Spatially resolved mass flux measurements with dual comb spectroscopy

TL;DR

This paper demonstrates that dual comb spectroscopy with mode-locked frequency combs can accurately and spatially resolve mass flux components in large open areas, offering a non-intrusive alternative to traditional sensors.

Contribution

The study introduces a novel application of dual comb spectroscopy for spatially resolved mass flux measurement with low uncertainty, in a hypersonic engine environment.

Findings

Mass flux measurements are within 3.6% of engine air supply values.

Instrument uncertainty is approximately 0.4%.

Total measurement uncertainty is around 7%.

Abstract

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.