Imaging of Fuel-Film Evaporation and Combustion in a Direct-Injection Model Experiment

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/air-mixture. 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 while schlieren imaging visualizes the gradients associated with fuel/air mixing and combustion. 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 shows that soot formation occurs in small regions but always close to the fuel films.