Measurements of OH radical concentration in combustion environments by wavelength-modulation spectroscopy with a 1.55-microm distributed-feedback diode laser

Wavelength-modulation spectroscopy with a standard commercial 1.55-microm distributed-feedback diode laser was applied to in situ quantitative measurements of OH radical concentration in combustion environments. The second-harmonic (2f) signal was generated from absorption by the P11.5 (nu', nu") = (2, 0) overtone vibrational transition of OH at 6421.354 cm(-1). The absorption occurred in the postflame region of a two-dimensional laminar counterflow burner (Tsuji burner) with a 60-mm line-of-sight path length. The postflame region lies between propane-air premixed twin flames stabilized in the Tsuji burner at various equivalence ratios (phi = 0.65-1.0). The OH concentrations were determined by least-squares fitting of theoretical f line shapes to the experimental counterparts. The measured OH concentrations were in general agreement with adiabatic chemical equilibrium predictions. The lower limit of OH detectivity by multiline deconvolution was limited by ubiquitous unidentified high-temperature H(2)O transitions.     

Diode-Laser Wavelength-Modulation Absorption Spectroscopy for Quantitative in situ Measurements of Temperature and OH Radical Concentration in Combustion Gases

The in situ quantitative profiles of temperature and OH radical concentration in a postflame region of methane-air premixed counterflow flames were measured by wavelength modulation spectroscopy with a 1.5-mum external cavity diode laser. The second harmonic (2f) signal was generated from absorption by overtone vibrational-rotational transitions of OH: the ?(3/2) (v?, v?) = (2, 0) P11.5e (nu(0) = 6421.35 cm(-1)) or the ?(3/2) (v?, v?) = (3, 1) P5.5f (nu(0) = 6434.61 cm(-1)) transitions. The absorption occurred in the postflame region between methane-air premixed twin flames stabilized in a two-dimensional laminar counterflow burner (Tsuji burner) with a 60-mm line-of-sight path length. The temperature and OH concentration profiles at an equivalence ratio of phi = 0.85 were determined by least-squares fitting of theoretical 2f line shapes to the experimental counterparts and by calculation of the ratio of the line intensities of the two different OH transitions (two-line thermometry). The measured temperature and OH concentration profiles were cross checked by Rayleigh scattering thermometry, thermocouple measurements, and two-dimensional numerical prediction of premixed combustion by use of a detailed chemical kinetic mechanism. The measurements and the prediction showed reasonable agreement.     

Experimental investigation of saturated polarization spectroscopy for quantitative concentration measurements

Polarization-spectroscopy (PS) line shapes and signal intensities are measured in well-characterized hydrogen-air flames operated over a wide range of equivalence ratios. We use both low (perturbative) and high (saturating) pump beam intensities in the counterpropagating pump-probe geometry. The effects of saturation on the line-center signal intensity and the resonance linewidth are investigated. The PS signal intensities are used to measure relative OH number densities in a series of near-adiabatic flames at equivalence ratios (phi) ranging from 0.5 to 1.5. The use of saturating pump intensities minimizes the effect of pump beam absorption, providing more accurate number density measurements. When calibrated to the calculated OH concentration in the phi = 0.6 flame, the saturated PS number density measurements probing the P(1)(2) transition are in excellent agreement with OH absorption measurements, equilibrium calculations of OH number density, and previous saturated degenerate four-wave mixing OH number density measurements.     

Experimental investigation of saturated degenerate four-wave mixing for quantitative concentration measurements

Degenerate four-wave mixing (DFWM) line shapes and signal intensities are measured experimentally in well-characterized hydrogen-air flames operated over a wide range of equivalence ratios. We use both low (perturbative) and high (saturating) beam intensities in the phase-conjugate geometry. Resonances in the A 2Sigma+ -X 2II (0,0) band of OH are probed with multiaxial-mode laser radiation. The effects of saturation on the line-center signal intensity and the resonance linewidth are investigated. The DFWM signal intensities are used to measure OH number densities in a series of near-adiabatic flames at equivalence ratios ranging from 0.5 to 1.5. Use of saturating pump intensities minimizes the effects of beam absorption, providing more-accurate number density measurements. The saturated DFWM results are in excellent agreement with OH absorption measurements and equilibrium calculations of OH number density. The polarization dependence of the P(1)(2) and R(2)(1) resonances is investigated in both laser intensity regimes. There is a significant change in relative reflectivities for different polarization configurations when saturated.     

