LIF images of CO2 are demonstrated using these two excitation wavelengths and the measured fluorescence quantum yield.

Development of atmospheric NO analyzer by using a laser-induced fluorescence NO 2 detector

In this study, we propose a new principle for measuring atmospheric NO using a laser-induced fluorescence (LIF) NO 2 detector. NO is chemically converted into NO 2; then, NO 2 is detected by the LIF technique. To convert NO into NO 2, ozone is added to sample air. Since there exists un-reacted NO and loss of NO 2 due to further reaction of NO 2 with ozone depending on both the concentration of ozone and reaction time, the enhanced NO 2 concentration is less than the initial NO concentration. Using chemical kinetic analysis, we successfully derive the relationship between NO concentration and LIF signals. We also propose a unique calibration method for an LIF NO 2 detector without using a chemiluminescent NO x analyzer; this method is based on the gas phase titration technique. The limits of the detection of the developed instrument for NO 2 and NO measurements are 14 ppt and 22 ppt, respectively. We perform ambient air measurements in a suburb of Tokyo using the developed LIF NO analyzer and a commercial chemiluminescent NO analyzer. A comparison of the measurement results reveals that they are in excellent agreement, with a slope of 0.99 and a correlation coefficient of 0.99.

Prompt-NO formation in methane/oxygen/nitrogen flames seeded with oxygenated volatile organic compounds: Methyl ethyl ketone or ethyl acetate

In the present work, CH and NO profiles are determined using laser-induced fluorescence (LIF) measurements in eight low-pressure laminar flames of CH 4/O 2/N 2 containing various amounts of methyl ethyl ketone or ethyl acetate with respect to the equivalence ratio. Relative CH LIF signals are calibrated using cavity ring-down spectroscopy (CRDS), while NO LIF calibration is performed in the burned gases of NO seeded flames. Temperature measurements are obtained using a coated Pt/Rh thermocouple and serve as temperature profiles input for Chemkin modeling. Volatile organic compound (VOC) submechanisms, previously validated upon major and intermediate species profiles measured using sampling techniques, are incorporated into the GDF-Kin 3.0_NCN for the CH 4 oxidation. This mechanism is adjusted and validated to predict the effect of VOCs seeding on CH and NO formation. It takes into account not only the CH and NO profiles but those previously measured. When methane is replaced by either MEK or EA, the NO mole fraction in the burned gases and the CH peak value are found to jointly decrease, indicating that NO is mainly formed according to the prompt-NO mechanism. According to the kinetic analysis, the VOC impact on NO formation is demonstrated. It is shown that CH radical formation, through the C1 sequence, mainly involves the CH 3 radical, which is formed with the same propensity by one molecule of methane, MEK, or EA. Due to the methane replacement by VOC while the equivalence ratio is maintained constant, we found that the CH peak value decrease follows the total fuel volumetric flow rate decrease, yielding, an NO decrease in the burned gases.

Absorption spectroscopy diagnostics of amidogen in ammonia-doped methane/air flames

The NH 2 absolute concentration profiles were measured by intracavity laser absorption spectroscopy in methane/air laminar flat flames doped with trace amounts of ammonia (∼0.1% of the total gas flow) at a pressure of 30 Torr. The radial distribution of NH 2 in the flame was obtained using tomographic reconstruction. The obtained results are compared with one-dimensional model predictions. The temperature profiles, experimentally measured by OH and NO laser-induced fluorescence, were used as an input for the model. Several mechanisms were used. GRI-Mech 3.0 predicts the appearance of an additional maximum for the NH 2 profile in the vicinity of the burner surface, which is not observed in the experiment. The effect of different reactions of the NH 2 radical on its concentration profile at close distances from the burner is discussed. All the mechanisms successfully predict the peak concentration of amidogen in ammonia-doped flames, which is almost unaffected by the equivalence ratio in contrast to flames doped with nitrogen oxides. The maxima of the experimental profiles are located ∼1 mm farther from the burner surface than the simulated ones, which is probably caused by applying a 1-D model for describing the real 3-D flame.

Absorption of NO laser-induced fluorescence by hot O 2 and CO 2

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.

Rotational energy transfer in NO (A 2Sigma+,v'=0) by N2 and O2 at room temperature.

State-to-state rotational energy transfer (RET) rate coefficients for NO (A 2Sigma+, v'=0, J=5.5, 11.5, 17.5) were measured for N2 and O2 at room temperature using a pump-probe method. The NO A 2Sigma+ state is prepared by 226 nm light and the RET is monitored by fluorescence from the D 2Sigma+ v'=0 state, following excitation by a time-delayed laser at approximately 1.1 microm. Additionally, total collisional removal and final state distributions were measured exciting in the Q1+P21 band head, to simulate an NO laser-induced fluorescence atmospheric monitoring scheme. Time-resolved modeling is used to understand relaxation mechanisms and predict relaxation times in ambient air. H2O at atmospherically relevant concentrations does not affect the degree of RET in ambient air.

Strategies for laser-induced fluorescence detection of nitric oxide in high-pressure flames. II. A-X(0,1) excitation.

