Simultaneous lateral- and end-view visualization of the auto-ignition of primary reference fuels in a rapid compression machine

Abstract

Rapid compression machines (RCMs) are intensively used in auto-ignition investigations for their high similarity to real engines and their convenience for optical visualization. However, previous works using RCMs usually visualized the combustion processes from only a single view, which is limited in capturing critical details regarding ignition in other directions due to line-of-sight integration, given that ignition is actually three-dimensional. This work presents a comprehensive investigation of auto-ignition behaviors in a novel dual-window optical RCM. By simultaneously providing fully-optical accesses into the entire combustion chamber from both the end- and lateral-view, the auto-ignition of primary reference fuels (PRFs) under various initial conditions were captured and the impacts of vortices were examined. Images obtained from dual perspectives demonstrated that auto-ignition under vortex-minimized conditions always initiated from the end-walls and then propagated in the chamber axial direction, exhibiting obvious “near-wall initiation, off-wall propagation” characteristic. Such feature was insignificantly impacted by fuel reactivity, initial thermodynamic condition, and the existence of initial spark-ignited flame. Under vortex-existing conditions, however, the initiation and development of auto-ignition was highly dependent on the interaction between the turbulent field and the fuel reactivity. Only for cases with Da >1, were vortices observed to have distinct impacts on auto-ignition characteristics. At last, the influences of ignition delay time gradient were also discussed.