Fiber-based emission spectroscopy on a cylindrical body in the T-ADFA hypersonic shock tunnel

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 testing. Two air conditions are tested, with total enthalpies of approximately 3.9 and 5.4 MJ/kg respectively. The testing occurs mostly at the 3.9 MJ/kg condition, and the repeatability of measurement is assessed at that condition. A novel method, involving a bare multi-mode fiber embedded at the surface of the model, is used for non-intrusive light collection in the streamwise direction. The flow effects on the fiber face are investigated quantitatively, indicating very little 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 fiber 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, with better accuracy than the more common approach of imaging from outside the flow facility. Spectra are seen to be self-similar with time for fixed tunnel conditions and between tunnel runs. However, differences in intensity, up to 100%, are observed between subsequent runs in the facility. A strong presence of H, Fe and Na impurities is detected in the emission spectrum, as well as a blackbody continuum source, attributed to soot in the shock layer. Measurements taken of flow above the stagnation streamline 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 measurements are made from the emitting particulates. These temperatures are consistently a few hundred Kelvin above CFD predictions, yet the transient temperature profiles are of similar shape. This paper presents the first blackbody-derived temperature measurements of its kind in a shock tunnel facility and provides evidence in contradiction to a radiating nozzle reservoir hypothesis.