A-X(0,1) excitation is a promising new approach for NO laser-induced fluorescence (LIF) diagnostics at elevated pressures and temperatures. We present what to our knowledge are the first detailed spectroscopic investigations within this excitation band using wavelength-resolved LIF measurements in premixed methane/air flames at pressures between 1 and 60 bar and a range of fuel/air ratios. Interference from O2 LIF is a significant problem in lean flames for NO LIF measurements, and pressure broadening and quenching lead to increased interference with increased pressure. Three different excitation schemes are identified that maximize NO/O2 LIF signal ratios, thereby minimizing the O2 interference. The NO LIF signal strength, interference by hot molecular oxygen, and temperature dependence of the three schemes are investigated.

Strategies for laser-induced fluorescence detection of nitric oxide in high-pressure flames. I. A-X(0,0) excitation.

Three different high-pressure flame measurement strategies for NO laser-induced fluorescence (LIF) with A-X(0,0) excitation have been studied previously with computational simulations and experiments in flames up to 15 bars. Interference from O2 LIF is a significant problem in lean flames for NO LIF measurements, and pressure broadening and quenching lead to increased interference with increased pressure. We investigate the NO LIF signal strength, interference by hot molecular oxygen, and temperature dependence of the three previous schemes and for two newly chosen excitation schemes with wavelength-resolved LIF measurements in premixed methane and air flames at pressures between 1 and 60 bars and a range of fuel/air ratios. In slightly lean flames with an equivalence ratio of 0.83 at 60 bars, the contribution of O2 LIF to the NO LIF signal varies between 8% and 29% for the previous schemes. The O2 interference is best suppressed with excitation at 226.03 nm.

Rate coefficients for photoinitiated NO 2 unimolecular decomposition: energy dependence in the threshold regime

Rate coefficients k( E) for photoinitiated NO 2 unimolecular decomposition have been obtained by recording the product NO laser-induced fluorescence signal intensity versus the delay between the pump and probe pulses. A 10 ps pump-probe cross-correlation temporal width provides a useful compromise between time and frequency resolution (3 cm −1) . k( E) increases rapidly: from ∼2×10 10 to ⩾1.3×10 11 s −1 within 25 cm −1 of reaction threshold. In this regime, long-range interfragment interactions play an important role. A high level of theory will be needed to reconcile these results.

Fast LIF Approach to NO Rotational Temperature and Density Measurement: Application to a Gas-Dynamic Expansion

An excitation–detection laser-induced fluorescence (LIF) scheme is proposed for the measurement of the NO rotational temperature and density. It is based on the simultaneous excitation of three NO ro-vibronic transitions from only two—one low and one high—J-rotational levels, with the laser set at a suitable fixed wavelength. The procedure does not require the measurement of the excitation spectrum but only of a well-resolved small spectral region of the fluorescence spectrum. The temperature and the density measurements are based on a reference NO LIF spectrum obtained at a known pressure and temperature that is used as a standard. This method is easy and not time consuming. It has been applied to a gas-dynamic expansion.

Accurate Thermometry Using NO and OH Laser-Induced Fluorescence in an Atmospheric Pressure Flame. Checked by Narrow-Band N2 Coherent Anti-Stokes Raman Scattering.

Planar laser-induced fluorescence(PLIF) using OH and NO facilitates noncontact two-dimensional temperature measurements, and is therefore expected to be an effective technique for combustion thermometry. However, the accuracy of the measurement has not been clarified. The relaxation in the upper rotational levels of the tracer molecule is not easy to be predicted, and the rotational dependence of the molecular constants, such as absorption and emission coefficient, has not been fully clarified. Furthermore, it is not confirmed whether the rotational distribution of the tracer molecule maintains the quasi-equilibrium states or not. In this study, we investigated the accuracy of thermometry by means of laser-induced fluorescence (LIF) using NO:A-X(0, 0)and OH:A-X(3, 0) excitation for a premixed methaneair laminar flame in atmospheric pressure as a measuring target. These were checked by narrow-band N2 CARS with high spectral resolution, which is believed to be the most reliable temperature standard in flames. NO LIF temperature showed several % lower temperature than N2 CARS one, and OH LIF temperature 30% lower. Both methods were calibrated under exponential dependence assumption of molecular constans, and the calibrated temperature of NO and OH LIF agreed well with N2 CARS one. Furthermore, two-dimensional thermometry in a flame cross section using calibrated NO PLIF was demonstrated.

Direct studies on the decomposition of the tert-butoxy radical and its reaction with NO

The first laser induced fluorescence (LIF) spectrum for the tert-butoxy radical is reported following radical generation by excimer laser photolysis of tert-butyl nitrite. The laser flash photolysis-LIF technique is used to measure the temperature dependence of the rate coefficient for the reaction with NO (T=200–390 K): which can be represented in either Arrhenius or AT-n format: and the tert-butoxy decomposition. Whilst the former reaction shows no pressure dependence (p=70–500 Torr of helium), the latter reaction is in the fall-off regime over the entire range of experimental conditions (T=303–393 K, p=13–600 Torr helium). The fall-off data were fitted using an inverse Laplace transformation/master equation model to give the following Arrhenius parameters: These Arrhenius parameters are significantly lower than previous indirect measurements or calculations and the atmospheric and mechanistic implications are discussed.Finally, reaction (3) was also studied by monitoring the temporal dependence of the NO LIF signal following the photolysis of tert-butylnitrite with no additional NO. The results are in good agreement with the tert-butoxy monitoring and allow for an estimation of the rate parameters for the tert-butoxy self reaction.