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. III. Comparison of A-X excitation schemes

Laser-induced fluorescence (LIF) has proven a reliable technique for nitric oxide (NO) diagnostics in practical combustion systems. However, a wide variety of different excitation and detection strategies are proposed in the literature without giving clear guidelines of which strategies to use for a particular diagnostic situation. We give a brief review of the high-pressure NO LIF diagnostics literature and compare strategies for exciting selected transitions in the A-X(0, 0), (0, 1), and (0, 2) bands using a different detection bandpass. The strategies are compared in terms of NO LIF signal strength, attenuation of laser and signal light in the hot combustion gases, signal selectivity against LIF interference from O2 and CO2, and temperature and pressure sensitivity of the LIF signal. The discussion is based on spectroscopic measurements in laminar premixed methane-air flames at pressures between 1 and 60 bars and on NO and O2 LIF spectral simulations.     

Background corrections for laser-induced-fluorescence measurements of nitric oxide in lean, high-pressure, premixed methane flames

An experimental technique is presented that both minimizes and accounts for the interference background when laser-induced-fluorescence (LIF) measurements are made of NO in lean, high-pressure, premixed, CH(4)/O(2)/N(2) flames. Measurement interferences such as fluorescence and Raman scattering from secondary species become increasingly important for high-pressure LIF studies. O(2) fluorescence interferences are particularly troublesome in lean premixed flames. An excitation-detection scheme that minimizes the effects of these interferences is identified. A procedure that corrects the resulting LIF signal so as to account for any remaining interference signal is then developed. This correction is found to vary from less than 10% of the overall NO signal at atmospheric pressure to over 40% of the overall signal at 14.6 atm for LIF measurements of NO in a series of worst-case flames (phi = 0.6, dilution ratio 2.2). The correction technique is further demonstrated to be portable over a useful range of flame conditions at each pressure.     

Quantitative temperature measurements in high-pressure flames with multiline NO-LIF thermometry

An accurate temperature measurement technique for steady, high-pressure flames is investigated using excitation wavelength-scanned laser-induced fluorescence (LIF) within the nitric oxide (NO) A-X(0, 0) band, and demonstration experiments are performed in premixed methane/air flames at pressures between 1 and 60 bars with a fuel/air ratio of 0.9. Excitation spectra are simulated with a computational spectral simulation program (LIFSim) and fit to the experimental data to extract gas temperature. The LIF scan range was chosen to provide sensitivity over a wide temperature range and to minimize LIF interference from oxygen. The fitting method is robust against elastic scattering and broadband LIF interference from other species, and yields absolute, calibration-free temperature measurements. Because of loss of structure in the excitation spectra at high pressures, background signal intensities were determined using a NO addition method that simultaneously yields nascent NO concentrations in the postflame gases. In addition, fluorescence emission spectra were also analyzed to quantify the contribution of background signal and to investigate interference in the detection band-width. The NO-LIF temperatures are in good agreement with intrusive single-color pyrometry. The proposed thermometry method could provide a useful tool for studing high-pressure flame chemistry as well as provide a standard to evaluate and validate fast-imaging thermometry techniques for practical diagnostics of high-pressure combustion systems.     

Experimental assessment of O(2) interferences on laser-induced fluorescence measurements of NO in high-pressure, lean premixed flames by use of narrow-band and broadband detection

We experimentally investigate the influence of O(2) interferences on laser-induced fluorescence measurements of NO in lean methane-fueled flames at a range of pressures for both narrow-band and broadband fluorescence detection. We identify NO excitation schemes that minimize O(2) interferences. From detection scans we obtain interference spectra for the different NO excitation schemes. We then identify optimum excitation-detection schemes for narrow-band detection measurements of NO. To simulate broadband detection experiments, we numerically apply five different filter combinations to the experimentally obtained detection scans. We develop filter-assessment parameters to judge the effectiveness of the different filtering schemes, and we establish a methodology for evaluating broadband excitation-detection strategies. From this research we identify optimum excitation-detection schemes for broadband detection measurements of NO.     

