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Accurate diagnostics of the combustor region of ramjet engines can improve engine design and create benchmarks for computational fluid dynamics models. Previous works demonstrate that dual frequency comb spectroscopy can provide low uncertainty diagnostics of multiple flow parameters in the non-combusting regions of ramjets. However, the high temperatures present in the combustor present a challenge for broadband spectroscopic absorption models that are used to interpret measurements in these regions. Here, we utilize a new water vapor absorption database created for high temperature water-air mixtures to fit spectra measured in a ground-test ramjet engine with a broadband near-infrared dual comb absorption spectrometer. We extract 2D profiles of pressure, temperature, water mole fraction, and velocity using this new database. We demonstrate that the new database provides the lowest fit residuals compared to other water vapor absorption databases. We compare computational fluid dynamics simulations of the combustor with the measured data to demonstrate that the simulations overpredict heat release and water vapor production., Comment: This work has been cleared by the Air Force under case number AFRL-2023-4205

Laser absorption Doppler velocimeters use a crossed-beam configuration to cancel error due to laser frequency drift and absorption model uncertainty. This configuration complicates the spatial interpretation of the measurement since the two beams sample different volumes of gas. Here, we achieve single-beam velocimetry with a portable dual comb spectrometer (DCS) with high frequency accuracy and stability enabled by GPS-referencing, and a new high-temperature water vapor absorption database. We measure the inlet flow in a supersonic ramjet engine and demonstrate single-beam measurements that are on average within 19 m/s of concurrent crossed-beam measurements. We estimate that the DCS and the new database contribute 1.6 and 13 m/s to this difference respectively., Comment: This work has been cleared by the Air Force under case number AFRL-2023-4805

We present broadband dual frequency comb laser absorption measurements of 2% H$_2$O (natural isotopic abundance of 99.7% H$_2^{16}$O) in air from 6600-7650 cm$^{-1}$ (1307-1515 nm) with a spectral point spacing of 0.0068 cm$^{-1}$. Twenty-nine datasets were collected at temperatures between 300 and 1300 K ($\pm$0.82% average uncertainty) and pressures ranging from 20 to 600 Torr ($\pm$0.25%) with an average residual absorbance noise of 8.0E-4 across the spectrum for all measurements. We fit measurements using a quadratic speed-dependent Voigt profile to determine 7088 absorption parameters for 3366 individual transitions found in HITRAN2020. These measurements build on the line strength, line center, self-broadening, and self-shift parameters determined in the Part I companion of this work. Here we measure air-broadened width (with temperature- and speed-dependence) and air pressure shift (with temperature dependence) parameters. Various trends are explored for extrapolation to weak transitions that were not covered in this work. Improvements made in this work are predominantly due to the inclusion of air pressure shift temperature dependence values. In aggregate, these updates improved RMS absorbance error by a factor of 4.2 on average, and the remaining residual is predominantly spectral noise. This updated database improves high temperature spectroscopic knowledge across the 6600 7650 cm$^{-1}$ region of H$_2$O absorption., Comment: Database files available upon request. Will be included with published manuscript following review process

Developing accurate computational models of wildfire dynamics is increasingly important due to the substantial and expanding negative impacts of wildfire events on human health, infrastructure, and the environment. Wildfire spread and emissions depend on a number of factors, including fuel type, environmental conditions (moisture, wind speed, etc.) and terrain/location. However, there currently exist only a few experimental facilities that enable testing of the interplay of these factors at length scales <1 m with carefully controlled and characterized boundary conditions and advanced diagnostics. Experiments performed at such facilities are required for informing and validating computational models. Here we present the design and characterization of a novel tilting wind tunnel (the 'WindCline') for studying wildfire dynamics. The WindCline is unique in that the entire tunnel platform is constructed to pivot around a central axis, which enables sloping of the entire system without compromising the quality of the flow properties. In addition, this facility has a configurable design for the test section and diffuser to accommodate a suite of advanced diagnostics to aid in the characterization of 1) the parameters needed to establish boundary conditions and 2) flame properties and dynamics. The WindCline thus allows for measurement and control of several critical wildfire variables and boundary conditions, especially at the small length scales important to the development of high fidelity computational simulations (10 - 100 cm). Computational modeling frameworks developed and validated under these controlled conditions can expand understanding of fundamental combustion processes, promoting greater confidence when leveraging these processes in complex combustion environments., Comment: 27 pages, 12 figures + 6 in supplemental material, submitted to Review of Scientific Instruments

Supersonic engine development requires accurate and detailed measurements of fluidic and thermodynamic parameters to optimize engine designs and benchmark computational fluid dynamic (CFD) simulations. Here, we demonstrate that dual frequency comb spectroscopy (DCS) with mode-locked frequency combs can provide simultaneous absolute measurements of several flow parameters with low uncertainty across a range of conditions owing to the broadband and ultrastable optical frequency output of the lasers. We perform DCS measurements across a 6800-7200 cm-1 bandwidth covering hundreds of H2O absorption features resolved with a spectral point spacing of 0.0067 cm-1 and point spacing precision of 1.68 x 10-10 cm-1. We demonstrate 2D profiles of velocity, temperature, pressure, water mole fraction, and air mass flux in a ground-test dual-mode ramjet at Wright-Patterson Air Force Base. The narrow angles of the measurement beams offer sufficient spatial resolution to resolve properties across an oblique shock train in the isolator and the thermal throat of the combustor. We determine that the total measurement uncertainties for the various parameters range from 1% for temperature to 9% for water vapor mole fraction, with the absorption database/model that is used to interpret the data typically contributing the most uncertainty (leaving the door open for even lower uncertainty in the future). CFD at the various measurement locations show good agreement, largely falling within the DCS measurement uncertainty for most profiles and parameters.

