Christopher S. Goldenstein

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

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.

Analysis of calibration-free wavelength-scanned wavelength modulation spectroscopy for practical gas sensing using tunable diode lasers

A novel strategy has been developed for analysis of wavelength-scanned, wavelength modulation spectroscopy (WMS) with tunable diode lasers (TDLs). The method simulates WMS signals to compare with measurements to determine gas properties (e.g., temperature, pressure and concentration of the absorbing species). Injection-current-tuned TDLs have simultaneous wavelength and intensity variation, which severely complicates the Fourier expansion of the simulated WMS signal into harmonics of the modulation frequency (fm). The new method differs from previous WMS analysis strategies in two significant ways: (1) the measured laser intensity is used to simulate the transmitted laser intensity and (2) digital lock-in and low-pass filter software is used to expand both simulated and measured transmitted laser intensities into harmonics of the modulation frequency, WMS-nfm (n = 1, 2, 3,...), avoiding the need for an analytic model of intensity modulation or Fourier expansion of the simulated WMS harmonics. This analysis scheme is valid at any optical depth, modulation index, and at all values of scanned-laser wavelength. The method is demonstrated and validated with WMS of H2O dilute in air (1 atm, 296 K, near 1392 nm). WMS-nfm harmonics for n = 1 to 6 are extracted and the simulation and measurements are found in good agreement for the entire WMS lineshape. The use of 1f-normalization strategies to realize calibration-free wavelength-scanned WMS is also discussed.

Two-color absorption spectroscopy strategy for measuring the column density and path average temperature of the absorbing species in nonuniform gases

A two-color absorption spectroscopy strategy has been developed for measuring the column density and density-weighted path-average temperature of the absorbing species in nonuniform gases. This strategy uses two transitions with strengths that scale nearly linearly with temperature. In addition, measured lineshapes are used to accurately model absorbance spectra. As a result, the column density and density-weighted path-average temperature of the absorbing species can be inferred from a comparison of signals measured across a nonuniform line of sight (LOS) with simulated signals calculated using a uniform LOS. This strategy is demonstrated with simulations of water-vapor absorption across a nonuniform LOS with temperature and composition gradients comparable to those in hydrogen-air diffusion flames. In this demonstration, both the column density and density-weighted path-average temperature of water vapor are recovered to within 0.5%.

Fiber-coupled 2.7 µm laser absorption sensor for CO2 in harsh combustion environments

A tunable diode laser absorption sensor near 2.7 μm, based on 1f-normalized wavelength-modulation spectroscopy with second-harmonic detection (WMS-2f), was developed to measure CO2 concentration in harsh combustion flows. Wavelength selection at 3733.48 cm −1 exploited the overlap of two CO2 transitions in the ν1 + ν3 vibrational band at 3733.468 cm −1 and 3733.498 cm −1 . Primary factors influencing wavelength selection were isolation and strength of the CO2 absorption lines relative to infrared water absorption at elevated pressures and temperatures. The HITEMP 2010 database was used to model the combined CO2 and H2O absorption spectra, and key line-strength and line-broadening spectroscopic parameters were verified by high-temperature static cell measurements. To validate the accuracy and precision of the WMS-based sensor, measurements of CO2 concentration were carried out in non-reactive shock-tube experiments (P ∼ 3‐12 atm, T ∼ 1000‐2600 K). The laser was then free-space fiber-coupled with a zirconium fluoride single-mode fiber for remote light delivery to harsh combustion environments, and demonstrated on an ethylene/air pulse detonation combustor at pressures up to 10 atm and temperatures up to 2500 K. To our knowledge, this work represents the first time-resolved in-stream measurements of CO2 concentration in a detonation-based engine.

Wavelength-modulation spectroscopy near 1.4 µm for measurements of H2O and temperature in high-pressure and -temperature gases

The development, validation and demonstration of a two-color tunable diode laser (TDL) absorption sensor for measurements of temperature and H2O in high-pressure and high-temperature gases are presented. This sensor uses first-harmonic-normalized wavelength-modulation spectroscopy with second-harmonic detection (WMS-2f/1f) to account for non-absorbing transmission losses and emission encountered in harsh, high-pressure environments. Two telecommunications-grade TDLs were used to probe H2O absorption transitions near 1391.7 and 1469.3 nm. The lasers were frequency-multiplexed and modulated at 160 and 200 kHz to enable a measurement bandwidth up to 30 kHz along a single line-of-sight. In addition, accurate measurements are enabled at extreme conditions via an experimentally derived spectroscopic database. This sensor was validated under low-absorbance (<0.05) conditions in shock-heated H2O–N2 mixtures at temperatures and pressures from 700 to 2400 K and 2 to 25 atm. There, this sensor recovered the known temperature and H2O mole fraction with a nominal accuracy of 2.8% and 4.7% RMS, respectively. Lastly, this sensor resolved expected transients with high bandwidth and high precision in a reactive shock tube experiment and a pulse detonation combustor.

Multispecies Midinfrared Absorption Measurements in a Hydrocarbon-Fueled Scramjet Combustor

Spatially resolved laser absorption measurements of CO, CO2, and H2O within an ethylene-fueled direct-connect model scramjet combustor are presented. The sensors employ a variety of laser sources at midinfrared wavelengths to provide access to fundamental vibrational band absorption transitions for each species. Both scanned-wavelength-modulation spectroscopy and scanned-wavelength direct-absorption are used, with particular attention paid to employing these methods in a manner that accounts for expected nonuniformities in temperature and composition throughout the combustor. Results for product temperatures and column densities offer insight on the ongoing combustion process downstream of fuel injection throughout the combustion-product plume, and on the significant temporal variations in the combustor. Additional tests measure the temperature and concentration of H2O in the cavity flameholder during a flame extinction event, which gives an upper bound of the cavity residence time. These measurements are...