Single diode laser sensor for wide-range H2O temperature measurements

A single diode laser absorption sensor (near 1477 nm) useful for simultaneous temperature and H2O concentration measurements is developed. The diode laser tunes approximately 1.2 cm(-1) over three H2O absorption transitions in each measurement. The line strengths of the transitions are measured over a temperature range from 468 to 977 K, based on high-resolution absorption measurements in a heated static cell. The results indicate that the selected transitions are suitable for sensitive temperature measurements in atmospheric pressure combustion systems using absorption line ratios. Comparing the results with HITRAN 96 data, it appears that these transitions will be sensitive over a wide range of temperatures (450-2000 K), suggesting applicability for combustion measurements.     

In situ combustion measurements of CO with diode-laser absorption near 2.3 microm

In situ measurements of CO concentration were recorded with tunable diode-laser absorption spectroscopy techniques in both the exhaust and the immediate post-flame regions of an atmospheric-pressure flat-flame burner operating on ethylene air. Two room-temperature cw single-mode InGaAsSb/AlGaAsSb diode lasers operating near 2.3 microm were tuned over individual transitions in the CO first overtone band (v' = 2 <-- v" = 0) to record high-resolution absorption line shapes in the exhaust duct [79 cm above the burner, approximately 470 K; R(15) transition at 4311.96 cm(-1)] and the immediate postflame zone [1.5 cm above the burner, 1820-1975 K; R(30) transition at 4343.81 cm(-1)]. The CO concentration was determined from the measured absorption and the gas temperature, which was monitored with type-S thermocouples. For measurements in the exhaust duct, the noise-equivalent absorbance was approximately 3 x 10(-5) (50-kHz detection bandwidth, 50-sweep average, 0.1-s total measurement time), which corresponds to a CO detection limit of 1.5 ppm m at 470 K. Wavelength modulation spectroscopy techniques were used to improve the detection limit in the exhaust to approximately 0.1 ppm m (approximately 500-Hz detection bandwidth, 20-sweep average, 0.4-s total measurement time). For measurements in the immediate postflame zone, the measured CO concentrations in the fuel-rich flames were in good agreement with chemical equilibrium predictions. These experiments demonstrate the utility of diode-laser absorption sensors operating near 2.3 microm for in situ combustion emission monitoring and combustion diagnostics.     

Comparison of nanosecond and picosecond excitation for interference-free two-photon laser-induced fluorescence detection of atomic hydrogen in flames

Two-photon laser-induced fluorescence (TP-LIF) line imaging of atomic hydrogen was investigated in a series of premixed CH4/O2/N2, H2/O2, and H2/O2/N2 flames using excitation with either picosecond or nanosecond pulsed lasers operating at 205 nm. Radial TP-LIF profiles were measured for a range of pulse fluences to determine the maximum interference-free signal levels and the corresponding picosecond and nanosecond laser fluences in each of 12 flames. For an interference-free measurement, the shape of the TP-LIF profile is independent of laser fluence. For larger fluences, distortions in the profile are attributed to photodissociation of H2O, CH3, and/or other combustion intermediates, and stimulated emission. In comparison with the nanosecond laser, excitation with the picosecond laser can effectively reduce the photolytic interference and produces approximately an order of magnitude larger interference-free signal in CH4/O2/N2 flames with equivalence ratios in the range of 0.5< or =Phi< or =1.4, and in H2/O2 flames with 0.3< or =Phi< or =1.2. Although photolytic interference limits the nanosecond laser fluence in all flames, stimulated emission, occurring between the laser-excited level, H(n=3), and H(n=2), is the limiting factor for picosecond excitation in the flames with the highest H atom concentration. Nanosecond excitation is advantageous in the richest (Phi=1.64) CH4/O2/N2 flame and in H2/O2/N2 flames. The optimal excitation pulse width for interference-free H atom detection depends on the relative concentrations of hydrogen atoms and photolytic precursors, the flame temperature, and the laser path length within the flame.     