Providing an accurate, representative sample of mass flux across large open areas for atmospheric studies or the extreme conditions of a hypersonic engine is challenging for traditional intrusive or point-based sensors. Here, we demonstrate that laser absorption spectroscopy with mode-locked frequency combs can simultaneously measure all of the components of mass flux (velocity, temperature, pressure, and species mole fraction) with low uncertainty, spatial resolution corresponding to the laser line of sight, and no supplemental sensor readings. The low uncertainty is provided by the broad spectral bandwidth, high resolution, and extremely well-known and controlled frequency axis of stabilized, mode-locked frequency combs. We demonstrate these capabilities using dual frequency comb spectroscopy (DCS) in the isolator of a ground-test supersonic propulsion engine at Wright-Patterson Air Force Base. The mass flux measurements are consistent within 3.6% of the facility-level engine air supply values. A vertical scan of the laser beams in the isolator measures the spatially resolved mass flux, which is compared with computational fluid dynamics simulations. A rigorous uncertainty analysis demonstrates a instrument uncertainty of ~0.4%, and total uncertainty (including non-instrument sources) of ~7% for mass flux measurements. These measurements demonstrate DCS with mode-locked frequency combs as a low-uncertainty mass flux sensor for a variety of applications.

Accurate absorption models for gases at high pressure and temperature support advanced optical combustion diagnostics and aid in the study of harsh planetary atmospheres. Developing and validating absorption models for these applications requires recreating the extreme temperature and pressure conditions of these environments in static, uniform, well-known conditions in the laboratory. Here, we present the design of a new gas cell to enable reference-quality absorption spectroscopy at high pressure and temperature. The design centers on a carefully controlled quartz sample cell housed at the core of a pressurized ceramic furnace. The half-meter sample cell is relatively long compared to past high- pressure and temperature absorption cells, and is surrounded by a molybdenum heat spreader that enables high temperature uniformity over the full length of the absorbing gas. We measure the temperature distribution of the sample gas using in situ thermocouples, and fully characterize the temperature uniformity across a full matrix of conditions up to 1000 K and 50 bar. The results demonstrate that the new design enables highly uniform and precisely known conditions across the full absorbing path length. Uniquely, we test the new gas cell with a broadband, high-resolution dual frequency comb spectrometer that enables highly resolved absorption spectroscopy across a wide range of temperature and pressure conditions. With this system, we measure the spectrum of CO$_2$ between 6800 and 7000 cm$^{-1}$ at pressures between 0.2 and 20 bar, and temperatures up to 1000 K. The measurements reveal discrepancies from spectra predicted by the HITRAN2016 database with a Voigt line shape at both low- and high-pressure conditions. These results motivate future work to expand absorption models and databases to accurately model high-pressure and -temperature spectra in combustion and planetary science research.

Continuous monitoring of oil and gas infrastructure is of interest for improving emissions and safety by enabling rapid identification and repair of emission sources, especially large sources that are responsible for the bulk of total emissions. We have previously demonstrated dual frequency comb spectroscopy (DCS) coupled with atmospheric modeling and inversion techniques (the DCS Observing System) as a viable and accurate approach for detection, attribution and quantification of methane emissions at distances of more than 1 km under controlled, steady emissions scenarios. Here, we present the results of validation testing designed to mimic the complexity of operational well pad emissions from oil and gas production, and the first field measurements at an active oil and gas facility. The validation tests are performed single-blind (the measurement and data analysis team are not given information about the emissions) at the Methane Emissions Technology Evaluation Center (METEC) test facility. They consist of a series of scenarios ranging from a single, steady-rate emission point to multiple emission points that include intermittent releases (the METEC "R2" tests). Additionally, we present field measurements at an active natural gas storage facility demonstrating that the system can remotely and autonomously monitor methane emissions in a true industrial setting. This field verification is in a configuration designed for continuous and long-term characterization of operational and fugitive emissions. These demonstrations confirm that the DCS Observing System can provide high-confidence continuous monitoring of emissions from complex, operational facilities among natural gas infrastructure., Comment: 13 pages (main text), 4 pages (supplemental information), 5 figures (main text), 2 figures (supplemental information)

Identification and quantification of trace gas sources is a major challenge for understanding and regulating air quality and greenhouse gas emissions. Current approaches either provide continuous but localized monitoring, or quasi-instantaneous 'snapshot-in-time' regional monitoring. There is a need for emissions detection that provides both continuous and regional coverage, because sources and sinks can be episodic and spatially variable. We field deploy a dual frequency comb laser spectrometer for the first time, enabling an observing system that provides continuous detection of trace gas sources over multiple-square-kilometer regions. Field tests simulating methane emissions from oil and gas production demonstrate detection and quantification of a 1.6 g min^-1 source (approximate emissions from a small pneumatic valve) from a distance of 1 km, and the ability to discern two leaks among a field of many potential sources. The technology achieves the goal of detecting, quantifying, and attributing emissions sources continuously through time, over large areas, and at emissions rates ~1000x lower than current regional approaches. It therefore provides a useful tool for monitoring and mitigating undesirable sources and closes a major information gap in the atmospheric sciences., Comment: 9 pages, 4 main text figures, 2 main text tables, 5 SI figures