High-speed imaging of n-heptane ignition in a high-pressure shock tube

Homogeneous and inhomogeneous ignition modes of n-heptane were investigated using high-speed imaging in a high-pressure shock tube. n-Heptane, a fuel with strong negative temperature coefficient behavior, was mixed with 4%-21% O₂ in Ar or N₂ and ignited over a wide temperature range (700 K–1250 K) and at elevated pressures ( > 10 atm). Ultraviolet images of OH* emission were captured through a sapphire shock-tube end wall using a high-speed camera and a UV intensifier. The results demonstrates the capability to study auto-ignition modes using high-speed imaging in a high-pressure shock tube. Both homogeneous and inhomogeneous auto-ignition events were observed with the latter generally confined to intermediate temperatures and reactive n-heptane mixtures. Conventional sidewall diagnostic signals were, in many cases, sufficient to identify inhomogeneous ignitions that are not accurately modeled under the assumption of spatially uniform chemistry.