Christopher L. Strand

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.

Supersonic Mass-Flux Measurements via Tunable Diode Laser Absorption and Nonuniform Flow Modeling

Measurements of mass flux are obtained in a vitiated supersonic ground test facility using a sensor based on line-of-sight (LOS) diode laser absorption of water vapor. Mass flux is determined from the product of measured velocity and density. The relative Doppler shift of an absorption transition for beams directed upstream and downstream in the flow is used to measure velocity. Temperature is determined from the ratio of absorption signals of two transitions (lambda(sub 1)=1349 nm and lambda(sub 2)=1341.5 nm) and is coupled with a facility pressure measurement to obtain density. The sensor exploits wavelength-modulation spectroscopy with second-harmonic detection (WMS-2f) for large signal-to-noise ratios and normalization with the 1f signal for rejection of non-absorption related transmission fluctuations. The sensor line-of-sight is translated both vertically and horizontally across the test section for spatially-resolved measurements. Time-resolved measurements of mass flux are used to assess the stability of flow conditions produced by the facility. Measurements of mass flux are within 1.5% of the value obtained using a facility predictive code. The distortion of the WMS lineshape caused by boundary layers along the laser line-of-sight is examined and the subsequent effect on the measured velocity is discussed. A method for correcting measured velocities for flow non-uniformities is introduced and application of this correction brings measured velocities within 4 m/s of the predicted value in a 1630 m/s flow.

Thermometry and Velocimetry in Supersonic Flows via Scanned Wavelength-Modulation Absorption Spectroscopy

A tunable diode laser-based H2O absorption diagnostic has been developed to characterize transient phenomena within the Stanford Expansion Tube Flow Facility. Key parameters provided by the diagnostic include the temperature, H2O mole-fraction and velocity time-histories of the supersonic test gas, as well as the arrival time of the test gas contact surfaces. The presence of water within the test gas affords well-established rovibrational transitions near 1.4 µm that are probed using scanned wavelength-modulation absorption spectroscopy with second harmonic detection (scanned-WMS-2f). The scannedWMS-2f technique offers the sensitivity and noise resistance necessary for temperature and mole-fraction measurements within the harsh conditions of the expansion tube test section. A rigorous scanned-WMS-2f theory has been developed to account for non-ideal laser characteristics prompted by short test times and high flow velocities and is employed for the first time in an absorption diagnostic. Furthermore, by implementing a crossed beam laser configuration, second harmonic Doppler shift velocimetry is performed in concert with the temperature and mole-fraction measurements to provide flow velocity data. The measurements carried out with this sensor provide a quantitative characterization of the test gas conditions produced in the expansion tube and thus provide an essential foundation for supersonic combustion experimentation within the facility.

SpectraPlot.com: Integrated Spectroscopic Modeling of Atomic and Molecular Gases

Abstract SpectraPlot is a web-based application for simulating spectra of atomic and molecular gases. At the time this manuscript was written, SpectraPlot consisted of four primary tools for calculating: (1) atomic and molecular absorption spectra, (2) atomic and molecular emission spectra, (3) transition linestrengths, and (4) blackbody emission spectra. These tools currently employ the NIST ASD, HITRAN2012, and HITEMP2010 databases to perform line-by-line simulations of spectra. SpectraPlot employs a modular, integrated architecture, enabling multiple simulations across multiple databases and/or thermodynamic conditions to be visualized in an interactive plot window. The primary objective of this paper is to describe the architecture and spectroscopic models employed by SpectraPlot in order to provide its users with the knowledge required to understand the capabilities and limitations of simulations performed using SpectraPlot. Further, this manuscript discusses the accuracy of several underlying approximations used to decrease computational time, in particular, the use of far-wing cutoff criteria.

Hypersonic Scramjet Testing via Diode Laser Absorption in a Reflected Shock Tunnel

A wavelength-multiplexed two-color tunable diode laser absorption spectroscopy sensor probing transitions near 1.4  μm was developed to measure H2O temperature and column density simultaneously across three lines of sight in a ground-based model scramjet combustor. High-enthalpy scramjet conditions equivalent to Mach 10 flight were generated with a reflected shock tunnel. The sensor hardware development and optical engineering to overcome noisy combustor conditions, including short test time, beam steering, and mechanical vibration, are discussed. A new scanned-wavelength-modulation spectroscopy technique was used to acquire the complete spectral lineshape while maintaining the high signal-to-noise ratio characteristic of wavelength-modulation spectroscopy measurements. Two combusting flow experiments were conducted, and the results compared with steady-state computational fluid dynamics calculations, with both the simulations and the measurements finding formation of H2O from H2 combustion during the tes...

