  Shock tubes are widely used in the study of chemical kinetics. Its benefits
rely on the almost ideal shock-heating process that provides high temperatures
and pressures to a chemical system for a limited test time. Just like any
reactor, shock tubes are not immune to non-ideal effects. The study of
conditions that might deviate experiments from ideal conditions is thus of the
utmost importance. High-speed imaging has been proven to be a powerful bytool
to analyze non-ideal / non-homogenous combustion in shock tubes. In this work,
dual-camera high-speed imaging experiments were performed at 10, 15 and 20 bar
in a high-pressure shock tube (HPST). An optical section was designed as an
extension of the HPST which enabled simultaneous visualization from the endwall
and the sidewall of the driven section of the shock tube. n-Hexane, a fuel with
a negative temperature coefficient (NTC) behavior that has been identified as
prone to non-homogenous ignition, is used as a test fuel. Reactive mixtures and
thermodynamic conditions were selected to visually analyze ignition processes
at the high-temperature, NTC and low-temperature regimes. Non-homogeneous
ignition was observed mostly at the local maximum of the IDT, which is
comprised by the high-temperature and NTC regions. Stoichiometric n-hexane
mixture with high fuel loading (5% n-hexane) presented the highest deviation
from constant volume chemical kinetic simulations. The inclusion of helium as a
bath gas to mitigate preignition was tested and it showed to improve the
susceptibility of the mixtures to develop reaction fronts. The modified
Sankaran criterion for the identification of ignition regimes in shock tubes
was tested and it showed an overall good agreement against the experimental
observations.
