Laser-saturated Fluorescence Research Articles

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.

Laser-saturated fluorescence of nitric oxide and chemiluminescence measurements in premixed ethanol flames

In this study, nitric oxide laser-saturated fluorescence (LSF) measurements were acquired from premixed ethanol flames at atmospheric pressure in a burner. NO-LSF experimental profiles for fuel-rich premixed ethanol flames ( Φ = 1.34 and Φ = 1.66) were determined through the excitation/detection scheme of the Q 2(26.5) rotational line in the A 2 Σ + − X 2 Π (0, 0) vibronic band and γ(0, 1) emission band. A calibration procedure by NO doping into the flame was applied to establish the NO concentration profiles in these flames. Chemiluminescent emission measurements in the (0, 0) vibronic emission bands of the OH * ( A 2 Σ + − X 2 Π) and CH * ( A 2 Δ − X 2 Π) radicals were also obtained with high spatial and spectral resolution for fuel-rich premixed ethanol flames to correlate them with NO concentrations. Experimental chemiluminescence profiles and the ratios of the integrated areas under emission spectra (A CH*/A CH*(max.) and A CH*/A OH*) were determined. The relationships between chemiluminescence and NO concentrations were established along the premixed ethanol flames. There was a strong connection between CH * radical chemiluminescence and NO formation and the prompt-NO was identified as the governing mechanism for NO production. The results suggest the optimum ratio of the chemiluminescence of two radicals (A CH*/A OH*) for NO diagnostic purposes.

Quantitative laser-saturated fluorescence measurements of nitric oxide in counter-flow diffusion flames under sooting oxy-fuel conditions

We report quantitative, spatially resolved, laser-saturated fluorescence measurements of nitric oxide ([NO]) concentration in sooting, high-temperature, oxygen-methane, counter-flow diffusion flames at atmospheric pressure. Six different flames containing 1%, 3%, and 10% N2 in either the oxidizer or fuel streams are investigated at a global strain rate of 20 s−1. Excitation of NO is obtained at 224.45 nm in the γ(0,0) band and detection is performed in a 2-nm region centered at 235.78 nm in the γ(0,1) band. Numerical computations for all counter-flow diffusion flames are conducted using OPPDIF with GRI Mech-3.0. The effect of gas-phase radiation is considered in the modeling. Excitation scans indicate no significant change in background for oxygen-rich as compared to air-rich flames and detection scans verify the absence of interferences from other species. Quantitative axial profiles of [NO] are presented for all six flames. Comparisons with modeling indicate good agreement in those regions of each flame having predicted temperatures below 2600 K. Enhanced radiative heat loss caused by soot formation leads to poorer agreement between predicted and measured NO concentrations in regions at higher flame temperatures (T > 2600 K), thus indicating the need for a combined soot formation and radiation model.

Laser-saturated and linear laser-induced fluorescence measurements of nitric oxide in counterflow diffusion flames under non-sooting oxygen-enriched conditions

We report quantitative, spatially resolved, laser-induced fluorescence (LIF) measurements of NO concentration ([NO]) in non-sooting, oxygen-enriched counterflow diffusion flames at atmospheric pressure. Three different flames containing 25%, 50%, and 100% CH 4 , respectively, in the fuel stream and 100%, 50%, and 35% O 2 , respectively, in the oxidizer stream are investigated at a global strain rate of 20 s m 1 . Excitation of NO is achieved at 224.45 nm in the n (0,0) band and detection is performed in a 2-nm region centered at 235.78 nm in the n (0,1) band. Excitation scans ensure no spectral background problems under the high-temperature conditions of oxygen enrichment. Detection scans obtained at the excitation wavelength verify no interferences from any other species, particularly O 2 . Quantitative axial profiles of [NO] are presented for all three flames. Linear LIF measurements are compared to laser-saturated fluorescence (LSF) measurements to assess the utility of a broadband LSF technique at temperatures approaching 3000 K. Numerical computations for all counterflow diffusion flames are conducted using OPPDIF with GRI Mech-3.0. The effect of gas-phase radiation is considered in the modeling. The results indicate excellent agreement between the measured and predicted NO concentrations for all three flames. A comparison of the linear LIF and LSF measurements also yields excellent agreement.

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.

