Infrared laser-absorption sensing for combustion gases

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.

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

Wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments

The availability of new lasers in the nearand mid infrared spectral region has led to the development of sensors for gas measurements that are now applied extensively in the process industries. Based on tunable diode laser absorption spectroscopy (TOLAS) molecules like O2, CH* H2O, CO, CO2, NH3, HC1 and HF can be detected in-situ with high selectivity and sensitivity in continuous, real time operation. Using sensitive detection techniques like wavelength modulation spectroscopy (WMS), often low ppb and ppm concentration measurements with Is integration time are feasible. Detection limits can be improved by using extractive sampling and a long multi-pass cell. TOLAS has become an accepted technique in the process industries for difficult measurement tasks, because it is compatible with high temperatures, pressures, dust levels and corrosive media. Gas concentrations, temperatures, velocities and pressures can be determined. TOLAS is used widely for continuous emission monitoring and process control with over 1,000 field instruments worldwide. In this article, after an introduction to the basics of TOLAS, several interesting applications and installations in various process industrial units with some examples from other industries are reviewed.

Tunable diode laser absorption spectroscopy (tdlas) in the process industries – a review

The development and initial demonstration of a scanned-wavelength, first-harmonic-normalized, wavelength-modulation spectroscopy with nf detection (scanned-WMS-nf/1f) strategy for calibration-free measurements of gas conditions are presented. In this technique, the nominal wavelength of a modulated tunable diode laser (TDL) is scanned over an absorption transition to measure the corresponding scanned-WMS-nf/1f spectrum. Gas conditions are then inferred from least-squares fitting the simulated scanned-WMS-nf/1f spectrum to the measured scanned-WMS-nf/1f spectrum, in a manner that is analogous to widely used scanned-wavelength direct-absorption techniques. This scanned-WMS-nf/1f technique does not require prior knowledge of the transition linewidth for determination of gas properties. Furthermore, this technique can be used with any higher harmonic (i.e., n>1), modulation depth, and optical depth. Selection of the laser modulation index to maximize both signal strength and sensitivity to spectroscopic parameters (i.e., gas conditions), while mitigating distortion, is described. Last, this technique is demonstrated with two-color measurements in a well-characterized supersonic flow within the Stanford Expansion Tube. In this demonstration, two frequency-multiplexed telecommunication-grade TDLs near 1.4 μm were scanned at 12.5 kHz (i.e., measurement repetition rate of 25 kHz) and modulated at 637.5 and 825 kHz to determine the gas temperature, pressure, H2O mole fraction, velocity, and absorption transition lineshape. Measurements are shown to agree within uncertainty (1%–5%) of expected values.

Fitting of calibration-free scanned-wavelength-modulation spectroscopy spectra for determination of gas properties and absorption lineshapes.

Crank angle-resolved measurements of gas temperature and water vapor concentration are made during the compression stroke of an internal combustion (IC) engine with a novel diode laser absorption sensor. The temperature is determined from the ratio of optical absorption for two overtone transitions of water vapor in the intake gas mixture, and the H2O concentration is determined from this inferred temperature and the absorption for one of the transitions. The measurements sample a short-path region (6 mm) of the in-cylinder gases near the spark plug, which has been modified to provide optical access. Hence, the sensor can be installed in nearly any engine via the spark plug port. Wavelength modulation spectroscopy is employed with second harmonic detection (WMS-2f) to enable the short-path measurements over a range of temperatures and pressures from 500 to 1050 K and 1 to 50 atm at a bandwidth of 7.5 kHz. The accuracy of the sensor is validated in a static cell, giving RMS errors of less than 3% in temperature and less than 3.6% in H2O concentration over a wide range of conditions. Crank angle-resolved measurements are performed in unfired and fired engine cylinders, illustrating the potential of this sensor for investigating a range of difficult-to-model trends in current and proposed IC engine combustion schemes.

Rapid measurements of temperature and H2O concentration in IC engines with a spark plug-mounted diode laser sensor

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Rapid measurements of temperature and H2O concentration in IC engines with a spark plug-mounted diode laser sensor