Numerical modeling of gaseous n-hexane autoignition in an ASTM-E659 apparatus

Numerical simulation of three-dimensional motion, molecular transport, and chemical reactions is reported for gaseous nC6H14 autoignition in an ASTM-E659 apparatus, a 500 mL round-bottom, heated flask filled with hot air and the top open to the atmosphere. The injected fuel is heavier than air, initially flows to the bottom of the flask and diffuses up over time as a stratified layer. Inflow of cold air from the open end of the flask penetrates downward and mixes with the flask contents. The inflow and convection due to the heated flask are observed to create a vortical motion. Chemical reactions were modeled using a reduced kinetic mechanism suitable for low-temperature oxidation of nC6H14. Reaction begins slowly and more substantially near the walls where the local gas temperature is hottest. A ring-like ignition kernel forms followed by a localized two-stage ignition event in a rich fuel/air mixture. Critical reaction pathways were identified for both ignition stages. The resultant flame propagation, which is inhibited towards the bottom by lack of oxygen molecules in this region of the flask, convects upwards, chasing unburnt fuel expelled from the flask due to gas expansion from the ignition event itself. Implications are discussed for refinement of autoignition testing.