Laser-induced fluorescence measurements in leandirect-injection spray flames: technique development andapplication

The combustion diagnostics community has recently begun to focusits efforts toward practical combustion devices. One impetusbehind this effort is the need to develop aeropropulsion gasturbine combustors with ultra-low NOx emissions. For thepast several years, the Flame Diagnostics Laboratory at PurdueUniversity has been advancing optically non-intrusive techniquesto measure concentrations of nitric oxide [NO] in leandirect-injection (LDI) spray flames. LDI flames offer thepossibility of reducing NOx emissions from gas turbines byrapid mixing of the liquid fuel and air so as to drive the flamestructure toward partially premixed conditions. In this paper,we review the technical approach required to utilizelaser-induced fluorescence (LIF) methods for quantitativelymeasuring [NO] in LDI spray flames. In the progression fromatmospheric to high-pressure measurements, the LIF methodrequires a shift from the saturated to the linear regime offluorescence measurements. As such, we discuss quantitative,spatially resolved laser-saturated fluorescence (LSF), linearlaser-induced fluorescence (LIF) and planar laser-inducedfluorescence (PLIF)measurements of NO concentration in atmospheric, LDI sprayflames. In general, the results are comparable, although novelfiltering techniques are required at higher flame pressures.

Quantification of Planar Laser-Induced Fluorescence Measurements of Nitric Oxide in Laminar Partially Premixed Flames

Planar laser-induced fluorescence (PLIF) measurements of nitric oxide (NO) have been obtained in atmospheric pressure, laminar, partially premixed ethane-air flames. The objective was to quantify PLIF measurements in partially premixed flames across which there exists a significant range of quenching environments. In particular, we investigated two flame configurations with an overall equivalence ratio of 0.5 and fuel-tube equivalence ratios of 1.33 and 2.22, respectively. Each PLIF image was made semi-quantitative by scaling the entire image based on a single laser-saturated fluorescence (LSF) point measurement. The quantified PLIF measurements were then assessed by comparison with separate two-dimensional arrays of LSF measurements. Good agreement is found between quantified PLIF and LSF measurements in both flames at all elevations. In addition, a majority of the PLIF measurements fall within the uncertainty of the LSF measurements.

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

Quantitative measurements of NO concentrations ([NO]) have been obtained along the centerline of atmospheric ethane–air counterflow diffusion flames by using saturated and linear laser-induced fluorescence (LIF). In particular, four flames with strain rates varying from 5 to 48 s −1 were investigated while maintaining a constant fuel dilution in all cases. The utility of a broad-band laser-saturated fluorescence (LSF) technique is assessed by comparison to similar measurements of NO using linear LIF. The linear LIF measurements are corrected for variations in the local electronic quenching rate coefficient by using major species profiles generated by a diffusive flame code and available correlations for the quenching cross-sections of NO. The corrected LIF profiles compare favorably with the LSF profiles. A four-level model is used to investigate the effects of rotational energy transfer (RET) on the LSF measurements. The excellent comparison between the quenching-corrected linear LIF and the LSF measurements at locally fuel-lean to greater than stoichiometric mixture fractions verifies the validity of the LSF technique for these conditions. The slight but consistent discrepancy between the LSF and linear LIF measurements at local equivalence ratios above 1.6 may be attributed to a change in the collisional branching ratio from lean to rich stoichiometries and/or the need for further work on the electronic quenching cross-sections required for quantitative NO measurements under fuel-rich conditions.

Laser-saturated fluorescence measurements of nitric oxide in an inverse diffusion flame

Inverse diffusion flames (IDFs) are a primary component of staged-air combustion processes, which are often used to achieve ultralow NO x emissions. Unfortunately, such IDFs created by mixing air with hot fuel-rich combustion products rather than the usual unreacted fuel have received little previous attention in the literature. Here, we use laser-saturated fluorescence (LSF) to make nonintrusive point measurements of nitric oxide concentration throughout a single atmospheric IDF of this type; thermocouple-based temperature measurements are also made at specific locations including those of the concentration measurements. These measurements demonstrate similarities between IDF and normal diffusion flame behavior, and indicate that the majority of the IDF-attributable NO is generated at the IDF tip. The results suggest that the dominant NO mechanism varies throughout the flame, and provide data relevant to assessment of future kinetics models.

Laser-induced Fluorescence Triple-integration Method Applied to Hydroxyl Concentration and Fluorescence Lifetime Measurements

We report quantitative hydroxyl concentrations obtained by using a new laser-induced fluorescence triple-integration method (LIFTIME), which is capable of rapid and continuous fluorescence lifetime measurements via a unique photon-counting technique. LIFTIME has been convolved with picosecond time-resolved laser-induced fluorescence to permit the rapid monitoring of instantaneous species concentrations in flames. Here, LIFTIME is used to measure hydroxyl concentrations and fluorescence lifetimes at a sampling rate of 1 Hz in eight premixed laminar flat flames and in one laminar opposed flow diffusion flame. Fluorescence lifetimes as a function of axial position are generally obtained with less than 5% uncertainty, while concentrations at the same locations are obtained with less than 10% uncertainty (95% confidence interval). The hydroxyl concentration measurements are shown to agree well with modeling predictions and with previous laser-saturated fluorescence measurements. The measurements are also compared with predictions based on existing quenching cross-section correlations.