Determination of H2O and CO2 concentrations in fluid inclusions in minerals using laser decrepitation and capacitance manometer analysis

Water and carbon dioxide concentrations within individual and selected groups of fluid inclusions in quartz were analyzed by using laser decrepitation and quantitative capacitance manometer determination. The useful limit of detection (calculated as ten times the typical background level) is about 5 x 10(-10) mol of H2O and 5 x 10(-11) mol of CO2; this H2O content translates into an aqueous fluid inclusion approximately 25 micrometers in diameter. CO2/H2O determinations for 38 samples (100 separate measurements) have a range of H2O amounts of 5.119 x 10(-9) to 1.261 x 10(-7) mol; CO2 amounts of 7.216 x 10(-10) to 1.488 x 10(-8) mol, and CO2/H2O mole ratios of 0.011 to 1.241. Replicate mole ratio determinations of CO2/H2O for three identical (?) clusters of inclusions in quartz have average mole ratios of 0.0305 +/- 0.0041 1 sigma. Our method offers much promise for analysis of individual fluid inclusions, is sensitive, is selective when the laser energy is not so great as to melt the mineral (laser pits approximately 50 micrometers in diameter), and permits rapid analysis (approximately 1 h per sample analysis).     

N2 and CO vibrational CARS and H2 rotational CARS spectroscopy of CH4/N2O flames

Broadband CARS spectra of N2 and CO have been obtained from the postflame gases of rich CH4/N2O flames using the nonplanar BOXCARS technique. The temperature and concentration of both N2 and CO in these flames were estimated from CARS spectra with the aid of model calculations and agreed with standard thermochemical predictions. In addition, several pure rotational H2 CARS transitions, certain of which had been previously unobserved, were seen in several spectral regions, most notably in both the CO and NO CARS regions. These observations are important in future modeling of CARS data.     

Shock-tube study of high-pressure H2O spectroscopy

Water-vapor absorption features near 7117, 7185, and 7462 cm(-1) were probed at pressures to 65 atm (1 atm = 760 Torr) and temperatures to 1800 K in shock-heated mixtures of H(2)O in N(2) and Ar with a diode-laser source. Calculated absorbances based on Voigt line shapes and measured line parameters were in good agreement, within 10%, with measured absorbances at 7185.4 and 7117.4 cm(-1). We obtained temperature-dependent N(2) and Ar shift parameters for H(2)O absorption features by shifting the calculated spectra to match the recorded absorption scan. Absorbance simulations based on line parameters from HITRAN and HITEMP were found to be similar over the range of temperatures 600-1800 K and were within 25% of the measurements. The combined use of Toth's [Appl. Opt. 36, 4851 (1994)] line positions and strengths and HITRAN broadening parameters resulted in calculated absorption coefficients that were within 15% of the measurements at all three probed wavelengths.     

Diode laser sensor for measurements of CO, CO(2), and CH(4) in combustion flows

A diode laser sensor has been applied to monitor CO, CO(2), and CH(4) in combustion gases with absorption spectroscopy and fast extraction-sampling techniques. Survey spectra of the CO 3nu band (R branch) and the 2nu(1) + 2nu(2)(0) + nu(3) CO(2) band (R branch) near 6350 cm(-1) and H(2)O lines from the nu(1) + 2nu(2) and 2nu(2) + nu(3) bands in the spectral region from 6345 to 6660 cm(-1) were recorded and compared with calculated spectra (from the HITRAN 96 database) to select optimum transitions for species detection. Species concentrations above a laminar, premixed, methane-air flame were determined from measured absorption in a fast-flow multipass absorption cell containing probe-sampled combustion gases; good agreement was found with calculated chemical equilibrium values.     

