External hot surface axisymmetric models for supersonic combustion

External hot surface axisymmetric models for supersonic combustion

An external axisymmetric configuration is introduced for supersonic combustion research
in hypersonic wind tunnel flow. In this configuration, high quality data can be
generated for validation of computational simulations. The external axisymmetric geometry
offers the important advantages of easy optical diagnostic access to the physical
fields of interest and a geometry that can be visualised as two dimensional, but is free of
non two dimensional edge effects. The application of quantitative OH* measurements in
the axisymmetric configuration is introduced in this work. A resistively-heated graphite
model with a water cooling system was devised for the axisymmetric arrangement and
was commissioned to simulate the hot surface environments typically encountered in
hypersonic flight. The model was fueled with pure hydrogen and premixed hydrogen-air
mixtures through the fuel delivery system that was constructed for the experimental
work. Hot wall temperatures within the range of 1500 to 1800K were achieved during
the combustion testing.

Several optical techniques were used for the experimental measurements: Two Colour
Ratio Pyrometry (TCRP) and Visible near Infrared (VnIR) Spectrometer methods
were used for the hot surface temperature measurement; high-speed schlieren was used
for the flowfield visualization and an ICCD camera fitted with a narrow-band filter
at approximately 310 nm was used for two-dimensional imaging of the OH* chemiluminescence.
The TCRP with the wavelength ratio of I(850nm) I(700nm) was used
for time-resolved temperature determination of the hot surface. The ICCD camera
setup was used to detect and quantify the OH* chemiluminescence. The quantitative
chemiluminescence measurements were achieved by using the Abel inversion and a new
method which is proposed for the first time in this thesis for the calibration of absolute
number density of the radiating OH*. This is a convenient approach when adequate signal
magnitudes are emitted from the hot surface radiation and OH* chemiluminescence
is acquired through the ICCD device simultaneously during testing.

A set of experimental conditions at Mach 2, Mach 4 and Mach 6 flow were examined
over a range of total pressure varying from 0.2 MPa to 1.9 MPa and a total temperature
of approximately 570K. No evidence of combustion was observed from the ICCD
images during the hot surfaces testing at the supersonic and hypersonic conditions.
The flow environments produced by the TUSQ facility and the models were evidently
not sufficient for ignition. The optical diagnostic techniques developed in this study
for external axisymmetric configurations were demonstrated based on combustion results
acquired in the nominally quiescent test section environment (without hypersonic
flow). These tests indicated that the ignition process was initiated when the background
pressure was elevated to about 10 kPa. The combustion flow was reconstructed
numerically using a CFD Solver - Eilmer3 with a hydrogen oxidation chemistry model
and the addition of a OH* sub-scheme reaction mechanism. The measured peak level
of OH* chemiluminescence was over-predicted by the numerical simulation by a factor
of about 10. The results of the numerical simulations show that in the supersonic
and hypersonic cases, the poor mixing also contributed to suppression of the ignition
process.