Comparisons of laser-saturated, laser-induced, and planar laser-induced fluorescence measurements of nitric oxide in a lean direct-injection spray flame

We report quantitative, spatially resolved laser-saturated fluorescence (LSF), linear laser-induced fluorescence (LIF), and planar laser-induced fluorescence (PLIF) measurements of nitric oxide (NO) concentration in a preheated, lean direct-injection spray flame at atmospheric pressure. The spray is produced by a hollow-cone, pressure-atomized nozzle supplied with liquid heptane, and the overall equivalence ratio is unity. NO is excited by means of the Q(2)(26.5) transition of the gamma(0, 0) band. LSF and LIF detection are performed in a 2-nm region centered on the gamma(0, 1) band. PLIF detection is performed in a broad ~70-nm region with a peak transmission at 270 nm. Quantitative radial NO profiles obtained by LSF are presented and analyzed so as to correct similar LIF and PLIF profiles. Excellent agreement is achieved among the three fluorescence methodologies.

Quantitative laser-saturated fluorescence measurements of nitric oxide in a heptane-fueled lean direct-injection spray flame at varying global equivalence ratios

We report the influence of equivalence ratio on nitric oxide (NO) concentration profiles in an unconfined heptane spray flame at atmospheric pressure. The associated burner is based on the lean direct-injection design and incorporates a pressure-atomized, hollow-cone spray nozzle. Helical vanes in the air passage coupled with a divergent exit and preheated air produce a strongly swirling, clean, blue flame. NO concentration profiles are measured using laser-saturated fluorescence (LSF) at four axial heights above the burner. Because of the liquid droplet environment, an extension of LSF utilizing a Mie-scattering back-ground subtraction technique is required to make quantitative measurements of NO concentration. The NO profiles are measured in three flames with global equivalence ratios of 1.0, 0.9, and 0.8. Probe measurements of CO 2 and O 2 mole fractions are used to determine representative effective equivalence ratios, which are lower than the global equivalence ratios because of significant entrainment of ambient air Through an analysis of area-averaged NO concentrations at various axial heights, the downstream [NO] is found to be mainly redistributed as the flame spreads owing to the intense recirculation of combustion products. Moreover, comparisons of [NO] profiles as a function of equivalence ratio demonstrate that dilution air is a large influence in the reduction of NO concentration at equivalence ratios leaner than stoichiometric.

Quantitative Laser-Saturated Fluorescence Measurements of Nitric Oxide in a Heptane Spray Flame

We report spatially resolved laser-saturated fluorescence measurements of NO concentration in a pre-heated.lean-direct injection (LDI) spray flame at atmospheric pressure. The spray is produced by a hollow-cone, pressure-atomized nozzle supplied with liquid heptane. NO is excited via the Q2(26.5) transition of the γ(O, 0) band. Detection is performed in a 2-nm region centered on the γ(O, 1) band. Because of the relatively close spectral spacing between the excitation (226 nm) and detection wavelengths (236 nm), the γ(O, 1) band of NO cannot be isolated from the spectral wings of the Mie scattering signal produced by the spray. To account for the resulting superposition of the fluorescence and scattering signals, a background subtraction method has been developed that utilizes a nearby non-resonant wavelength. Excitation scans have been performed to locate the optimum off-line wavelength. Detection scans have been performed at problematic locations in the flame to determine possible fluorescence interferences from UHCs and PAHs at both the on-line and off-line excitation wavelengths. Quantitative radial NO profiles are presented and analyzed so as to better understand the operation of lean-direct injectors for gas turbine combustors.

