Visualization of soot formation from evaporating fuel films by laser-induced fluorescence and incandescence
Late-evaporating liquid fuel wall films are considered a major source of soot in gasoline direct-injection engines. In this study, a direct-injection model experiment was developed to visualize soot formation in the vicinity of evaporating fuel films. Isooctane is injected by a multi-hole injector into a wind tunnel with an optically accessible test section. Air flows continuously at low speed and ambient pressure through the test section. Some of the liquid fuel impinges on the quartz-glass windows and forms fuel films. After spark ignition, a turbulent flame front propagates through the chamber and ignites pool fires near the fuel films, leading to locally sooting combustion. A laser light sheet with a wavelength 532 nm excites laser-induced fluorescence (LIF) of large polycyclic aromatic hydrocarbons (PAH) with five or more aromatic rings, considered as soot precursors, near the evaporating fuel films. Additionally, a light sheet at 1064 nm excites laser-induced incandescence (LII) of soot particles. Two intensified CCD cameras simultaneously detect the LII and LIF signals, and thus visualize PAH and soot. In complementary line-of-sight imaging, the fuel spray, chemiluminescence of the flame, and soot incandescence are captured with a high-speed color camera. In addition to this fuel-injection experiment, a sooting laminar coflow flame burning ethylene in air (Santoro burner) is used for preliminary in-situ measurements.