Visualization study of ignition modes behind bifurcated-reflected shock waves

This study was a numerical and experimental investigation of low-temperature auto-ignitions behind reflected shock waves in which a shock tube was employed as the experimental system. We used a high-speed video camera and the Schlieren method to visualize the ignition phenomena. Experiments were performed over a temperature range from 549±10 to 1349±11K and a pressure range from 56±2 to 203±13kPa, and a non-diluted stoichiometric acetylene–oxygen mixture was chosen as the combustible gas. We introduced a numerical simulation to help us understand the disturbed temperature distribution behind bifurcated shock waves due to interference between reflected shock waves and the boundary layer developed behind incident shock waves. Additionally, we experimentally observed and evaluated quantitatively a tendency for ignition positions to be located farther from the reflecting wall as the temperature decreased behind reflected shock waves. To focus our attention on the ignition positions, we classified the ignition types behind reflected shock waves as near-wall ignition and far-wall ignition by 4.7mm distance from reflecting wall. The criterion for these ignition types was estimated to be -1.0⩽(∂ti/∂T5t)p5t⩽-0.5. As a main object in this manuscript, we proposed an ignition model in which local ignition is induced at some distance from reflecting wall based on the numerical simulation and results; the local ignitions at a point distant from the reflecting wall are induced by the temperature rise, with the distance from the reflecting wall, immediately behind concave reflected shock waves due to developing of bifurcated shock waves. We confirmed that there is no discrepancy between the proposed model and experimental results.