R. Mitchell Spearrin

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...

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.

Fiber-coupled diode-laser sensors for calibration-free stand-off measurements of gas temperature, pressure, and composition.

A fiber-coupled near-infrared diode-laser sensor for stand-off measurements of gas temperature, pressure, and composition is presented. This sensor utilizes a fiber bundle with six multimode catch fibers surrounding one single-mode pitch fiber to transmit and receive backscattered laser light in a handheld transmitter/receiver. Scanned-wavelength-modulation spectroscopy with 1f-normalized 2f-detection and fast (80-200 kHz) wavelength modulation were used to provide calibration-free measurements and reduce the influence of spurious cavity noise formed by the overlapping transmitted and reflected laser light. Demonstrations include two-color measurements of temperature, pressure, and H(2)O near 1.4 μm in a propane flame at 2 kHz (SNR=200) and measurements of CH(4) near 1.65 μm (SNR=20 to 1500) at stand-off distances of 15 cm and 10 m, respectively. The fraction of photons collected ranged from 10(4) to 1 parts per million at stand-off distances from 10 cm to 10 m, respectively, and is similar for aluminum and paper reflectors.

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.

Quantitative Emission and Absorption

We are ready to begin discussion of quantitative emission and absorption spectroscopy, with the goal of being able to specify emission and absorption as a function of wavelength. Two steps are involved in this treatment. In the first step, a simple form of the equation of radiative transfer will be used to identify a new parameter, known as the spectral absorption coefficient, which will be seen as the governing quantity which characterizes emission and absorption of light, as a function of wavelength. In the second step, the Einstein theory of radiation is employed to show that the spectral absorption coefficient is given simply by the product of the line strength and a lineshape function; the relationship of the line strength to fundamental quantities known as Einstein coefficients is also shown. With these relationships in hand, it will become evident how spectrally resolved absorption (or emission) can be used as a nonintrusive means of measuring a variety of gasdynamic parameters, including: species concentration, pressure, temperature, density, and even flow velocity.

Rayleigh and Raman Spectra

Unlike absorption and emission, Rayleigh and Raman spectroscopy are based on how a molecule scatters photons. One key difference to keep in mind as we explore scattering processes further is that while absorption requires that the molecule have energy-level spacings (for allowed transitions) corresponding to the energy of the interacting photon, scattering can occur with an incident photon of almost any energy. That is, for a molecule to absorb light, the light must generally be at specific wavelengths or frequencies. Scattering, on the other hand, can occur at almost any wavelength.

Laser-Induced Fluorescence

This chapter discusses the theory and practical application of laser-induced fluorescence (LIF). Fluorescence, generally, is just another name for spontaneous emission. LIF is a two-step process: (1) absorption of the laser photon, followed by (2) emission.

Diatomic Molecular Spectra

The primary interactions of light and matter take the form of emission, absorption, or scattering.

Polyatomic Molecular Spectra

The mechanism for electromagnetic radiation to interact with polyatomic molecules is similar to the process discussed previously for diatomics. Molecular vibrations and rotations cause changes in electric dipole moments that occur at resonant frequencies. At these frequencies, molecules can interact with radiation (via emission, absorption, or scattering). The difference is that polyatomic molecules have more rotational and vibrational modes, and each of these modes gives rise to additional possible resonances.

Bond Dissociation Energies

The bond dissociation energy, D e , is a critical parameter in thermodynamics, spectroscopy, and kinetics. For example, in thermodynamics, bond energies directly affect heats of formation and reaction. Dissociation energies thus also play key roles determining rates of reaction as a function of temperature. This chapter will give several examples of how spectroscopic information can reveal D e (of ground and excited electronic states).