Time-resolved in situ measurements of thermodynamic properties (pressure, temperature and species density) were performed in the annular exhaust of a methane-oxygen rotating detonation rocket engine (RDRE) using high-speed laser absorption spectroscopy. Bias-tee circuitry was coupled with a distributed feedback quantum cascade near 5 μm to spectrally-resolve a cluster of rovibrational transitions in the fundamental vibrational band of CO at MHz measurement rates, from which temperature and density were inferred from the line areas and pressure from line-width. The laser source was fiber-coupled for remote light delivery to the exhaust plane of the RDRE combustor. A single-ended optical interface with retro-reflection off of the annulus center body was utilized for stand-off in situ detection. The test article consisted of a 76.2 mm diameter annulus with a annular width of 0.5 cm and doublet impingement injection. Time-resolved CO absorption data was analyzed to examine the evolution of gas properties during engine test firings of 0.5–1.0 s in duration. Start-up transients were examined and intra-cycle profiles of gas properties were compared over a range of equivalence ratios and mass fluxes comprising approximately 25 test conditions. The effects of variable mixing were also examined through a staggered doublet injector configuration. The novel thermodynamic dataset was compared with thrust measurements, wave speed visualizations and first-order detonation models, revealing non-ideal behavior such as parasitic deflagration and modulating oxidizer-to-fuel ratio related to post-detonation injector recovery.

Mid-infrared laser absorption spectroscopy has been used to measure gas properties (pressure, temperature and species density) at MHz measurement rates in the annular exhaust of a methane-oxygen rotating detonation rocket engine. Bias-tee circuitry is used to modulate laser injection current into distributed feedback quantum cascade and interband cascade lasers in the 4-5 micron range enabling radio frequency (RF) wavelength modulation on the order of several MHz while yielding sufficient scan depth to capture multiple rovibrational transitions in the fundamental vibrational bands of CO and CO2. Sub-microsecond spectrally-resolved CO absorption lineshapes provide for inference of temperature and species from a two-line area ratio and pressure from collision line-width. Time-resolved CO absorption data is used to examine the complete time history of gas properties during a representative engine test firing. Demonstrations of MHz CO2 column density measurements and higher speed measurements (2-3~MHz) of pressure, temperature, and CO column density are also shown.

Temperature-dependent line mixing and line broadening parameters were empirically-determined for rovibrational transitions (J = 99-145) in the (00(0)0 -> 00(0)1) and (01(1)0 -> 01(1)1) bandheads of carbon dioxide near 4.2 mu m. Collisional effects by argon on the high rotational energy lines (E '' = 3920-8090 cm(-1)) in the R-branch were studied over a range of temperatures from 1200-3000 K in a shock tube. Measured absorption spectra comprising the target lines in an argon bath gas at near-atmospheric pressures were fit with Voigt profiles to determine line-broadening coefficients, with temperature dependence accounted by a power law. With line broadening established, line-mixing effects were examined at elevated pressures up to 58 atm and similar temperatures, reflecting conditions in high-pressure combustion environments. A modified exponential gap model for line mixing was developed to capture the pressure and temperature dependence of collisional transfer rates for the bandhead region using the relaxation matrix formalism. (C) 2020 Elsevier Ltd. All rights reserved.

A mid-infrared laser absorption sensing method has been developed to quantify gas properties (temperature, pressure, and species density) at MHz measurement rates, with application to annular rotating detonation rocket flows. Bias-tee circuitry is integrated with distributed feedback quantum cascade and interband cascade lasers in the $$4{-}5~\mu \hbox {m}$$ range enabling diplexed radio frequency (RF) wavelength modulation on the order of several MHz while yielding sufficient scan depth to capture multiple rovibrational transitions in the fundamental vibrational bands of $${\text {CO}}$$ and $${\text {CO}}_{2}$$ . Sub-microsecond spectrally-resolved $${\text {CO}}$$ absorption lineshapes provide for inference of temperature and species from a two-line area ratio and pressure from collision line-width. $${\text {CO}}_{2}$$ column density is inferred from peak-to-valley differential absorption at the bandhead near $$4.19~\mu \hbox {m}$$ . A field demonstration on a methane-oxygen rotating detonation rocket engine was performed utilizing an in situ single-ended retro-reflection optical configuration aligned at the exhaust plane. The target gas properties are temporally-resolved at up to 3 MHz across rotating detonations with up to 20 kHz cycle frequency.

