Jay B. Jeffries

Calibration-free wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments

We present a practical implementation of calibration-free wavelength-modulation spectroscopy with second harmonic detection (WMS-2f) for measurements of gas temperature and concentration in harsh environments. The method is applicable to measurements using lasers with synchronous wavelength and intensity modulation (such as injection current-tuned diode lasers). The key factors that enable measurements without the on-site calibration normally associated with WMS are (1) normalization of the WMS-2f signal by the first harmonic (1f) signal to account for laser intensity, and (2) the inclusion of laser-specific tuning characteristics in the spectral-absorption model that is used to compare with measured 1f-normalized, WMS-2f signals to infer gas properties. The uncertainties associated with the calibration-free WMS method are discussed, with particular emphasis on the influence of pressure and optical depth on the WMS signals. Many of these uncertainties are also applicable to calibrated WMS measurements. An example experimental setup that combines six tunable diode laser sources between 1.3 and 2.0 mum into one probe beam for measurements of temperature, H(2)O, and CO(2) is shown. A hybrid combination of wavelength and frequency demultiplexing is used to distinguish among the laser signals, and the optimal set of laser-modulation waveforms is presented. The system is demonstrated in the harsh environment of a ground-test scramjet combustor. A comparison of direct absorption and 1f-normalized, WMS-2f shows a factor of 4 increase in signal-to-noise ratio with the WMS technique for measurements of CO(2) in the supersonic flow. Multidimensional computational fluid-dynamics (CFD) calculations are compared with measurements of temperature and H(2)O using a simple method that accounts for the influence of line-of-sight (LOS) nonuniformity on the absorption measurements. The comparisons show the ability of the LOS calibration-free technique to gain useful information about multidimensional CFD models.

Collisional Quenching of CH(A), OH(A), and NO(A) in Low Pressure Hydrocarbon Flames

Abstract Excited state lifetimes have been measured for the A-states of CH, OH, and NO in a number of low-pressure, premixed, laminar flow methane flames. From these lifetimes, collisional quenching rates were determined as a function of height above the burner and thus as a function of flame temperature and composition. The results were compared with values calculated using a model of the flame chemistry to predict collider mole fractions, together with parameterizations of quenching rate coefficients for each collider. Measured OH and NO quenching rates agree well with those calculated from these quenching rate coefficients and modeled flame composition data. This indicates that collisional quenching corrections for laser-induced fluorescence measurements can be calculated from knowledge of major species mole fractions and gas temperature. Predicted quenching rates for CH range from agreement with measured values to 27% higher than measured values. This discrepancies suggest insufficient knowledge of high temperature quenching by H2O and N2.

Extension of wavelength-modulation spectroscopy to large modulation depth for diode laser absorption measurements in high-pressure gases

Tunable diode laser absorption measurements at high pressures by use of wavelength-modulation spectroscopy (WMS) require large modulation depths for optimum detection of molecular absorption spectra blended by collisional broadening or dense spacing of the rovibrational transitions. Diode lasers have a large and nonlinear intensity modulation when the wavelength is modulated over a large range by injection-current tuning. In addition to this intensity modulation, other laser performance parameters are measured, including the phase shift between the frequency modulation and the intensity modulation. Following published theory, these parameters are incorporated into an improved model of the WMS signal. The influence of these nonideal laser effects is investigated by means of wavelength-scanned WMS measurements as a function of bath gas pressure on rovibrational transitions of water vapor near 1388 nm. Lock-in detection of the magnitude of the 2f signal is performed to remove the dependence on detection phase. We find good agreement between measurements and the improved model developed for the 2f component of the WMS signal. The effects of the nonideal performance parameters of commercial diode lasers are especially important away from the line center of discrete spectra, and these contributions become more pronounced for 2f signals with the large modulation depths needed for WMS at elevated pressures.

Development of a sensor for temperature and water concentration in combustion gases using a single tunable diode laser

The water vapour spectrum in the 1–2 µm near-infrared region is systematically analysed to find the best absorption transitions for sensitive measurement of H2O concentration and temperature in combustion environments using a single tunable diode laser with typical distributed feedback single-mode scanning range (1 cm−1). The use of a single laser, even with relatively narrow tuning range, can offer distinct advantages over wavelength-multiplexing techniques. The strategy and spectroscopic criteria for selecting optimum wavelength regions and absorption line combinations are discussed. It should be stressed that no single figure of merit can be derived to simplify the selection process, and the optimum line pair should be chosen case by case. Our investigation reveals that the 1.8 µm spectral region is especially promising, and we have identified 10 of the best water line pairs in this spectral region for temperature measurements in flames. Based on these findings, a pair of H2O transitions near 1.8 µm was targeted for the design and development of an initial single-laser sensor for simultaneously measuring H2O concentration and temperature in atmospheric-pressure flames. As part of the sensor development effort, fundamental spectroscopic parameters including the line strength, line-centre frequency and lower state energies of the probed transitions were measured experimentally to improve the current databases. We conclude with demonstration results in a steady and a forced atmospheric-pressure laboratory combustor.

