Speaker
Description
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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 reactions
for 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.s1 and
10 km.s1 [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
(103 Pa range) 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 (NeAr) 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
(103 Pa range) up to the test section’s window using a
dry scroll pump (Edwards nXDS10i) and a turbopump
(Edwards nEXT300D) mounted in series.