Two-photon nitric oxide laser-induced fluorescence measurements in a diesel engine

A two-photon nitric oxide (NO) laser-induced fluorescence (LIF) technique was developed and applied to study in-cylinder diesel combustion. The technique prevents many problems associated with in-cylinder, single-photon NO planar-laser-induced fluorescence measurements, including fluorescence interference from the Schumann-Runge bands of hot O2, absorption of a UV excitation beam by in-cylinder gases, and difficulty in rejecting scattered laser light while simultaneously attempting to maximize fluorescence signal collection. Verification that the signal resulted from NO was provided by tuning of the laser to a vibrational off-resonance wavelength that showed near-zero signal levels, which resulted from either fluorescence or interference at in-cylinder pressures of as much as 20 bar. The two-photon NO LIF signal showed good qualitative agreement with NO exhaust-gas measurements obtained over a wide range of engine loads.     

Comparison of nanosecond and picosecond excitation for two-photon laser-induced fluorescence imaging of atomic oxygen in flames

Two-photon laser-induced fluorescence (LIF) imaging of atomic oxygen is investigated in premixed hydrogen and methane flames with nanosecond and picosecond pulsed lasers at 226 nm. In the hydrogen flame, the interference from photolysis is negligible compared with the LIF signal from native atomic oxygen, and the major limitations on quantitative measurements are stimulated emission and photoionization. Excitation with a nanosecond laser is advantageous in the hydrogen flames, because it reduces the effects of stimulated emission and photoionization. In the methane flames, however, photolytic interference is the major complication for quantitative O-atom measurements. A comparison of methane and hydrogen flames indicates that vibrationally excited CO2 is the dominant precursor for laser-generated atomic oxygen. In the methane flames, picosecond excitation offers a significant advantage by dramatically reducing the photolytic interference. The prospects for improved O-atom imaging in hydrogen and hydrocarbon flames are presented.     

Quantification of NO A-X (0, 2) laser-induced fluorescence: investigation of calibration and collisional influences in high-pressure flames

Laser-induced-fluorescence techniques have been used successfully for quantitative two-dimensional measurements of nitric oxide. NO A-X(0, 2) excitation at 248 nm recently found applications in internal-combustion engines. We assess the collisional processes that influence quantification of signal intensities in terms of saturation, rotational energy transfer, and line broadening, using laminar high-pressure methane/air and n-heptane/air flames at pressures as high as 80 bars (8 x 10(6) Pa). A calibration method that is applicable in technical combustion systems based on addition of NO to the burning flame is investigated for various air/fuel ratios and pressures and yields information about the influence of NO reburn processes.     

Two-Line Laser-Induced Fluorescence Imaging of Vibrational Temperatures in a NO-Seeded Flame

Two-dimensional temperature fields are measured in lean and sooting flames by means of two-color laser-induced fluorescence (LIF) imaging that uses seeded NO. Vibrational thermometry is performed by the probing of different vibrational ground-state levels. Spectral properties of the excited transitions within the A (2)?(+)-X (2)? system are well known from previous studies. The energy difference of 1974 cm(-1) between the (0, 0)Q(1) + P(21)(33.5) and the (0, 2)O(12)(5.5) lines offers great sensitivity in the temperature range that is relevant for combustion processes. Excitation is possible by use of a tunable KrF excimer laser on its fundamental (248-nm) and Raman shifted (in H(2), 225-nm) wavelengths. An excitation scheme for instantaneous two-line measurements by use of a single laser is developed. The possibility of single-shot measurements is discussed.     

Laser-induced-fluorescence detection of nitric oxide in high-pressure flames with A-X(0, 2) excitation

Laser-induced fluorescence techniques have been used successfully for quantitative two-dimensional measurements of nitric oxide. The commonly applied D-X(0, 1) or A-X(0, 0) schemes are restricted to atmospheric-pressure flames and engines driven with gaseous fuels because of strong attenuation of the exciting laser beam by combustion intermediates. The properties of a detection scheme for which excitation in the nitric oxide A-X(0, 2) band was used were investigated. We discuss the advantages of the A-X(0, 2) system (excited at 247.95 nm) based on measurements in laminar premixed methane/air flames at 1-40 bars.     

