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

Homogeneous and inhomogeneous ignition modes of n-heptane were studied using high-speed imaging in a high-pressure shock tube (HPST). n-Heptane, a fuel with strong negative temperature coefficient (NTC) behavior, was mixed with 4%-21% oxygen in argon or nitrogen and ignited over a wide temperature range (700–1250 K) and at elevated pressures (> 10 atm). Ultraviolet (UV) images of OH* emission were captured through a sapphire shock-tube end wall using a high-speed camera and a UV intensifier. The current study 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. We also observed that conventional sidewall diagnostic signals are, in many cases, sufficient to identify inhomogeneous ignitions that are not accurately modeled under the assumption of spatially uniform chemistry.