Line intensities and temperature-dependent line broadening coefficients of Q-branch transitions in the v2 band of ammonia near 10.4 μm

We report measured line intensities and temperature-dependent broadening coefficients of NH3 with Ar, N2, O2, CO2, H2O, and NH3 for nine sQ(J,K) transitions in the ν2 fundamental band in the frequency range 961.5-967.5 cm-1. This spectral region was chosen due to the strong NH3 absorption strength and lack of spectral interference from H2O and CO2 for laser-based sensing applications. Spectroscopic parameters were determined by multi-line fitting using Voigt lineshapes of absorption spectra measured with two quantum cascade lasers in thermodynamically-controlled optical cells. The temperature dependence of broadening was measured over a range of temperatures between 300 and 600 K. These measurements aid the development of mid-infrared NH3 sensors for a broad range of gas mixtures and at elevated temperatures.

Rapid Chemiluminescent Imaging Behind Reflected Shock Waves

Current shock tube combustion experiments generally assume that the test environment behind a reflected shock wave is quiescent and that ignition processes progress uniformly over the entire test volume. However, various past investigations, including those based on schlieren data and sidewall imaging [1, 2], have observed nonuniform ignition in certain test regimes. Here, we use both conventional diagnostics (pressure, emission, and laser absorption) and a high-speed chemiluminescent imaging system to investigate the ignition behavior of n-heptane/oxygen/argon in shock tubes at long test times (greater than 2 ms), in an attempt to map the boundary of uniform and nonuniform ignition behavior in one of our shock tubes.

Supersonic Mass Flux Measurements via Tunable Diode Laser Absorption and Non-Uniform Flow Modeling

Measurements of mass flux are obtained in a vitiated supersonic ground-test facility using a sensor based on lineof-sight diode laser absorption of water vapor. Mass flux is determined from the product of measured velocity and density. The relative Doppler shift of an absorption transition for beams directed upstream and downstream in the flow is used to measure velocity. Temperature is determined from the ratio of absorption signals of two transitions ( 1 1349 nm and 2 1341:5 nm) and is coupled with a facility pressure measurement to obtain density. The sensor exploits wavelength-modulation spectroscopywith second-harmonic detection for large signal-to-noise ratios and normalization with the 1f signal for rejection of non-absorption-related transmission fluctuations. The sensor line of sight is translated both vertically and horizontally across the test section for spatially resolvedmeasurements. Time-resolved measurements of mass flux are used to assess the stability of flow conditions produced by the facility. Measurements ofmassfluxarewithin 1.5%of the value obtainedusing a facility predictive code.The distortion of the wavelength-modulation spectroscopy lineshape caused by boundary layers along the laser line of sight is examined and the subsequent effect on themeasured velocity is discussed. Amethod for correctingmeasured velocities for flow nonuniformities is introduced and application of this correction brings measured velocities within 4 m=s of the predicted value in a 1630 m=s flow.

Hypersonic scramjet testing via TDLAS measurements of temperature and column density in a reflected shock tunnel

A wavelength-multiplexed two-color TDLAS sensor probing transitions near 1.4 μm was developed to measure H2O temperature and column density simultaneously across three lines-of-sight in a ground-based model scramjet combustor. High-enthalpy scramjet conditions equivalent to Mach 10 flight were generated with a reflected shock tunnel. The sensor hardware development and optical engineering to overcome noisy combustor conditions including short test time, beam steering, and mechanical vibration are discussed. A new scanned wavelength-modulation spectroscopy (WMS) technique was used to acquire the complete spectral lineshape while maintaining the high signal-to-noise ratio characteristic of WMS measurements. Two combusting flow experiments were conducted and the results compared with steady-state CFD calculations, with both the simulations and the measurements finding formation of H2O from H2 combustion during the test time. To our knowledge, this work represents the first use of WMS in a reflected shock tunnel and the first use of scanned-WMS in a scramjet combustor.