Saturated Laser-induced Fluorescence Research Articles

Experimental and kinetic modeling study of NO formation in premixed CH4+O2+N2 flames

The nitric oxide (NO) formation in methane (CH 4 ) flames has been widely investigated, with quite a few kinetic mechanisms available in the literature. However, studies have shown that there are often discrepancies between the simulations using various mechanisms and the experimental results. To elucidate reactions leading to these discrepancies, experiments were designed to measure the NO formation in the post flame zone of CH 4 +O 2 +N 2 flames with the oxygen ratio, x O 2 = O 2 /(O 2 +N 2 ), varying from 0.2 to 0.27. The experiments were carried out on a heat flux burner at atmospheric pressure and 298 K using saturated Laser-induced fluorescence. The equivalence ratio, ϕ , was changed from 0.7 to 1.6. The corresponding laminar burning velocity, S L , for each condition was also measured using the heat flux method. A comparison was made for the present experimental data and simulation results using the Konnov, Glarborg, NOMecha 2.0, and San Diego mechanisms, and none of them well reproduced the new NO experimental data for all investigated conditions. Numerical analyses show that the increment of NO mole fraction in stoichiometric and fuel-lean flames when the x O 2 increases is mostly defined by the thermal-NO production, which is found to be over-predicted, especially by the Konnov and San Diego mechanisms. The rate constant of reaction NO+N = N 2 +O was derived as k = 1.529 × 10 13 T − 0.0027 exp ( 185.41 [ c a l / m o l e ] R T ) cm 3 / mol s over 225–2400 K temperature range. The rate constants of four reactions controlling CH mole fraction profiles and prompt-NO formation were updated based on the analysis of the literature data that yields an improved performance of the Konnov mechanism.

Experimental and modeling study of nitric oxide formation in premixed methanol + air flames

Validation and further development of models for alcohol combustion requires accurate experimental data obtained under well-controlled conditions. To this end, measurements of nitric oxide, NO, mole fractions in premixed laminar methanol + air flames have been performed using saturated laser-induced fluorescence, LIF. The methanol flames have been stabilized at atmospheric pressure and initial gas temperature of 318 K at equivalence ratios ɸ = 0.7–1.5 using the heat flux method that allows for simultaneous determination of their laminar burning velocity. The LIF signal is converted into NO mole fraction via calibration measurements, which have been performed in flames of methane, methanol and syngas seeded with known amounts of NO. The experimental approach is verified by the measurements of NO mole fractions in the post flame zone of methane flames, investigated in previous studies at similar conditions. Data on the NO formation together with burning velocities for methanol and methane obtained under adiabatic flame conditions provide highly valuable input for model validation. They have been compared with predictions of six different chemical kinetic mechanisms. Summarizing the behavior of all models tested with respect to burning velocities and NO formation in flames of methane and methanol, the mechanism of Glarborg et al. (2018) and the San Diego mechanism (2019) demonstrate uniformly satisfactory performance.

Saturated laser-induced fluorescence in liquid kerosene seeded with a dye: influence of temperature and excitation intensity

Fluorescence-based techniques are commonly implemented to characterize droplet sprays in flows at temperature above 293 K. Nevertheless, in the case of gas turbine altitude relight, aeroengines are exposed to critical operating conditions where the liquid fuel temperature can reach values as low as − 40 °C. As a result, it is necessary to address the influence of this parameter on fluorescence intensity for temperatures below room values. In this paper, an innovative saturated laser-induced fluorescence (SLIF) excitation scheme is proposed to overcome the strong temperature dependence of fluorescence in liquid kerosene seeded with a dye. The findings of this work are based on gated sequential images recorded with an intensified camera during and after the laser pulse duration so as to sample the temporal evolution of the fluorescence emission. The results show that, for low laser excitation intensity, the fluorescence intensity remains proportional to the laser power. However, a non-linear response is observed in the case of high excitation intensity. This difference can be used to limit the fluorescence dependence on temperature by carefully selecting the temporal window where fluorescence is detected. Finally, although the method presented in this paper is applied to cold liquid kerosene, its range of application can be extended to other liquids and other temperature ranges.