A unique spectroscopic strategy has been developed for laser absorption sensing of carbon monoxide (CO) and carbon dioxide (CO2) at extreme pressures (P > 50 atm) relevant to modern combustion devices. The strategy exploits the band narrowing effects of line mixing, which acutely impact spectrally dense regions, such as bandheads, where line spacing is small. Line mixing is shown to counter collisional line-broadening effects that reduce differential absorption at elevated pressures and often limit the pressure range of laser absorption methods. In this work, the R-branch bandheads of CO and CO2, which are only observed at high temperatures relevant to combustion, are targeted near 2.3 mu m and 4.2 mu m, respectively. Spectral line-mixing models were developed for each bandhead region to account for the collision-induced population transfer rates between rotational energy states over a wide range of elevated temperatures and pressures. Modified exponential-gap models using the relaxation matrix formalism were shown to capture the thermodynamic dependence of the population transfer rates and enabled scaling. Differential absorption at the bandheads was observed to increase by up to a factor of ten at high gas densities, due to line-mixing effects, enabling detection with relatively narrow-band tunable semi-conductor lasers. With refined spectroscopic models, laser absorption measurements of temperature, CO, and CO2 were demonstrated over a range of high pressures (up to 104 atm) in a sub-scale rocket combustor operated with kerosene and supercritical methane. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

A novel laser absorption sensing strategy has been developed to evaluate combustion progress through quantitative measurements of carbon dioxide (CO2) in high-pressure (> 50atm), high-temperature (> 3000 K) hydrocarbon-fueled rocket combustion flows. The sensor enables a broad range of operability by probing rovibrational transitions in the bandhead of CO2 near 4.2μm, accessed with an interband cascade laser. Under extreme rocket conditions, this targeted bandhead region experiences line-mixing effects that favorably distort the molecular spectra. A preliminary spectroscopic model of line-mixing effects has been developed utilizing a high-enthalpy shock tube to achieve scalability of spectral simulations over a range of high temperatures and high pressures. The model is employed for quantitative interpretation of measured absorption signals. The mid-infrared light source was fiber-coupled for remote light delivery at propulsion test facilities. A wavelength modulation spectroscopy technique utilizing normalized-second harmonic detection was implemented for acquiring differential absorption signals in a harsh rocket combustor environment. Using this method, measurements of CO2 concentration have been demonstrated over a range of operating conditions up to 83 bar in a single-element-injector RP-2/GOx rocket combustor at the Air Force Research Laboratory in Edwards, CA.

We present a heterodyne terahertz spectrometry platform based on plasmonic photomixing, which enables the resolution of narrow spectral signatures of gases over a broad terahertz frequency range. This plasmonic heterodyne spectrometer replaces the terahertz mixer and local oscillator of conventional heterodyne spectrometers with a plasmonic photomixer and a heterodyning optical pump beam, respectively. The heterodyning optical pump beam is formed by two continuous-wave, wavelength-tunable lasers with a broadly tunable terahertz beat frequency. This broadly tunable terahertz beat frequency enables spectrometry over a broad bandwidth, which is not restricted by the bandwidth limitations of conventional terahertz mixers and local oscillators. We use this plasmonic heterodyne spectrometry platform to resolve the spectral signatures of ammonia over a 1-4.5 THz frequency range.

