Experimental and numerical investigation of evaporating fuel films in combustion

Fuel films evaporating throughout combustion can cause soot formation in spark-ignited direct-injection (DI) engines. In this work, laser-induced fluorescence (LIF), combustion imaging, 3D computational fluid dynamics (CFD), and a low-dimensional model (LDM) were used to investigate fuel-film evaporation, combustion, and soot formation in an atmospheric-pressure constant-flow facility that serves as a DI model experiment. A six-hole injector laterally injects isooctane doped with toluene as a fluorescent tracer. Most of the fuel evaporates into the preheated air crossflow, but some impinges on a quartz window on the opposite side, forming fuel films. The fuel/air-mixture can be ignited by spark electrodes protruding into the chamber. LIF was used to image the fuel-film thickness at different times after start of injection (aSOI). From that, the film mass and evaporation rate are calculated. The large eddy simulation (LES) CFD employs a Lagrangian treatment for the dispersed phase in combination with models for spray/wall interaction and the wall film. CFD and LIF consistently find that evaporation rates are highest for early times aSOI and remain constant from about 10 ms aSOI. At ambient temperature the evaporation rates from LIF and CFD are almost the same while at elevated temperatures the CFD predicts about a two times higher evaporation rate than measured. LIF, CFD, and LDM reveal a strong dependence of the evaporation rate on the wall temperature, while there is very little influence of combustion and thus convective heat transfer. The simulations show that the fuel-film temperature rapidly reaches the wall temperature that remains approximately constant throughout evaporation. High-speed combustion imaging and CFD show the inception of soot pockets at similar times, close to the evaporating fuel films, and consistent in spatial extent.