Mixing Process in High Pressure Diesel Jets by Normalized Laser Induced Exciplex Fluorescence Part II: Wall Impinging Versus Free Jet

The effect of perpendicular jet wall impingement on the mixing process of high pressure Diesel jets is studied using normalized laser induced exciplex fluorescence (LIEF). A single hole common rail Diesel injector is used which allows high injection pressures up to 200MPa. Visualisations of the jet were performed in a high pressure, high temperature cell that reproduces the thermodynamic conditions which exist in the combustion chamber of a Diesel engine during injection. A LIEF technique is combined to a normalization method in order to obtain fuel vapor concentration fields. The jet-wall interaction configuration is compared to a free jet configuration at identical operating conditions in order to provide detailed information on the influence of wall impingement and its effects on the subsequent mixing process. A significant effect is observed on the mixing rate, which is weaker in the central part of the jet before impact, while it is higher in the jet tip due to the formation of a jet wall vortex. Both effects seem to compensate for one another at an injection pressure of 150MPa, so that the quantity of air entrained in the jet for the jet wall configuration is similar to that of a corresponding free jet. However, at an injection pressure of 200MPa the increased mixing rate at the jet tip becomes predominant due to the intensity of the wall jet vortex, so that the mass of air entrained in the jet is higher in the case of the impinging jet compared to the free jet.