Diode-laser absorption sensor for line-of-sight gas temperature distributions

Line-of-sight diode-laser absorption techniques have been extended to enable temperature measurements in nonuniform-property flows. The sensing strategy for such flows exploits the broad wavelength-scanning abilities (>1.7 nm approximately 30 cm(-1)) of a vertical cavity surface-emitting laser (VCSEL) to interrogate multiple absorption transitions along a single line of sight. To demonstrate the strategy, a VCSEL-based sensor for oxygen gas temperature distributions was developed. A VCSEL beam was directed through paths containing atmospheric-pressure air with known (and relatively simple) temperature distributions in the 200-700 K range. The VCSEL was scanned over ten transitions in the R branch of the oxygen A band near 760 nm and optionally over six transitions in the P branch. Temperature distribution information can be inferred from these scans because the line strength of each probed transition has a unique temperature dependence; the measurement accuracy and resolution depend on the details of this temperature dependence and on the total number of lines scanned. The performance of the sensing strategy can be optimized and predicted theoretically. Because the sensor exhibits a fast time response (~30 ms) and can be adapted to probe a variety of species over a range of temperatures and pressures, it shows promise for industrial application.

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.

Laser-induced fluorescence determination of temperatures in low pressure flames

Spatially resolved temperatures in a variety of low pressure flames of hydrogen and hydrocarbons burning with oxygen and nitrous oxide are determined from OH, NH, CH, and CN laser-induced fluorescence rotational excitation spectra. Systematic errors arising from spectral bias, time delay, and temporal sampling gate of the fluorescence detector are considered. In addition, we evaluate the errors arising from the influences of the optical depth and the rotational level dependence of the fluorescence quantum yield for each radical. These systematic errors cannot be determined through goodness-of-fit criteria and they are much larger than the statistical precision of the measurement. The severity of these problems is different for each radical; careful attention to the experimental design details for each species is necessary to obtain accurate LIF temperature measurements.

Low pressure flame determinations of rate constants for OH(A) and CH(A) chemiluminescence

Abstract Absolute excited state concentrations of OH(A), CH(A,B), and C 2 (d) were determined in three low pressure premixed methane-air flames. Two-dimensional images of chemiluminescence from these states were recorded by a filtered CCD camera, processed by Abel inversion, and calibrated against Rayleigh scattering. Using a previously validated 1-D flame model with known chemistry and excited state quenching rate constants, rate constants are extracted for the reactions CH + O 2 → OH(A) + CO and C 2 H + O → CH(A,B) + CO at flame temperatures. Variations of flame emission intensities with stoichiometry agree well with model predictions.

Vibrational relaxation of OH (X 2Πi, v=2)

Vibrational relaxation rates for the v=2 level of the X 2Πi state of the OH radical have been measured in a low pressure flow system, using a novel two‐laser pump‐and‐probe technique. The OH is prepared in the v=2 level by overtone pumping (2←0) and monitored by ultraviolet laser‐induced fluorescence in the (1,2) band of the A–X system. Scanning the time delay between the lasers at a given collider pressure produces exponential decay whose rate as a function of collider pressure yields the rate constant. We determine values (all cm3 s−1 units) for NH3: (1.20±0.15)×10−10; CH4: (2.3±0.2)×10−12; CO2: (6.7±1.1)×10−13; N2O: (4.6±0.6)×10−13; O2: (2.6±0.54)×10−13; N2 and H2: ≤10−14. Except for ammonia, these are two to three orders of magnitude smaller than those measured for relaxation of v=1 in the A 2Σ+ excited state of OH, where attractive forces appear to play a role.

LIF measurements in methane/air flames of radicals important in prompt-NO formation

Abstract Laser-induced fluorescence measurements have been made of the OH, CH, and NO radicals in slighly rich methane/air flames burning at 30, 70, and 120 torr. Absolute NO and OH concentrations were determined using separate calibration experiments. Temperature profiles were deduced from OH rotational excitation scans, and fluorescence quenching rates for NO and CH were determined. Predicted profiles of the concentrations of these radical species as a function of height above the burner were obtained from a computer model of the flame, and comparison made with experiment. Good agreement is achieved for the relative concentration profiles, the absolute NO concentration, and concentration ratios between 30 and 70 torr, although slight disagreement with the CH profile indicates there remain some unknown aspects of the flame chemistry.