Speaker
Description
Introduction
Atmospheric entry leads to extreme heat flux onto the flight vehicle’s surface due to the high enthalpy of the surrounding flowfield. Currently, thermal protection systems are severely oversized which makes vehicles too heavy,restricting performance and payload capacity. One large source of uncertainty is thermochemical non-equilibrium, which has been shown to have a strong effect on surface heat flux and shear stress, gas radiation, and flowfield characteristics. Improved design of hypersonic vehicles and re-entry capsules necessitates more accurate predictive capabilities of non-equilibrium flows. Ground testing is conducted to generate flows similar to those encountered in flight, replicating the essential features of non-equilibrium flows. One such canonical experimental setup is the shock tube, where a normal shock transiently passes through a straight tube [CB17]. Although subject to several facility-related artefacts [CMM23, CSMM22], shock tubes represent one of the most fundamental fluid mechanical processes, which allows the isolation of thermochemical non-equilibrium from other aspects of complex flowfields. As such, shock tubes provide the opportunity of studying fundamentals of thermochemical reactionsfor flight-relevant enthalpies. Previous datasets span different facilities, with NASA Ames’ EAST and Oxford University’s T6 representing some of the most recent examples [GCM22, CB17]. Both gas radiation and thermochemistry can be readily analysed using optical diagnostics. Optical emission spectroscopy (OES) collects the emitted light from highly energetic particles, which produce the majority of radiative heat flux. Data can be used to infer number densities of highly excited states, as well as internal excitation temperatures of electronic, vibration, and rotation modes. In cases of significant self-absorption, OES can be utilised to infer the population density of low energy states, or even the ground state, i.e. the most abundant energy states which make up the majority of the particles present [HLF+17]. Transitions suitable for this analysis can be found in the VUV spectral region. This region features
emission lines of atomic oxygen and nitrogen where the lower state, i.e. the absorbing state, corresponds to metastable and ground states. Furthermore, the VUV is the dominant spectral region for radiative heat flux to a reentry capsule’s surface. Existing data of emission spectra in the VUV is very sparse due to the experimental complexity associated with the respective measurement technique [CMGO09, MJM+23, HLF+17], e.g. the optical path needs to be either evacuated or flushed with a nonabsorbing
gas.
Methodology
Shock tube flows are generated in the Oxford T6 Stalker Tunnel in aluminium shock tube mode [GCM22]. The strong driver conditions of T6 enable high shock velocities while retaining a large tube diameter, providing a long optical path for a spectroscopic system. This enables a larger signal to noise ratio in the measurement of spectroscopic data while minimising exposure times in order to reduce the amount of spatial blurring due to shock movement. Optical measurements are taken through windows set in the shock tube wall, and acquisition is triggered to record as it passes this location. A number of flush-mounted piezoelectric pressure sensors along the length of the facility record the arrival time of the shock wave which allows subsequent rebuilding of the shock history. Recent work has shown that the shock history, and associated hydrodynamic behaviour of the flow has a significant influence on the thermodynamic state [CSMM22]. Furthermore, hydrodynamic effects have been shown to significantly affect the time of flight of reacting particles which necessitates a spatial transformation to match conditions between flight case and shock tube [CMM23]. Flow conditions investigated in this work
will consist of a set of velocities between 5.5 km/sand 10 km/s [GCM22]. The current work extends their characterisation to include the now accessible VUV spectral region. Some of the considered test conditions also directly correspond to data collected in the EAST facility, allowing cross-facility comparison [CB17]. The VUV spectroscopic system is designed to collect radiation between 116 nm and 900 nm. Even though longer wavelengths in the Ultraviolet, Visible and Near Infrared are accessible, the system is optimised towards wavelengths between 116 nm and 250 nm where the quantum efficiency of the detector, blazing angle of the dispersion grating are greatest. The detailed functionality of the system is presented in [MGC+22], and previous measurements in the OPG2 plasma wind tunnel facility are presented in [BCWH24]. Because molecular oxygen and water vapour absorb VUV wavelengths in ambient air, any acquisition set-up must operate either using oxygenfree gas or under a high level of vacuum, the latter being the approach of the current work. The collection optics system is designed to be contained in vacuum chambers fitted to the shock-tube via the test section window. The telecentric optical system images light onto the entry slit of a spatially resolving VUV spectrograph (McPherson 207V). An intensified P43 iStar sCMOS camera is connected
to the spectrograph to record the spectra. The whole system is pumped down to a low vacuum level $10^{-3}$ up to the test section’s window using a
dry scroll pump (Edwards nXDS10i) and a turbopump (Edwards nEXT300D) mounted in series. Wavelength calibration is performed by fitting easily identifiable atomic lines from an IntelliCal mercury (Hg) and neon-argon (Ne-Ar) calibration source to their expected wavelengths. The fitted wavelength axis is subsequently used to obtain the Spectral Instrument Line Shape (ILS) by fitting the most intense spectral line measured to the square-root of a Voigt profile, the most accurate shape function to capture the ILS with an intensified CCD array. The experimentally measured full-width half-maximum parameters are 0.86 nm and 0.01 nm for Gaussian and Lorentzian profiles respectively. Spatial smearing was measured using a knife edge placed in front of an integrating sphere at the centreline of the test-section. Images were recorded at different locations along the field of view to obtain edge spread functions (ESF) and resolve spatial smearing in space. The LSF is a good indication of signal intensity loss and spatial smearing of information, and dictates the practical spatial resolution limit of the optical system. The experimentally measured full-width half-maximum parameters varied from 0.556 to 1.75mm for Gaussian and 0.008 and 0.210mm for Lorentzian profiles, with averages of 0.8362 and 0.0575 mm.
Results
The full paper will present the data of the currently ongoing test campaign and will contain setup, calibration and initial post-processing. Calibration is carried out in all three dimensions of wavelength, space and absolute spectral radiance. This system will allow a cross-comparison to measurements taken in NASA’s EAST facility and will extend the measured wavelength range of previously investigated conditions in T6. Furthermore, the data will allow the investigation of chemical non-equilibrium, by utilising the characteristic features of emitted radiation behind strong shock waves.
Summary
The VUV spectroscopic system is designed to collect radiation between 116 nm and 900 nm. Even though longer wavelengths in the Ultraviolet, Visible and Near Infrared are accessible, the system is optimised towards wavelengths between 116 nm and 250 nm where the quantum efficiency of the detector, blazing angle of the dispersion grating are greatest. The detailed functionality of the system is presented in [MGC+22], and previous measurements in the OPG2 plasma wind tunnel facility are presented in [BCWH24]. Because molecular oxygen and water vapour absorb VUV wavelengths in ambient air, any acquisition set-up must operate either using oxygenfree gas or under a high level of vacuum, the latter being the approach of the current work. The collection optics system is designed to be contained in vacuum chambers fitted to the shock-tube via the test section window. The telecentric optical system images light onto the entry slit of a spatially resolving VUV spectrograph (McPherson 207V). An intensified P43 iStar sCMOS camera is connected to the spectrograph to record the spectra. The whole system is pumped down to a low vacuum level (10$^{-3}$ Pa range) up to the test section’s window using a dry scroll pump (Edwards nXDS10i) and a turbo pump (Edwards nEXT300D) mounted in series.