Temperature-dependent line mixing and line broadening parameters were empirically-determined for 17 rovibrational transitions in the v(1 -> 3) bandhead of carbon monoxide near 2.3 mu m. Collisional effects on the high rotational energy lines (E '' = 5500-8600 cm(-1)) in the R-branch were studied over a range of temperatures from 1200-3750 K in a shock tube and heated gas cell. Measured spectra comprising the target lines in Ar and CO bath gases were fit with Voigt profiles at near-atmospheric pressures to determine line-broadening coefficients, with temperature dependence accounted by a power law. With line broadening established, line-mixing effects were examined at elevated pressures up to 60 atm at similar temperatures, reflecting conditions in high-pressure combustion environments. A modified exponential gap model for line mixing was developed to capture the pressure and temperature dependence of collisional transfer rates for the bandhead region using the relaxation matrix formalism. (C) 2019 Elsevier Ltd. All rights reserved.

We present a heterodyne terahertz spectrometry platform based on plasmonic photomixing, which enables the resolution of narrow spectral signatures of gases over a broad terahertz frequency range. This plasmonic heterodyne spectrometer replaces the terahertz mixer and local oscillator of conventional heterodyne spectrometers with a plasmonic photomixer and a heterodyning optical pump beam, respectively. The heterodyning optical pump beam is formed by two continuous-wave, wavelength-tunable lasers with a broadly tunable terahertz beat frequency. This broadly tunable terahertz beat frequency enables spectrometry over a broad bandwidth, which is not restricted by the bandwidth limitations of conventional terahertz mixers and local oscillators. We use this plasmonic heterodyne spectrometry platform to resolve the spectral signatures of ammonia over a 1-5 THz frequency range.

A novel cross-band laser absorption spectroscopy technique has been developed for quantitative measurements of gas temperature and carbon monoxide (CO) in high-pressure, high-temperature rocket combustion flows. The strategy enables a broad range of sensor operability by simultaneously probing rovibrational transitions in both the fundamental and first overtone bands of CO near $$4.98\,\upmu \hbox {m}$$ and $$2.32\,\upmu \hbox {m}$$, respectively, which sustain large differences in temperature dependence despite collisional broadening. Scanned-wavelength modulation spectroscopy methods are integrated for noise rejection in the harsh rocket operating environment. Initial experiments using the cross-band thermometry technique have been conducted on a single-element-injector rocket combustor with RP-2/GOx and $${\hbox {CH}_4}/\hbox {GOx}$$ propellant combinations at pressures up to 75 bar. Measurements of the first overtone bandhead ($$2.32\,\upmu \hbox {m}$$) maintained adequate signal-to-noise at even higher pressures (up to 105 bar), although deviating significantly from spectral simulations. To account for collisional effects at high gas densities, empirical models for line mixing, developed via shock tube studies, were employed to enable quantitative interpretation of measured signals for temperature and CO mole fraction in the rocket combustor.

A laser absorption sensor was developed for carbon monoxide (CO) sensing in high-pressure, fuel-rich combustion gases associated with the internal conditions of hydrocarbon-fueled liquid bipropellant rockets. An absorption feature near 4.98 \(\upmu\)m, comprised primarily of two rovibrational lines from the P-branch of the fundamental band, was selected to minimize temperature sensitivity and spectral interference with other combustion gas species at the extreme temperatures (> 3000 K) and pressures (> 50 atm) in the combustion chamber environment. A scanned wavelength modulation spectroscopy technique (1f-normalized 2f detection) is utilized to infer species concentration from CO absorption, and mitigate the influence of non-absorption transmission losses and noise associated with the harsh sooting combustor environment. To implement the sensing strategy, a continuous-wave distributed-feedback (DFB) quantum cascade laser (QCL) was coupled to a hollow-core optical fiber for remote mid-infrared light delivery to the test article, with high-bandwidth light detection by a direct-mounted photovoltaic detector. The method was demonstrated to measure time-resolved CO mole fraction over a range of oxidizer-to-fuel ratios and pressures (20–70 atm) in a single-element-injector RP-2-GOx rocket combustor.