Effects of CH4 Content on NO Formation in One-Dimensional Adiabatic Flames Investigated by Saturated Laser-Induced Fluorescence and CHEMKIN Modeling

Experiments applying saturated laser-induced fluorescence (LSF) technology were performed focusing on the influence of equivalence ratio, syngas mixture contents, on NO formation in H2/CO/CH4/CO2/N2/O2 premixed flat flames supplied by a heat flux burner. Experimental data were extracted to validate calculation by CHEMKIN software. Both experiments and CHEMKIN calculation draw one conclusion that NO mole fraction in CH4–air flame peaks at stoichiometric and ϕ 1.3, due to thermal NO and prompt NO routes. All mechanisms applied can well-predict the NO mole fraction at the fuel lean side but failed at the fuel rich side. NO mole fraction in syngas flame is propotional to CH4 ratio as shown in the experimental data, but all the mechanisms failed to predict it with some even having a wrong tendency. A rate of NO production and sensitivity analysis suggests that thermal and prompt NO routes play different roles in each mechanism, and modifications to mechanisms are required to improve NO concnetration prediction...

Investigation of NO formation in premixed adiabatic laminar flames of H2/CO syngas and air by saturated laser-induced fluorescence and kinetic modeling

A series of experiments were performed to investigate the relationship between syngas composition (H2/CO/CO2/N2/O2) and NO formation. Laser-saturated fluorescence measurements of NO in a premixed syngas flat flame from a heat flux burner were recorded for various fuel compositions, dilution ratios and equivalence ratios. Quantitative measurements were compared with the predictions from CHEMKIN software using four different chemical mechanisms, i.e., GRI mech 3.0, Chemical Reaction Engineering and Chemical Kinetics 1407, GDF-Kin 3.0, and the Mendiara and Glarborg mechanism; each mechanism included reaction subsets for NOx formation. These models were validated against a CH4 flame; the GDF-Kin 3.0 mechanism produced the best agreement with measurements. NO measurements for CO/H2 syngas fuel with 50% CO2 dilution at various equivalence ratios found the maximum NO production near stoichiometry. The majority of NO produced for these conditions is predicted to come from the combination of the NNH and N2O pathways. Investigation of NO production during combustion of different CO/H2 ratio syngas diluted with 60% N2 showed that increasing the H2 fraction decreased the total NO concentration. However, normalizing the NO production rate by the fuel mass consumption rate showed H2 produces more NO per gram of fuel consumed. Predictions of temperature and NO rate of production along the flame axis showed that the syngas with high H2 content had a flame front closer to the burner exit and NO production rate greater than the high CO case. Dilution of CO/H2 syngas fuel with N2 and CO2 showed that CO2 has a stronger reduction in NO emission than N2. GDF-Kin 3.0 predicted that dilution with CO2 caused a greater reduction in flame temperature than the same volume of N2. Predicted NO rate of production and reaction sensitivity analysis predicted that the Zel’dovich pathway was dominant for undiluted syngas. However, as the flame temperature reduced, the Zel’dovich pathway was inhibited to a greater extent than the N2O and NNH pathways so that, at high dilution, NO production was driven by the N2O and NNH pathways.

Measurements of NO concentration in NH3-doped CH4 + air flames using saturated laser-induced fluorescence and probe sampling

An experimental study of methane+air flames doped with ammonia (4370ppm of the fuel) has been performed. The goal of this work was to analyze formation of NOx from fuel-N under well-controlled conditions. The Heat Flux method was used for stabilization of non-stretched adiabatic flames on a perforated plate burner at atmospheric pressure. Laser-saturated fluorescence (LSF) and probe sampling were adopted to measure NO concentrations in the post-flame zone. LSF experiments include two series of measurements: in methane+air flames doped with NO and then in flames doped with NH3. In the lean flames seeded with NO, LSF measurements clearly deviates from the model predictions at higher concentrations of NO seeded, that strongly corroborates existence of the lean NO reburning. The modeling accurately predicts [NO] in the neat flame and shows no consumption of NO up to 170ppm seeded. In (CH4+NH3)+air mixtures the NO concentrations measured by LSF are in good agreement with the probe sampling results in the whole range of equivalence ratios 0.65<Φ<1.45. No significant impact of the probe sampling on the NOx measurements in the post-flame zone was observed. Present ammonia conversion data are only in agreement with the measurements of Henshaw et al. and disagree with many other earlier experiments. On the other hand they are accurately reproduced by the flame modeling within experimental uncertainties. Therefore the earlier criticism of the models developed by Konnov and by Skreiberg et al. is not substantiated anymore; these mechanisms are capable in predicting ammonia conversion both in lean and in rich flames. Formation of metal complexes in gas cylinders containing ammonia was put forward to explain inconsistencies between present results and earlier experiments.

