Douglas S. Baer

MULTIPLEXED DIODE-LASER SENSOR SYSTEM FOR SIMULTANEOUS H2O, O2, AND TEMPERATURE MEASUREMENTS

A multiplexed diode-laser sensor system has been developed to monitor multiple species and measure multiple flowfield parameters along a single path with optical fibers and scanned-laser absorption techniques. Two InGaAsP lasers were tuned over H2O transitions near 1347 and 1392 nm, and an AlGaAs laser was tuned over an O2 transition near 760.65 nm to record fully resolved absorption concurrently at a repetition rate of 2 kHz. The system was applied to measure H2O, O2, and temperature simultaneously in a heated static cell and in room air. Wavelength modulation with second-harmonic detection, applied simultaneously in the O2 measurements, yielded a minimum detectable absorbance of 1.6 × 10−6/Hz1/2 and an O2 detectivity of 6.3 parts in 106 in a 1-m path length.

Diode-laser sensor for monitoring multiple combustion parameters in pulse detonation engines

Diode-laser absorption spectroscopy techniques have been adapted and, applied for in situ measurements of pertinent combustion parameters in pulse detonation engines (PDEs). A sensor employing five multiplexed diode lasers operating in the 1300–1800 nm spectral region has been developed for monitoring gas temperature, H2O concentration, liquid fuel concentration, and soot volume fraction. Gas temperature is determined from the ratio of H2O absorbances at different wavelengths: water mole fraction and fuel and soot volume fractions are determined from the measured gas temperature and absorbances at selected wavelengths. The sensors time response (0.5 μs) and non-intrusive, nature make it suitable for measurements in the hostile environment generated by PDEs. The sensor was used to monitor a 4 cm diameter PDE operating on a JP-10/oxygen aerosol. Measurements revealed charges of non-uniform equivalence ratio at ignition. Detonations processing such charges reached 95% of the Chapman-Jouget velocity and gas pressures predicted for a stoichiometric, uniform load. Gas temperature and H2O concentration, however, reached only ≈50% of the Chapman-Jouget predictions, as a result of the decreasing fuel concentration along the length of the engine. The sensor also revealed the presence of hot H2O for a long duration (>100 ms) relative to the duration of the pressure pulse (≈500 μs) in the blowdown following the detonation. The engine performance information recorded by the sensor is expected to enhance PDE modeling and optimization efforts, potentially enabling PDE combustion control.

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.

Scanned- and fixed-wavelength absorption diagnostics for combustion measurements using multiplexed diode lasers

A multiplexed diode-laser sensor system comprising two diode lasers and fiber-optic components has been developed to nonintrusively monitor temperature and species mole fraction over a single path using both scanned-and fixed-wavelength laser absorption spectroscopy techniques. In the scanned-wavelength method, two InGaAsP lasers were current tuned at a 2-kHz rate across H 2 O transitions near 1343 nm and 1392 nm in the 2v 1 and v 1 + v 3 bands. Gas temperature was determined from the ratio of single-sweep integrated line intensities. Species mole fraction was determined from the measured line intensity and the calculated line strength at the measured temperature. In the fixed-wavelength method, the wavelength of each laser was fixed near the peak of each absorption feature using a computer-controlled laser line-locking scheme. Rapid measurements of gas temperature were obtained from the determination of peak line-intensity ratios. The system was applied to measure temperature and species concentration in the postflame gases of an H 2 -O 2 flame. The good agreement between the laser-based measurements obtained using scanned- and fixed-wavelength methods with those recorded with thermocouples demonstrates the flexibility and utility of the multiplexed diode-laser sensor system and the potential for rapid, continuous measurements of gasdynamic parameters in high-speed or transient flows with difficult optical access.

Ammonia monitoring near 1.5 µm with diode-laser absorption sensors

We investigated ammonia spectroscopy near 1.5 mum to select transitions appropriate for trace ammonia detection in air-quality and combustion emissions-monitoring applications using diode lasers. Six ammonia features were selected for these trace-gas detection applications based on their transition strengths and isolation from interfering species. The strengths, positions, and lower-state energies for the lines in each of these features were measured and compared with values published in the literature. Ammonia slip was measured in the exhaust above an atmospheric pressure premixed ethylene-air burner to demonstrate the feasibility of the in situ diode-laser sensor.

