High-speed mid-infrared laser absorption spectroscopy of CO2 for shock-induced thermal non-equilibrium studies of planetary entry

Abstract A high-speed laser absorption technique is employed to resolve spectral transitions of CO $$_2$$ 2 in the mid-infrared at MHz rates to infer non-equilibrium populations/temperatures of translation, rotation and vibration in shock-heated CO $$_2$$ 2 - Ar mixtures. An interband cascade laser (DFB-ICL) resolves 4 transitions within the CO $$_2$$ 2 asymmetric stretch fundamental bands ( $$\Delta$$ Δ v $$_3$$ 3 = 1) near 4.19 \upmu \hbox {m} μ m . The sensor probes a wide range of rotational energies as well as two vibrational states (00 $$^0$$ 0 0 and 01 $$^1$$ 1 0). The sensor is demonstrated on the UCLA high enthalpy shock tube, targeting temperatures between 1250 and 3100 K and sub-atmospheric pressures (up to 0.2 atm). The sensor is sensitive to multiple temperatures over a wide range of conditions relevant to Mars entry radiation. Vibrational relaxation times are resolved and compared to existing models of thermal non-equilibrium. Select conditions highlight the shortcomings of modeling CO $$_2$$ 2 non-equilibrium with a single vibrational temperature

Spectroscopy and optical diagnostics for gases

This text provides an introduction to the science that governs the interaction of light and matter (in the gas phase). It provides readers with the basic knowledge to exploit the light-matter interaction to develop quantitative tools for gas analysis (i.e. optical diagnostics) and understand and interpret the results of spectroscopic measurements. The authors pair the basics of gas‐phase spectroscopy with coverage of key optical diagnostic techniques utilized by practicing engineers and scientists to measure fundamental flow‐field properties. The text is organized to cover three sub‐topics of gas‐phase spectroscopy: (1) spectral line positions, (2) spectral line strengths, and (3) spectral lineshapes by way of absorption, emission, and scattering interactions. The latter part of the book describes optical measurement techniques and equipment. Key subspecialties include laser induced fluorescence, tunable laser absorption spectroscopy, and wavelength modulation spectroscopy. It is ideal for students and practitioners across a range of applied sciences including mechanical, aerospace, chemical, and materials engineering

Solar-thermal production of graphitic carbon and hydrogen via methane decomposition

This work reports a process in which concentrated irradiation from a simulated solar source converts methane to high-value graphitic carbon and hydrogen gas. Methane flows within a photo-thermal reactor through the pores of a thin substrate irradiated by several thousand suns at the focal peak. The methane decomposes primarily into hydrogen while depositing highly graphitic carbon that grows conformally over ligaments in the porous substrate. The localized solar heating of the porous substrate serves to capture the solid carbon into a readily extractable and useful form while maintaining active deposition site density with persistent catalytic activity. Results indicate a strong temperature dependence with high decomposition occurring in the central heating zone with concentration factors and temperatures above 1000 suns and 1300 K, respectively. Even with a large flow area through regions of lower irradiation and temperature, methane conversion and hydrogen yields of approx. 70\% are achieved, and 58\% of the inlet carbon is captured in graphitic form

Extended tuning of distributed-feedback lasers in a bias-tee circuit via waveform optimization for MHz-rate absorption spectroscopy

International audienceVariations in injection-current waveform are examined using diplexed RF-modulation with continuouswave distributed-feedback (CW-DFB) lasers, with the aim to maximize the spectral tuning range and signal-tonoise ratio for MHz-rate laser absorption spectroscopy. Utilizing a bias-tee circuit, laser chirp rates are shown to increase by modulating the AC input voltage using square waves instead of sine waves and by scanning the laser below the lasing threshold during the modulation period. The effect of waveform duty cycle and leadingedge ramp rate are further examined. A spectral scan depth on the order of 1 cm −1 at a scan frequency of 1 MHz is achieved with a representative CW-DFB quantum cascade laser near 5 µm. Distortion of high-frequency optical signals due to detector bandwidth is also examined, and limitations are noted for applications with narrow spectral features and low-bandwidth detectors. Based on common detection system limitations, an optimization approach is established for a given detection bandwidth and target spectra. A representative optimization is presented for measurements of sub-atmospheric carbon monoxide spectra with a 200-MHz detection system. The methods are then demonstrated to resolve transient gas properties (pressure and temperature) via laser absorption spectroscopy at MHz rates in a detonation tube and shock tube facility. An appendix detailing a first-order model of high-speed distributed feedback laser tuning dynamics is also included to support the experimental observations of this work