Laser-induced breakdown spectroscopy of gas mixtures of air, CO2, N2, and C3H8 for simultaneous C, H, O, and N measurement

Laser-induced breakdown spectroscopy (LIBS) was applied for simultaneous measurement of the elements C, H, N, and O in CO2-air, C3H8-CO2, and C3H8-N2 gas mixtures at atmospheric pressure. A single 7-mm-diameter aperture at the sample chamber was used for 1064-nm Nd:YAG laser irradiation and plasma signal output to an echelle spectrometer. Double-pulse laser bursts of approximately 8-ns pulse width (FWHM) and 250-ns interpulse separation were applied to increase the plasma signal. Calibration curves of the LIBS signal versus the partial pressure or the atomic abundance ratios were taken by dilution series in intervals that are relevant in the combustion of heptane (C7H16) near an equivalence ratio of 1.     

Emission characteristics of coal combustion in different O2/N2, O2/CO2 and O2/RFG atmosphere

This study investigates the emission characteristics of CO(2), SO(2) and NOx in the flue gas of coal combustion by varying the compositions and concentrations of feed gas (O(2)/CO(2)/N(2)) and the ratios of recycled flue gas. The differences between O(2)/recycled flue gas (O(2)/RFG) combustion and general air combustion are also discussed. Experimental results indicate that the maximum concentration of CO(2) in O(2)/CO(2) combustion system is 95% as the feed gas is 30% O(2)/70% CO(2). The average concentration of CO(2) in the flue gas of O(2)/CO(2) coal combustion system is higher than 90% and much higher than that of O(2)/N(2) coal combustion system. This high concentration of CO(2) is beneficial for the separation of CO(2) from the flue gas by adsorption or absorption technologies. The maximum concentration of CO(2) in O(2)/N(2) combustion system is only 34% at the feed gas 50% O(2)/50% N(2), the concentration of CO(2) is increased with the concentration of O(2) in feed gas. By O(2)/CO(2) combustion technology, higher concentration of SO(2) is produced as the feed gas is 30% O(2)/70% CO(2) or 40% O(2)/60% CO(2), while higher concentration of NOx is produced as the feed gas is 20% O(2)/80% CO(2) or 50% O(2)/50% CO(2). The mass flow rates of CO(2), SO(2) and NOx in the flue gas are all increased with the ratio of recycled flue gas except for the feed gas 20% O(2)/80% CO(2). The enhanced mass flow rates of air pollutants in such O(2)/RFG combustion system are also beneficial for improving the control efficiencies of air pollution control devices. By O(2)/N(2) combustion technology, higher concentrations of SO(2) and NOx are produced as the feed gas is 21% O(2)/79% N(2). The results also indicate that the formation of NOx in general air combustion system is higher than that in O(2)/RFG or O(2)/CO(2) combustion system.     

Flame flow tagging velocimetry with 193-nm H2O photodissociation

In a new nonintrusive, instantaneous flow tagging method called hydroxyl tagging velocimetry (HTV), a molecular grid of hydroxyl (OH) radicals is written into a flame and the displaced grid is imaged at a later time to give the flame's velocity profile. Single-photon photodissociation of vibrationally excited H(2)O, when a 193-nm ArF excimer laser is used, produces a tag line of superequilibrium OH and H photoproducts in a high-temperature flow field that itself may contain ambient OH. The tag line OH concentration is composed mostly of direct OH photoproducts, but OH is also indirectly produced through H photoproduct reactions with oxygen-bearing species. For lean and modestly rich flames the OH tag lifetime is of the order of 1 ms. For very rich H(2)-air flames (equivalence ratio of 4.4) the lifetime drops to 200 ns. After displacement the position of the OH tag line is revealed through fluorescence caused by OH (A-X) (3 <-- 0) excitation by using a 248-nm tunable KrF excimer laser. A HTV grid of multiple tag lines, providing multipoint velocity information, is experimentally demonstrated in a turbulent H(2)/N(2)-air diffusion flame.     

