Mixing Process in High-Pressure Diesel Jets by Normalized Laser-Induced Exciplex Fluorescence Part I: Free Jet

摘要

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 200 MPa. The spray is observed 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. A detailed statistical analysis is then used to describe the jet mixing process. Mixing is strong in the stationary zone, located upstream, while it is much weaker at the tip of the jet. The effect of varying the injection parameters has also been investigated. In particular, it has been shown that the local mixing rate in the stationary zone remains constant despite an increase in the injection pressure. Also, it has been observed that the nozzle hole diameter has a significant effect on the global mixing rate. A reduction of nozzle hole diameter from 0.15 to 0.1 mm improves the local mixing rate in the stationary zone. However, an increase in hole diameter from 0.15 to 0.2 mm has similarly been shown to improve the global mixing, since the increase in mass momentum then compensates for the reduction in local mixing rate. Finally, a study of short injection timings shows that the mixing rate is conserved after the end of injection resulting in a rapid decrease of the maximum fuel jet concentration.