High-bandwidth scanned-wavelength-modulation spectroscopy sensors for temperature and H2O in a rotating detonation engine

The design and use of two-color tunable diode laser (TDL) absorption sensors for measurements of temperature and H2O in a rotating detonation engine (RDE) are presented. Both sensors used first-harmonic-normalized scanned-wavelength-modulation spectroscopy with second-harmonic detection (scanned-WMS-2f/1f) to account for non-absorbing transmission losses and emission encountered in the harsh combustion environment. One sensor used two near-infrared (NIR) TDLs near 1391.7 nm and 1469.3 nm that were modulated at 225 kHz and 285 kHz, respectively, and sinusoidally scanned across the peak of their respective H2O absorption transitions to provide a measurement rate of 50 kHz and a detection limit in the RDE of 0.2% H2O by mole. The other sensor used two mid-infrared (MIR) TDLs near 2551 nm and 2482 nm that were modulated at 90 kHz and 112 kHz, respectively, and sinusoidally scanned across the peak of their respective H2O transitions to provide a measurement rate of 10 kHz and a detection limit in the RDE of 0.02% H2O by mole. Four H2O absorption transitions with different lower-state energies were used to assess the homogeneity of temperature in the measurement plane. Experimentally derived spectroscopic parameters that enable temperature and H2O sensing to within 1.5–3.5% of known values are reported. The sensor design enabling the high-bandwidth scanned-WMS-2f/1f measurements is presented. The two sensors were deployed across two orthogonal and coplanar lines-of-sight (LOS) located in the throat of a converging-diverging nozzle at the RDE combustor exit. Measurements in the non-premixed H2-fueled RDE indicate that the temperature and H2O oscillate at the detonation frequency (≈3.25 kHz) and that production of H2O is a weak function of global equivalence ratio.

Spatially Resolved Water Measurements in a Scramjet Combustor Using Diode Laser Absorption

A two-color tunable diode laser sensor for detection of H2O absorption near 1.4  μm was used to simultaneously determine temperature, H2O column density, and velocity in a direct-connect scramjet combustor. Axial and transverse spatial resolutions were achieved by translating the measurement line of sight across and along the flowpath with measurements spanning six axial planes and 24 measurement lines of sight in each plane. Results are presented for a uniform noncombusting H2O-seeded case as well as for H2 air combustion at equivalence ratios of 0.17 and 0.46. A combination of wavelength-modulation spectroscopy and scanned-wavelength direct absorption were used to mitigate nonuniform gas conditions along the measurement line of sight present in the combustion cases. Comparisons of tunable diode laser absorption spectroscopy measurements with computational fluid dynamics simulations are included and illustrate the use of line-of-sight tunable diode laser absorption spectroscopy to validate computational ...

Quantum cascade laser absorption sensor for carbon monoxide in high-pressure gases using wavelength modulation spectroscopy

A tunable quantum cascade laser sensor, based on wavelength modulation absorption spectroscopy near 4.8 μm, was developed to measure CO concentration in harsh, high-pressure combustion gases. The sensor employs a normalized second harmonic detection technique (WMS-2f/1f) at a modulation frequency of 50 kHz. Wavelength selection at 2059.91  cm⁻¹ targets the P(20) transition within the fundamental vibrational band of CO, chosen for absorption strength and relative isolation from infrared water and carbon dioxide absorption. The CO spectral model is defined by the Voigt line-shape function, and key line-strength and line-broadening spectroscopic parameters were taken from the literature or measured. Sensitivity analysis identified the CO-N₂ collisional broadening coefficient as most critical for uncertainty mitigation in hydrocarbon/air combustion exhaust measurements, and this parameter was experimentally derived over a range of combustion temperatures (1100-2600 K) produced in a shock tube. Accuracy of the wavelength-modulation-spectroscopy-based sensor, using the refined spectral model, was validated at pressures greater than 40 atm in nonreactive shock-heated gas mixtures. The laser was then free-space coupled to an indium-fluoride single-mode fiber for remote light delivery. The fiber-coupled sensor was demonstrated on an ethylene/air pulse detonation combustor, providing time-resolved (~20  kHz), in situ measurements of CO concentration in a harsh flow field.

A scanned-wavelength-modulation absorption-spectroscopy sensor for temperature and H2O in low-pressure flames

The design and demonstration of a tunable diode laser sensor for in situ temperature and water (H2O) measurements in laboratory flames is presented. A newly-designed probe consisting of two single-crystal sapphire rods is used to guide 2.9 μm laser light across the core of a flame while avoiding absorption by water outside of the flame. The sensor probes two H2O transitions near 2.9 μm using a scanned-wavelength-modulation absorption-spectroscopy spectral-fitting technique, which enables measurements without calibration or knowledge of the mixture collisional-broadening coefficient. To demonstrate the sensor, temperature and water mole fraction measurements were acquired in stoichiometric, burner-stabilized flames of methane/oxygen/argon at pressures of 25 Torr and 60 Torr. Typical total uncertainties in the temperature and water mole fraction measurements were ±50 K and ±0.016, respectively. This sensor is simple, robust and accurate and enables improved chemical kinetics studies of low-pressure flames. Improvements to reduce temperature and H2O precision to ±20 K and ±0.008, respectively, are discussed.