Exhaust and in Situ Measurements of Nitric Oxide in Laminar Partially Premixed C2H6−Air Flames: Effect of Premixing Level at Constant Burner Tube Flow Rate

NO formation in laminar partially premixed ethane−air flames is investigated as a function of the amount of air introduced into the central fuel tube of an annular coflow burner. The NOx emission index at the exhaust is determined by chemiluminescent detection while the local NO number density is measured by laser-saturated fluorescence. The measurements are taken in flames with an overall equivalence ratio of 0.9, burner-tube equivalence ratios (φB) varying from 1.1 to 6.7, and a fixed burner-tube flow rate of 0.9 L(STP)/min. Local NO number densities are measured as a function of both radial position and height above the burner. Despite the increase in fuel flow rate with rising φB, an intermediate dual-flame pattern is identified which minimizes the NOx emission index. NO production is found to occur primarily between an inner premixed and an outer nonpremixed flame front, which constitutes the dual-flame structure. These results suggest that the optimum burner-tube equivalence ratio occurs due to a compromise between prompt and thermal formation of NO in the predominantly premixed and nonpremixed flame regions, respectively.

Advances in the chemistry of insect control, III

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.

Laser-Saturated Fluorescence Measurements of Nitric Oxide in Laminar, Flat, C2H6/O2/N2Flames at Atmospheric Pressure

Abstract We have performed both laser-saturated fluorescence (LSF) and linear laser-induced fluorescence (LIF) measurements of NO in lean and rich atmospheric-pressure C2H6/O2/N2 flames. Unlike previous LSF measurements of OH, NH, and CH, the LSF measurements of NO require a broadband detection scheme, and thus we include a comprehensive theory for broadband LSF. Saturation of NO is found to be easily attainable at atmospheric pressure. When high laser energies are used to insure saturation of NO, background fluorescence often occurs from additional flame species; hence, a subtraction technique is introduced to eliminate this fluorescence from the NO signal. Calibration of both the LSF and LIF techniques was accomplished by doping lean flames with known quantities of NO. A comparison of the LSF and LIF signals from postflame gases at ~ 1700 K as a function of equivalence ratio suggests that the influence of stoichiometry on fluorescence quenching is nearly negligible. Finally, we discuss the relative merits of the LSF and LIF methods, which both give a detection limit of ~1 ppm in atmospheric-pressure flames.

OH rotational temperature and number density measurements in high-pressure laminar flames using double phase-conjugate four-wave mixing

OH number densities and rotational temperatures up to 9 bar have been measured using double-phase-conjugate four-wave mixing (DPCFWM) in flat laminar premixed methane/air flames. By phase conjugating the backward pump to the forward pump with a phase-conjugate mirror conventional degenerate four-wave mixing (DFWM) becomes DPCFWM and quantitative measurements of OH radicals and OH rotational temperatures in flames at high pressure are possible. Our results show that conventional DFWM with a standard mirror is a low-biased measurement at high pressure primarly due to a fluctuating interaction volume which results from large fluctuating density gradients at the edge of the flame and from flame movement. A comparison is made between a laser-saturated fluorescence technique, conventional DFWM, and DPCFWM at 1, 5 and 9 bar.

Measurements of OH concentration in flames at high pressure by two-optical path laser-induced fluorescence

The two-optical path laser-induced fluorescence (TOPLIF) method was used to measure OH concentration profiles in flat, stoichiometric, premixed, methane-air flames burning at pressures ranging from 1 to 9 bars. The TOPLIF method does not require knowledge of the collisional quenching rates, which is a major advantage especially at these pressures. The population of a single rovibronic level is measured in a manner similar to many spectroscopic methods. TOPLIF measures this population at any location in flames at any pressure relative to this level's population in an arbitrary reference flame (here the 1-bar flame at 5 mm above the burner). Absolute values can therefore be obtained from the known value in the reference flame. In addition, the TOPLIF method is proposed as a technique to process standard laser-saturated fluorescence measurements. Using our approach, we can properly correct variations in the effective probe volume with pressure.

Saturated fluorescence measurements of the hydroxyl radical in laminar high-pressure C_2H_6/O_2/N_2 flames

We demonstrate saturation of a transition of the OH molecule in high-pressure flames by obtaining saturation curves in C(2)H(6)/O(2)/N(2) laminar flames at 1, 6.1, 9.2, and 12.3 atm. In addition we present quantitative fluorescence measurements of OH number density at pressures to 12.3 atm. To assess the efficacy of the balanced cross-rate model for high-pressure flames, we compare laser-saturated fluorescence measurements, which were calibrated in an atmospheric-pressure flame, with absorption measurements at 3.1 and 6.1 atm. At 3.1 atm the absorption and fluorescence measurements compare well. At 6.1 atm, however, the concentrations given by laser-saturated fluorescence are ~25% lower than the absorption values, indicating some depletion of the laser-coupled levels beyond that at atmospheric pressure. By using a reasonable estimate for the finite sensitivity to quenching, we anticipate that fluorescence measurements that are calibrated at 1 atm can be applied to flames at ~10 atm with absolute errors within +/-50%.