Dual-camera high-speed imaging of n-hexane oxidation in a high-pressure shock tube

Abstract

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 tool 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.