High-diodicity impinging injector design for rocket propulsion enabled by additive manufacturing

International audienceThis work examines novel impinging injector designs enabled by additive manufacturing that reduce forward pressure loss while maintaining high relative back-flow resistance (diodicity). A steady, non-reacting computational fluid dynamics (CFD) model is used to assess the hydraulic characteristics of fluidic diode features in a liquid bi-propellant impinging doublet-type injector configuration relevant to rocket propulsion applications. A design trade study is conducted to determine an effective fluidic diode feature to be implemented within the injector elements, constrained by practical considerations for additive manufacturing. Noteworthy increases in diodicity are achieved within the constraints of producibility relative to conventional designs. A complimentary transient, multiphase CFD model is used to evaluate propellant backflow behavior when subject to a high-pressure impulse within a downstream chamber. Preliminary results suggest that the diodicity is a relevant predictor of transient performance as injector stiffness decreases

Multi-line Boltzmann regression for near-electronvolt temperature and CO sensing via MHz-rate infrared laser absorption spectroscopy

International audienceA mid-infrared laser absorption technique is developed for sensing of temperature and carbon monoxide (CO) number density from 2000 K to above 9000 K. To resolve multiple rovibrational lines, a distributedfeedback quantum cascade laser (DFB-QCL) is modulated across 80% of its current range using a trapezoidal waveform via a bias-tee circuit. The laser attains a spectral scan depth of 1 cm −1 , at a scan frequency of 1 MHz, which allows for simultaneous measurements of four isolated CO transitions near 2011 cm −1 (4.97 µm) with lower-state energies spanning 3,000 to 42,000 cm −1. The number density and temperature are calculated using a Boltzmann regression of the four population fractions. This method leverages the information contained in each transition and yields a lower uncertainty than using a single line pair. The sensor is validated in shock tube experiments over a wide range of temperatures and pressures (2300-8100 K, 0.3-3 atm). Measurements behind reflected shock waves are compared to a kinetic model of CO dissociation up to 9310 K and are shown to recover equilibrium conditions. The high temperature range of the sensor is able to resolve rapid species and temperature evolution at near electronvolt conditions making it suitable for investigations of high-speed flows, plasma applications, and high-pressure detonations

Mars2020 entry shock layer thermochemical kinetics examined by MHz-rate laser absorption spectroscopy

International audienceA mid-infrared laser absorption diagnostic was deployed to examine the evolution of thermophysical properties across a simulated Mars2020 shock layer in the Electric Arc Shock Tube (EAST) facility at NASA Ames. Rapid laser tuning techniques using bias-tee circuitry enabled quantitative temperature and number density measurements of [Formula: see text] and CO with microsecond resolution over a shock velocity range of 1.30–3.75 km/s. Two interband cascade lasers were utilized at 4.17 and 4.19 μm to resolve rovibrational [Formula: see text] lines spanning across [Formula: see text] to [Formula: see text] in the asymmetric stretch fundamental bands. In test cases with enough energy to dissociate [Formula: see text], a quantum cascade laser scanned multiple transitions of the CO fundamental bands near [Formula: see text]. The results are compared to the Data Parallel Line Relaxation (DPLR) code and Lagrange shock tube analysis (LASTA) simulations of the shock layer. A numerical simulation of the compressible boundary layer is used to account for measurement sensitivities to this flow feature in the EAST facility. Temperature and species transients are compared to multiple chemical kinetic models. The laser absorption data presented in this work can be used to refine the models used to simulate the aerothermal environment encountered during Mars entry

Transient analysis of solar pyrolysis and hydrogen yield via interband cascade laser absorption spectroscopy of methane, acetylene, ethylene, and ethane

A mid-infrared laser absorption sensing method has been developed to measure species concentrations of four hydrocarbons and gas temperature over a range of temperatures in mixture compositions relevant to the pyrolytic decomposition of natural gas. The four measured species (methane, acetylene, ethylene, and ethane) are the most abundant hydrocarbons during the pyrolysis of natural gas, and provide a means to monitor decomposition progress and hydrogen yield through molar balance. In this work, time-division multiplexed signals of three distributed-feedback interband cascade lasers are used to make simultaneous measurements of select C-H stretch rovibrational transitions of the target hydrocarbons in the 3.00-3.34 μm range. The sensor was validated over a range of temperatures and pressures (300-1000 K, 0.03-1 atm) at relevant mixture compositions, with correction methods developed to mitigate cross-species interference. The sensor was demonstrated on a solar-thermal pyrolysis reactor, where time-resolved measurements of species mole fractions were performed across a range of insolation conditions to capture the transient response of the reactor