HIGH-TEMPERATURE METHANE ABSORPTION WITH A DUAL FREQUENCY COMB SPECTROMETER

Quantitative measurements of combustion system fueling and hot-Jupiter exoplanets require accurate methane absorption data at elevated temperatures. The ExoMol and HITRAN spectral databases in the near-infrared 6500-9000cm-1 range are based on the 10to10 potential energy surface, and the 80K and 296K empirical WKLMC linelist, respectively, which do not empirically constrain all elevated-temperature behavior. We present spectra of the near-infrared methane overtone band around 1400nm at temperatures from 296 K to 900 K. The spectra are taken using a three-zone tube furnace and a dual-frequency comb spectrometer with 600 cm-1 bandwidth and .00667cm-1 resolution. These measurements are targeted toward providing a compact, accurate methane absorption linelist for 300-900K

Spatially resolved mass flux measurements with dual comb spectroscopy

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 frequency combs can simultaneously measure all of the components of mass flux (velocity, temperature, pressure, and species concentration) with low uncertainty, spatial resolution corresponding to the span of 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 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 DCS instrument uncertainty of ~0.4%, and total uncertainty (including non-instrument sources) of ~7% for mass flux measurements. These measurements demonstrate DCS as a low-uncertainty mass flux sensor for a variety of applications.Comment: Main Text: 15 pages, 7 figure; Supplement: 6 pages, 4 figures; Submitted to Optic

Hypersonic Combustion Diagnostics with Dual Comb Spectroscopy

Hypersonic 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