Trace detection of atmospheric NO2 by laser-induced fluorescence using a GaN diode laser and a diode-pumped YAG laser

We report on the development of a highly sensitive detection system for measuring atmospheric NO(2) by means of a laser-induced fluorescence (LIF) technique at 473 nm using a diode-pumped Nd:YAG laser. A GaN-based laser diode emitting at 410 nm is also used as an alternative fluorescence-excitation source. For laboratory calibrations, standard NO(2) gas is diluted with synthetic air and is introduced into a fluorescence-detection cell. The NO(2) LIF signal is detected by a photomultiplier tube and processed by a photon-counting method. The minimum detectable limits of the NO(2) instrument developed have been estimated to be 0.14 ppbv and 0.39 ppbv (parts per billion, 10(-9), by volume) in 60 s integration time (signal-to-noise ratio of 2) for 473 and 410 nm excitation systems, respectively. Practical performance of the instrument has been demonstrated by the 24 hour continuous measurements of ambient NO(2) in a suburban area.     

High-sensitivity instrument for measuring atmospheric NO2

We report on the development of a high-sensitivity detection system for measuring atmospheric NO2 using a laser-induced fluorescence (LIF) technique around 440 nm. A tunable broad-band optical parametric oscillator laser pumped by the third harmonic of a Nd:YAG laser is used as a fluorescence excitation source. The laser wavelength is tuned at peak and bottom wavelengths around 440 nm alternatively, and the difference signal at the two wavelengths is used to extract the NO2 concentration. This procedure can give a good selectivity for NO2 and avoid interferences of fluorescent or particulate species other than NO2 in the sample air. The NO2 instrument developed has a sensitivity of 30 pptv in 10 s and S/N = 2. The practical performance of the detection system is tested in the suburban area for 24 h. The intercomparisons between the LIF instrument and a photofragmentation chemiluminescence (PF-CL) instrument have been performed under laboratory conditions. The correlation between the two instruments is measured up to 1000 pptv. A good linear relationship between the LIF measurements and the PF-CL measurements is obtained.     

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.     

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.     

Application of 266-nm and 355-nm Nd:YAG laser radiation for the investigation of fuel-rich sooting hydrocarbon flames by raman scattering

We describe the use of linear Raman scattering for the investigation of fuel-rich sooting flames. In comparison, the frequency-tripled and -quadrupled fundamental wavelengths of a Nd:YAG laser have been used as an excitation source for study of the applicability of these laser wavelengths for analysis of sooting flames. The results obtained show that, for the investigation of strongly sooting flames, 266-nm excitation is better than 355-nm excitation. Although the entire fluorescence intensity of polycyclic aromatic hydrocarbons (PAHs) decreases with rising excitation wavelength, there is increased interference with the Raman signals by displacement of the spectral region of the Raman signals toward the fluorescence maximum of the laser-induced fluorescence emissions. Besides the broadband signals of PAHs, narrowband emissions of laser-produced C2 occur in the spectra of sooting flames and affect the Raman signals. These C2 emission bands are completely depolarized and can be separated by polarization-resolved detection. A comparison of the laser-induced fluorescence emissions of an ethylene flame with those of a methane flame shows the same spectral features, but the intensity of the emissions is larger by a factor of 5 for the ethylene fuel. Using 266-nm radiation for Raman signal excitation makes possible measurements in the ethylene flame also.     

Pressure dependence of laser-induced fluorescence from acetone

The use of laser-induced fluorescence (LIF) from acetone is becoming increasingly widespread as a diagnostic of mixing processes in both reacting and nonreacting flows. One of the major reasons for its increasing use is that the acetone LIF signal is believed to be nearly independent of pressure because of fast intersystem crossing from the first excited singlet state, from which the fluorescence signal originates, to the first excited triplet state, which does not fluoresce. To evaluate the use of acetone LIF at pressures higher than atmospheric, we have performed a study of acetone LIF in a flowing gas cell at pressures up to 8 atm. We used four different buffer gases: air, nitrogen, methane, and helium. Surprisingly, we find that the acetone fluorescence quantum efficiency increases slightly (~30%-50%) as the buffer-gas pressure increases from 0.6 to 5 atm for all four buffer gases. When the buffer gas is air, we observe a decrease in the acetone fluorescence quantum efficiency as the buffer-gas pressure is increased from 5 to 8 atm; for the other three buffer gases the quantum efficiency is constant to within experimental error in this pressure regime. The observed pressure dependence of the acetone fluorescence signal is explained by use of a four-level model. The increase in the fluorescence quantum efficiency with pressure is probably the result of incomplete vibrational relaxation coupled with an increase in the intersystem crossing rate with increasing vibrational excitation in the first excited singlet manifold.     