The Measurement of Aerosol Optical Properties Using Continuous Wave Cavity Ring-Down Techniques

Abstract Large uncertainties in the effects that aerosols have on climate require improved in situ measurements of extinction coefficient and single-scattering albedo. This paper describes the use of continuous wave cavity ring-down (CW-CRD) technology to address this problem. The innovations in this instrument are the use of CW-CRD to measure aerosol extinction coefficient, the simultaneous measurement of scattering coefficient, and its small size, suitable for a wide range of aircraft applications. The prototype instrument measures extinction and scattering coefficient at 690 nm and extinction coefficient at 1550 nm. The instrument itself is small (60 cm × 48 cm × 15 cm) and relatively insensitive to vibrations. The prototype instrument has been tested in the lab and used in the field. While improvements in performance are needed, the prototype has been shown to make accurate and sensitive measurements of extinction and scattering coefficients. Combining these two parameters, one can obtain the single-s...

Tunable diode-laser absorption measurements of methane at elevated temperatures

A diode-laser sensor system based on absorption spectroscopy techniques has been developed to monitor CH(4) nonintrusively in high-temperature environments. Fundamental spectroscopic parameters, including the line strengths of the transitions in the R(6) manifold of the 2ν(3) band near 1.646 μm, havebeen determined from high-resolution absorption measurements in a heated static cell. In addition, acorrected expression for the CH(4) partition function has been validated experimentally over thetemperature range from 400 to 915 K. Potential applications of the diode-laser sensor system includeprocess control, combustion measurements, and atmospheric monitoring.

A diode-laser absorption sensor system for combustion emission measurements

A diode-laser sensor system has been developed to measure the concentrations of NO, O, CO, and in combustion gases using absorption spectroscopy and fast-extraction sampling techniques. Measured survey spectra of the NO 3 band (the R branch) and O lines from the band in the spectral region from 5556 to 5572 were recorded and compared with calculated spectra to select optimum transitions for detection of NO. Similarly, measured survey spectra of the O 3 band from 6535 to 6600 were used to identify optimum transitions for detection of O. High-resolution NO absorption measurements ((7.5) and (7.5) lines) were recorded in a fast-flow multipass cell containing probe-sampled combustion gases to determine NO concentrations in a laminar, premixed -air flame, seeded with . For fuel-lean conditions, the measured NO mole fractions corresponded to 68% of the injected . For fuel-rich conditions, the fraction of converted to NO decreased with increasing equivalence ratio. In additional experiments, CO, and absorption measurements (the R(13) line of the 3 band, the R(16) line of the band and the RQ(7, 8) line of the b band, respectively) were used to determine species concentrations above a laminar, premixed -air flame. Good agreement was found between measured CO, and concentrations and calculated chemical equilibrium values.

In situ combustion measurements of CO 2 by use of a distributed-feedback diode-laser sensor near 2.0 mm

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.

Diode-laser absorption measurements of CO 2 near 2.0 μm at elevated temperatures

A diode-laser sensor system based on absorption spectroscopy techniques has been developed for nonintrusive measurements of CO(2) in high-temperature environments. Survey spectra of the CO(2) (20 degrees 1,04 degrees 1)(I)-00 degrees 0 and (20 degrees 1,04 degrees 1)(II)-00 degrees 0 bands between 1.966 and 2.035 mum (4915-5085 cm(-1)) were recorded at temperatures between 296 and 1425 K in a heated static cell and compared with calculated spectra (by using the HITRAN 96/HITEMP database) to find candidate transitions for CO(2) detection. High-resolution measurements of the CO(2) R(56) line shape [(20 degrees 1,04 degrees 1)(II)-00 degrees 0 band] were used to determine the transition line strength, the self-broadening half-width, and the coefficient of temperature dependence of the self-broadening half-width. The results represent what are believed to be the first measurements of CO(2) absorption near 2.0 mum with room-temperature diode lasers. Potential applications of the diode-laser sensor system include in situ combustion measurements and environmental monitoring.