Near-infrared diode laser absorption diagnostic for temperature and water vapor in a scramjet combustor

Tunable diode laser absorption measurements of gas temperature and water concentration were made at the exit of a model scramjet combustor fueled on JP-7. Multiplexed, fiber-coupled, near-infrared distributed feedback lasers were used to probe three water vapor absorption features in the 1.34-1.47 microm spectral region (2v1 and vl + v3 overtone bands). Ratio thermometry was performed using direct-absorption wavelength scans of isolated features at a 4-kHz repetition rate, as well as 2f wavelength modulation scans at a 2-kHz scan rate. Large signal-to-noise ratios demonstrate the ability of the optimally engineered optical hardware to reject beam steering and vibration noise. Successful measurements were made at full combustion conditions for a variety of fuel/air equivalence ratios and at eight vertical positions in the duct to investigate spatial uniformity. The use of three water vapor absorption features allowed for preliminary estimates of temperature distributions along the line of sight. The improved signal quality afforded by 2f measurements, in the case of weak absorption, demonstrates the utility of a scanned wavelength modulation strategy in such situations.     

Quantitative CARS spectroscopy of CO2 and N2O

Experimental and theoretical investigations of the CARS spectroscopy of CO2(2v2) and N2O(v3) were carried out. The experimental spectra were measured in a heated test cell, and excellent agreement with the observed temperature dependences was obtained from numerical simulations. Assignments were made for all hot bands, and the role of collisional narrowing was quantified. Observed nonresonant susceptibility effects in pure N2O have made it possible to estimate the nonresonant background susceptibility for this molecule by using the resonant contribution as a reference calibration.     

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.     

稀土Sm2O3掺杂钙硼硅发光玻璃的光致发光行为研究

用高温熔融法制备了掺杂Sm2O3的CaO-B2 O3-SiO2(CBS)发光玻璃材料,并对其光谱学特性进行了研究。紫外-可见(UV-Vis)吸收光谱表明Sm2O3掺杂发光玻璃在紫外区有较强吸收并在可见光区具有良好的透过率。光谱学测试表明,掺杂发光玻璃在404nm激发下出现Sm3+的特征发射峰,峰值波长分别位于565.8、602.8和650.4nm。同时,Sm2O3掺杂发光玻璃的荧光发射强度随Sm2O3掺杂摩尔分数的增加出现浓度猝灭效应,其Sm2O3掺杂猝灭浓度约为0.10%(摩尔分数)。此外,在365nm紫外光照射下,Sm2 O3掺杂发光玻璃呈现出红橙色发光,表明其具有将紫外光转换成红橙光的能力,可以进一步应用于光转换和光发射领域。     

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.     

In Situ Combustion Measurements of CO(2) by Use of a Distributed-Feedback Diode-Laser Sensor Near 2.0 mum

High-resolution absorption measurements of CO(2) were made in a heated static cell and in the combustion region above a flat-flame burner for the development of an in situ CO(2) combustion diagnostic based on a distributed-feedback diode laser operating near 2.0 mum. Calculated absorption spectra of high-temperature H(2)O and CO(2) were used to find candidate transitions for CO(2) detection, and the R(50) transition at 1.997 mum (the nu(1) + 2nu(2) + nu(3) band) was selected on the basis of its line strength and its isolation from interfering high-temperature water absorption. Measurements of spectroscopic parameters such as the line strength, the self-broadening coefficient, and the line position were made for the R(50) transition, and an improved value for the line strength is reported. The combustion-product populations of CO(2) in the combustion region above a flat-flame burner were determined in situ to verify the measured spectroscopic parameters and to demonstrate the feasibility of the diode-laser sensor.