Late-evaporating liquid fuel films within the combustion chamber are considered a major source of soot in gasoline direct-injection engines. In this study a direct-injection model experiment was developed to visualize and investigate the evaporation of fuel films and their contribution to soot formation with different diagnostic techniques. A mixture of isooctane (surrogate fuel) and toluene (fluorescent tracer) 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. Combustion is initiated by a pair of electrodes within the fuel/airmixture. The turbulent flame front propagates through the chamber and ignites pool fires near the fuel films, leading to locally sooting combustion. Laser-induced fluorescence (LIF) of the toluene, excited by laser pulses at 266 nm, is used to image the fuel-film thickness and to visualize the fuel vapor, while laser-induced incandescence (LII), excited at 1064 nm, is used to visualize soot. In complementary line-of-sight imaging, the natural flame luminosity, mainly from soot incandescence, is captured with a high-speed camera and schlieren imaging, combining visualization of the fuel vapor and the sooting flame. The LIF images show that the fuel films remain on the wall surface long after the flame front has passed. The evaporation rate of the individual fuel films seems to be unaffected by combustion, indicating that conductive heat transfer from the wall is the limiting factor in evaporation. The visualization of both natural flame luminosity and LII show that soot formation occurs in small regions but always close to the fuel films.
Late-evaporating liquid fuel wall films are considered a major source of soot in gasoline directinjection 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. In the latter, PAH LIF is detected in a hollow cone region, indicating the transition from fuel to small PAH in the dark region and the growth of the latter into large PAH in the cone region. The edges of the PAH LIF cone are covered by regions of high soot LII signal, indicating the nucleation region where soot forms from PAH. In the wind tunnel, layers of PAH are found in the close vicinity of the evaporating fuel films. Soot forms mostly spatially separated from the PAH and with high spatial intermittency. The chemiluminescence in the bottom of the images indicates the oxidation of soot and PAH, being transported downstream.
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