Measurements of absolute CH concentrations by cavity ring-down spectroscopy and linear laser-induced fluorescence in laminar, counterflow partially premixed and nonpremixed flames at atmospheric pressure

We report quantitative, spatially resolved measurements of methylidyne concentration ([CH]) in laminar, counterflow partially premixed and nonpremixed flames at atmospheric pressure by using both cavity ring-down spectroscopy (CRDS) and linear laser-induced fluorescence (LIF) in the A-X (0, 0) band. Three partially premixed (phiB = 1.45, 1.6, 2.0) flames plus a single nonpremixed methane-air flame are investigated at a global strain rate of 20 s(-1). These quantitative measurements are compared with predictions from an opposed-flow flame code when utilizing two GRI chemical kinetic mechanisms (versions 2.11 and 3.0). The LIF measurements of [CH] are corrected for variations in the electronic quenching rate coefficient by using predicted major species concentrations and temperatures along with quenching cross sections for CH that are available in the literature. The peak CH concentration obtained by CRDS is used to calibrate the quenching-corrected LIF measurements. Excellent agreement is obtained between CH concentration profiles measured by using the CRDS and LIF techniques. The spatial location of the CH layer is very well predicted by GRI 3.0; moreover, the measured and predicted CH concentrations are in good agreement for all the flames of this study.     

Laser-induced incandescence for soot diagnostics at high pressures

The influence of pressure on laser-induced incandescence (LII) is investigated systematically in premixed, laminar sooting ethylene/air flames at 1-15 bar with wavelength-, laser fluence-, and time-resolved detection. In the investigated pressure range the LII signal decay rate is proportional to pressure. This observation is consistent with the prediction of heat-transfer models in the free-molecular regime. Pressure does not systematically affect the relationship between LII signal and laser fluence. With appropriate detection timing the pressure influence on LII signal's proportionality to soot volume faction obtained by extinction measurements is only minor compared with the variation observed in different flames at fixed pressures. The implications for particle sizing and soot volume fraction measurements using LII techniques at elevated pressures are discussed.     

Single-pulse, two-line temperature-measurement technique using KrF laser-induced O(2) fluorescence

A new single-pulse, two-line laser-induced O(2) fluorescence (LIF) temperature-measurement technique was demonstrated. The fluorescence spectrum obtained with multichannel detection following simultaneous excitation of two coincident transitions in the 0-6 and the 2-7 bands of the B(3)Σ(-)(u)-X(3)Σ(-)(g) Schumann-Runge system was used to determine the gas temperature. The rms error of 100-pulse average LIF temperature measurements, referenced to their corresponding thermocouple measurements, was 1.3% over a temperature range of 1300-1800 K in atmospheric air. Photon shot noise was found to be the primary source of uncertainty for these measurements in a quiescent environment. Single-pulse temperature-measurement uncertainties (1 σ) ranged from approximately 13% at 1300 K to 7% at 1800 K.     

Investigation of optical fibers for gas-phase, ultraviolet laser-induced-fluorescence (UV-LIF) spectroscopy

We investigate the feasibility of transmitting high-power, ultraviolet (UV) laser pulses through long optical fibers for laser-induced-fluorescence (LIF) spectroscopy of the hydroxyl radical (OH) and nitric oxide (NO) in reacting and non-reacting flows. The fundamental transmission characteristics of nanosecond (ns)-duration laser pulses are studied at wavelengths of 283 nm (OH excitation) and 226 nm (NO excitation) for state-of-the-art, commercial UV-grade fibers. It is verified experimentally that selected fibers are capable of transmitting sufficient UV pulse energy for single-laser-shot LIF measurements. The homogeneous output-beam profile resulting from propagation through a long multimode fiber is ideal for two-dimensional planar-LIF (PLIF) imaging. A fiber-coupled UV-LIF system employing a 6 m long launch fiber is developed for probing OH and NO. Single-laser-shot OH- and NO-PLIF images are obtained in a premixed flame and in a room-temperature NO-seeded N(2) jet, respectively. Effects on LIF excitation lineshapes resulting from delivering intense UV laser pulses through long fibers are also investigated. Proof-of-concept measurements demonstrated in the current work show significant promise for fiber-coupled UV-LIF spectroscopy in harsh diagnostic environments such as gas-turbine test beds.