Flame structure measurements of NO in premixed hydrogen–nitrous oxide flames

Experimental NO flame profiles were measured for N2-diluted, H2–N2O premixed flames stabilized on a flat flame apparatus at equivalence ratios ranging from 0.95 to 1.55. NO measurements were obtained using saturated laser-induced fluorescence and calibrated by microprobe post-flame sampling utilizing Fourier transform infrared (FTIR) spectroscopy. These experimental data were compared with a burner-stabilized flame model using three different published chemical mechanisms incorporating H2/O2/NxOy chemistry. For the stoichiometric flame condition, peak NO model predictions using the three chemical mechanisms differed by 38%, whereas these model predictions varied by over 47% for the fuel-rich conditions examined. NO predictions in the post-flame region using the Konnov chemical mechanism were generally within the estimated experimental uncertainty, whereas the model predictions using the other two mechanisms fell below the lower uncertainty bounds. The normalized spatial NO profile predictions for all three chemical mechanisms showed reasonable agreement with the normalized experimental NO profiles for near-stoichiometric H2–N2O flames; however, there was appreciable disparity between the experimental and model predictions for fuel-rich, H2–N2O flames. Sensitivity and rate-of-production analyses show that NHx/NO chemical interactions, as well as NHx/NxHy recombination reactions under fuel-rich conditions, are important to accurately modeling NO profiles in hydrogen-nitrous oxide flames. The experimental data provide constraints for the rate constants of important NHx/NxHy chemical interactions, and some modifications were considered for improving comparisons between model predictions and experiments.

Infrared Action Spectroscopy and Dissociation Dynamics of the HOOO Radical

The HOOO radical has long been postulated to be an important intermediate in atmospherically relevant reactions and was recently deemed a significant sink for OH radicals in the tropopause region. In the present experiments, HOOO radicals are generated in a pulsed supersonic expansion by the association of O(2) and photolytically generated OH radicals, and the spectral signature and vibrational predissociation dynamics are investigated via IR action spectroscopy, an IR-UV double resonance technique. Rotationally resolved IR action spectra are obtained for trans-HOOO in the fundamental (nu(OH)) and overtone (2nu(OH)) OH stretching regions at 3569.30 and 6974.18 cm(-1), respectively. The IR spectra exhibit homogeneous line broadening, characteristic of a approximately 26-ps lifetime, which is attributed to intramolecular vibrational redistribution and/or predissociation to OH and O2 products. In addition, an unstructured feature is observed in both the OH fundamental and overtone regions of HOOO, which is likely due to cis-HOOO. The nascent OH X(2)Pi, v = 0 or v = 1, products following vibrational predissociation of HOOO, nu(OH) or 2nu(OH), respectively, have been investigated using saturated laser-induced fluorescence measurements. A distinct preference for population of Pi(A') Lambda-doublets in OH was observed and is indicative of a planar dissociation of trans-HOOO in which the symmetry of the bonding orbital is maintained.

Development of a highly intense state-selected CH radical beam source for the study on the collision energy dependent reaction dynamics

A velocity-variable highly intense pulsed supersonic CH (X 2Π) radical beam source was newly developed for the study on the collision energy dependent reaction dynamics of the state-selected CH radical. CH radicals were state-selected by an electrostatic hexapole field. The focusing curves and the CH flux intensities for the ∣ J, F i, M〉 states were measured by a saturated laser-induced fluorescence spectroscopy. As a demonstration, the rotational state dependence of the reaction cross section of CH + O 2 → OH(A) + CO reaction was determined at the collision energy of 0.06 eV.

