A study of mixture formation in direct injection diesel like conditions using quantitative fuel concentration visualizations in a gaseous fuel jet

摘要

Quantitative fuel concentration visualizations are carried out to study the mixing process between fuel and air in Direct Injection (DI) diesel-like conditions, and generate high quality data for the validation of mixing models. In order to avoid the particular complication connected with fuel droplets, a gaseous fuel jet is investigated. Measurements are performed in a high-pressure chamber that can provide conditions similar to those in a diesel engine. A gas injection system able to perform injections in a high-pressure chamber with a good control of the boundary conditions is chosen and characterized. Mass flow rates typical of DI diesel injection are reproduced. A Laser-Induced Fluorescence technique requiring the mixing at high pressure of the fluorescent tracer, biacetyl, with the gaseous fuel, methane, is developed. This experimental technique is able to provide quantitative measurement of fuel concentration in high-pressure jets. Fuel concentration measurements are undertaken under various operating conditions for a large number of operating points in order to allow a statistical analysis of the results. This analysis includes fluctuations and Probability Density Function (PDF) determination. It is found that the high shear generated by the jet momentum produces a high turbulent mixing at the sides of the jet, while at the tip, the high-density jet impact on the high-density ambient gas produces little turbulence intensity. The shear-induced turbulent mixing that occurs at the sides of the jet controls mixing, so that when the jet momentum is increased, the increased turbulent mixing leads to higher air-entrainment, and therefore more fuel is mixed at about the same rate. Mixing is reduced at the tip of the jet, explaining the formation of homogeneous fuel concentration zones. Also, auto-ignition processes and soot cloud structure are direct consequences of that mixing process.