Developing a Minimally-Intrusive Fiber-optic Sensing System for High-Enthalpy Rocket Plumes

A minimally-intrusive optical sensing system for high-temperature/high-velocity gas-generator exhaust plumes has been developed. The overall goal of this effort is to perform a feasibility assessment of the associated non-intrusive measurement technologies and to establish a roadmap for the most effective practice. For this application glass fiber-optic cables, acting as radiation conduits, are inserted through the combustion chamber or nozzle wall and look directly into the flow core. The cable transmits data from the flame zone to externally-mounted spectrometers. In order to capture the full-optical spectrum, a blended dual-spectrum system was employed, with one spectrometer system tuned for best-response across the visible-light and near-infrared spectrum, and one spectrometer tuned for best-response in the near- and mid-infrared spectrum. The dual-band sensors are radiometrically-calibrated and the sensed-spectra are spliced together using an optimal Wiener filtering algorithm to perform the deconvolution. The merged spectrum is subsequently curve-fit to Planck's black-body radiation law, and flame temperature is calculated from associated curve maxima (Wien's law). The presented fiber-optic sensing systems performs a function that is analogous to Raman spectroscopy. The system is non-contact and does not interfere with the heat transfer processes. In this report data collected from a lab-scale (200 N) hybrid rocket system are analyzed using the described method. Optically-sensed flame-temperatures are correlated to analytical predictions, and shown to generally agree within a few degrees. Additionally, local maxima in the optical spectra are shown to correspond to emission frequencies all species known to exist in the hybrid combustion plume. The presented data makes clear that the approach works equally when the fiber optic protrudes from a solid boundary into the flow field. This result opens up the possibility for the presented fiber-optic techniques to be applied for a wide swath of gas-generators, including gas turbine engines.