Branching ratios for quenching of nitric oxide A2Σ+(ν′ = 0) to X2Π(ν″ = 0)

We describe experiments designed to measure the fraction of nitric oxide molecules that undergo quenching from A 2Sigma+ (nu' = 0) directly to X 2Pi(nu" = 0). This quenching channel was investigated for room temperature collisions with O2, CO, CO2, and H2O by measuring recovery of the ground-state population following intense laser excitation. Experiments were conducted in a room temperature flow cell containing dilute mixtures of NO, N2, and the quenching gases. An intense nanosecond laser pulse, tuned to the NO A 2Sigma(+) - X 2Pi(0,0) Q11 + pQ21 bandhead at 226.3 nm, depopulated more than 20% of the equilibrium population in the X 2Pi(nu'' = 0) manifold. A weak, time-delayed, picosecond laser pulse, tuned to the A 2Sigma(+) - 2Pi(1,0) Q11 + pQ21 bandhead at 214.9 nm, probed recovery of population in X(nu'' = 0) via subsequent LIF for each of the investigated quenchers. Remarkably large branching ratios were observed for direct quenching to X 2Pi(nu'' = 0). Water, carbon monoxide, and oxygen quench NO A 2Sigma+ (v' = 0) to X 2Pi(nu" = 0) with branching ratios that are approximately 0.3. The significantly higher branching ratio for quenching by carbon dioxide is 0.6. The results provide insight on the NO quenching process and represent an important step toward a detailed understanding of the effects of collisional energy transfer on saturated laser-induced fluorescence, which is necessary to properly model detection strategies based on high laser fluences.

Influence of the lifetime of the laser-excited level and the laser pulse duration on the saturated LIF signal during the prebreakdown phases of a pseudospark discharge

Ground state copper vapor number density, sputtered from the cathode of an experimental pseudospark switch is measured using laser-induced fluorescence (LIF) from the copper transition 4/sup 2/P/sub 3/2/-4/sup 2/D/sub 5/2/ at 510.55 nm. Measurements are made in the hollow cathode at several axial positions in the time interval between the trigger pulse and the voltage breakdown. At voltage 1 kV, this interval, referred to as a prebreakdown phase, has a random and variable shot to shot duration, but discharges for which this duration is the same reveal similar properties and can be arranged in groups. The presented results are for prebreakdown duration of 1 /spl mu/s. The metal vapor density is determined from the LIF signal by using both its maximum and integral. This approach follows from a widely used LIF model, which is examined in detail to reveal a less familiar feature, namely the possibility to obtain an estimate of the lifetime of the laser-excited level, which has been considered perturbed by collisions during the discharge. A comparison between this estimate and the lifetime, obtained by the exponential decay of the LIF signal, reveals good agreement.

A highly intense state-selected CH radical beam and its application to the CH + O 2 reaction

A highly intense pulsed supersonic CH(X 2Π) radical beam source was developed by use of the C( 1D) + H 2 reaction. CH radicals were state-selected by an electrostatic hexapole field. The focusing curves for the single state-selected CH were measured for the first time by a saturated laser-induced fluorescence (LIF) spectroscopy for the R-branch in A 2Δ 3/2 ← X 2Π 1/2 transition. The maximum CH flux intensity in the lowest ∣ J, F, M〉 state, ∣1/2, 2, 1/2〉 was measured by LIF to be 3.2 × 10 12 rad cm −2 s −1 at the focusing point of 150 cm downstream from the nozzle.

Spatial laser-wing suppression in saturated laser-induced fluorescence without spatial selection.

Spatial wings of laser beams are of great concern in saturated laser-induced fluorescence. Their contribution to fluorescence is customarily avoided by resolution of laser peaks in the interaction volume. An alternative and versatile approach is formulated, based on the derivative of fluorescence with respect to laser intensity. It turns out that wing-free data are possible, although they are obtained from wing-dependent fluorescence. The advantages of this approach are exact centerline detection and simplicity of the experimental setup and procedure.

A highly intense state-selected OH beam source by the pulsed electric DC discharge method

A high intensity pulsed DC discharge source for OH radical beam was developed by the systematic optimization for geometry of the discharge source and for the operation conditions. OH radicals produced in the DC discharge source were state-selected by an electrostatic hexapole field. The maximum OH flux in the lowest | J, Ω, M〉=|3/2,3/2,3/2〉 state was directly measured to be 8.5 × 10 14 radicals cm −2 s −1 at the beam crossing point of 150 cm downstream from nozzle by a saturated laser-induced fluorescence spectroscopy for the Q 1(1) transition in A 2Σ←X 2Π 3/2 transition.

