Application of Fiber-Based Emission Spectroscopy to the T-ADFA Hypersonic Shock Tunnel

Dawson Schuck


An optical-fiber-based emission spectroscopy technique is applied to a cylinder exposed to hypervelocity flow in the T-ADFA free-piston shock tunnel facility, for the purpose of re-entry vehicle ground test. Two air conditions are tested, with total enthalpies of approximately 3.8 and 5.2 MJ/kg respectively. The bulk of testing occurs at the 3.8 MJ/kg condition and the repeatability of measurement is assessed. A bare multi-mode fiber is embedded at the surface of the model and is used for non-intrusive light collection in the streamwise direction. The flow effects on the fiber face are investigated quantitatively, indicating very low permanent damage at the tested conditions. The technique of mechanical fiber cleaving is applied for each facility run, and is shown to offer a high run-to-run repeatability for preparation quality. Spectra are captured over the 350-800 nm spectral range at a rate of 2000 spectra per second, using an Avantes commercial digital spectrometer. The system is calibrated to offer absolute intensity measurements, more accurately than previous window-based methods. Spectra are seen to be self-similar with time for fixed tunnel conditions and between tunnel runs. However, large differences in intensity are observed between subsequent runs in the facility, as high as 100%. A strong presence of Fe and Ti impurities is detected, as well as a blackbody continuum source, attributed to soot in the shock layer. Measurements taken with angular rotation of the model indicate a significantly weaker influence of blackbody radiation than at the stagnation point, with an order of magnitude less incident energy recorded. A curve-fitting procedure is applied to the blackbody continuum of stagnation spectra and time-resolved temperature inferences are made about the emitting particulates.



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