Ian A. Schultz

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.

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%.

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...

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 ...

Tunable Diode Laser Absorption Sensor for Measurements of Temperature and Water Concentration in Supersonic Flows

The development and use of a tunable diode laser absorption sensor for gas temperature and water concentration in a high enthalpy supersonic flow is presented. This diagnostic enables non-intrusive, in situ measurements in harsh environments produced by propulsion ground test facilities. Water vapor was chosen as the target species to enable calculations of hydrogen-air combustion efficiency and to calibrate steam addition for simulated-vitiated testing. The sensor utilizes robust diode lasers capable of probing the nearinfrared absorption band of water near 1.4 microns. Two lasers are wavelength-multiplexed to enable twoline thermometry and both scanned-wavelength direct absorption and wavelength-modulation spectroscopy with second-harmonic detection (WMS-2f) techniques were used. Results are presented for measurements taken at the combustor entrance and tunnel exit of the University of Virginia’s dual-mode scramjet combustor for steam-injected and hydrogen-air combustion operation modes.

Diode Laser Absorption Sensor for Combustion Progress in a Model Scramjet

The development and use of a tunable diode laser absorption spectroscopy sensor for combustion product water vapor in a hydrogen-fueled model scramjet combustor are presented. A pair of absorption transitions was selected from the combination bands of water vapor near 1.4  μm exploiting telecommunications diode laser and fiber technologies. Wavelength-modulation spectroscopy with detection of the peak second-harmonic signal was used owing to its superior noise-rejection capabilities. The sensor measured temperature and H2O column density at two axial planes downstream of fuel injection with the absorption line-of-sight positioned at over 40 measurement locations using a translation stage system. The combustion product concentration and the gas temperature were not uniform along the line-of-sight, and the influence of these nonuniformities on the interpretation of the tunable diode laser measurements is discussed. The measurements are compared with published computational fluid dynamics simulations using t...

TDL Absorption Sensor for Temperature Measurements in High-Pressure and High-Temperature Gases

The development and demonstration of a tunable diode laser (TDL) absorption sensor for temperature measurements in high-pressure and high-temperature gases is presented. This sensor uses a wavelength modulation spectroscopy with second-harmonic detection (WMS-2f) technique with a sensor bandwidth of 10 kHz. Two-color thermometry was performed using water vapor absorption features near 1392 and 1469 nm. Sensor performance was validated with measurements in a heated static cell and shock-heated N2-H2O mixtures in the Stanford High-Pressure Shock Tube (HPST). Experiments were conducted at gas temperatures and pressures between 600-2200 K and 2-22 atm, respectively.

HyHypervelocity Testing of a Dual-mode Scramjet

At Mach numbers higher than about 6, the high speed flowpath of a hypersonic combined cycle vehicle operates in the supersonic mode. There are many aspects of this flowpath that makes it difficult to characterize analytically and numerically. Intense shockwave/boundary layer interaction, diffusion-limited mixing, and flame holding in short residence time flows are some of the most difficult phenomena to model. In order to examine such aspects, the NASA HyPulse shock tunnel at ATK GASL is being used to perform supersonic combustion experiments at conditions up to a hypervelocity vehicle Mach number of 10. The experimental team is adapting an existing wind tunnel model that was developed under funding of the OSD/TRMC SDPTE (Hy-V) Program. This model will first be tested at Mach 5, to reproduce the test conditions at UVa, and then extended to higher Mach numbers such as 7 and 10. The data is being generated under the auspices of the National Center for Hypersonic Combined Cycle Propulsion (NCHCCP) and it will be valuable for validating advanced numerical models and understanding fundamental flow physics in the hypervelocity flight regime. The scramjet flowpath is of a freejet test configuration and consists of an inlet, isolator and combustor. The flowpath retains many of the characteristics of the direct-connect dual-mode scramjet that has historically been tested at the University of Virginia.

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...