An alternative approach to temporal laser-wing effects in saturated laser-induced fluorescence

Temporal laser-wing effects are associated with the unavoidable time characteristics of laser pulses. To remove their contribution in saturated laser-induced fluorescence, time sampling is commonly adopted. In the present work, an alternative approach is formulated. It is based on power measurements obtained from the whole fluorescence response. Numerical simulations with a four-level dynamical model are described to test the method against the traditional time-sampling procedure. A greater amount of measured signal and independence from the fast electronic sampler are the main advantages of the proposed method.

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.

Spatially resolved measurements of CN, CH, NH, and H 2CO concentration profiles in a domestic gas boiler

Spatially resolved measurements of species profiles were performed for CN, CH, NH, and H2CO in a premixed methane/air flame as a function of height above the central slot of a seven-slot atmospheric pressure burner using laser-induced fluorescence (LIF). The burner was constructed to resemble as closely as possible a commercial household boiler with water heat exchanger. For several near-stoichiometric flame conditions, absolute number densities for CN, CH, and NH were obtained using partially saturated LIF with Raman or Rayleigh calibration. For CN, an independent measurement series using linear LIF with quenching corrections applied from known effective fluorescence lifetimes in atmospheric pressure flames verified the results from the saturated LIF measurements. Relative H2CO height profiles in the preheating zone of two flame cases were also obtained with applied quenching corrections from measured effective fluorescence lifetimes at each spatial location. H2CO lifetimes continuously decreased with increasing temperature in the preheating zone (i.e., increasing height above the burner exit). Absolute concentration values and peak concentrations showed striking differences in the two flame cases, which should provide valuable data for detailed numerical simulations of these flames.

00/00416 Comparison of saturated and linear laser-induced fluorescence measurements of nitric oxide in counterflow diffusion flames

00/00416 Comparison of saturated and linear laser-induced fluorescence measurements of nitric oxide in counterflow diffusion flames

The pressure and temperature dependence of the recombination reaction HO+SO2+M→HOSO2+M

The recombination of the HO radical with SO2 was studied in the bath gas helium at temperatures between 220 and 400 K and in the pressure range between 1 and 96 bar. HO radicals were generated by laser flash photolysis of CH4–O3 mixtures at 248 nm and their decay was detected using saturated laser induced fluorescence (SLIF) following excitation at 281 nm. Thermal second order rate constants were extracted from the SLIF–time profiles under pseudo-first order conditions. Due to our high pressure experiments we accessed a wide part of the falloff range which allowed for a reliable extrapolation towards the high pressure limiting rate constant. We constructed complete falloff curves at 220, 300 and 400 K incorporating the low-pressure experiments from the literature. From the temperature dependence of the high pressure limiting rate constant k1,∞=(1.2±0.3)×10-11 exp(-(360±100) K/T) cm3 molecule-1 s-1 we conclude on a small barrier for the recombination process. Taking into account this barrier, we express the temperature dependence of the low pressure limiting rate constants as k1,0=[He] (2.5±0.7)×10-32 (T/300 K)-(3.8±0.5) exp(-(360±100)K/T) cm6 molecule-2 s-1. With this new falloff analysis we reanalyzed the recombination rate constants in the literature for various third bodies (Ar, N2, O2, CO2, SO2, H2O and SF6), reliably extrapolated towards the corresponding low pressure limiting rate constants, and determined their collision efficiencies and the average energies 〈ΔE〉all transferred per collision. Implications for atmospheric chemistry are discussed.

The high-pressure range of the reaction CH(2Π)+CO+M⇒HCCO+M

The pulsed laser photolysis (PLP), time-resolved saturated laser-induced fluorescence (SLIF) technique has been used to study the reaction CH+CO+M⇒HCCO+M (1) in the temperature range between 300–800 K and at total pressures between 4 and 160 bar helium. The CH radicals have been generated using UV-multiphoton dissociation of CHBr3 at 248 nm. In the investigated temperature and pressure range, the usual falloff behavior of the reaction has been found. Falloff curves have been constructed and a high-pressure limiting rate constant with a weak negative temperature dependence of k1,∞=1.7×10−10(T/300 K)−0.4 cm3 molecule−1 s−1 has been extracted. This high-pressure limiting rate constant has been treated in terms of the statistical adiabatic channel model. Isobar Arrhenius curves have been constructed for pressures between 0.01 and 100 bar, including the formation of C2O+H at high temperatures.