Abstract

The effects of split-injection on the mixing and the fluid residence time distribution in turbulent gaseous jets are investigated using Direct Numerical Simulation (DNS). The mixing physics identified in this study are important for the understanding of split-injection compression-ignition engine operation, in which mixing rates and fuel residence time control the rate of heat release and pollutant formation. The configuration involves a round turbulent jet issuing from a flat plate, subject to single-pulse, double-pulse, and continuous injection. A novel analysis of fluid residence time is performed by solving a transport equation for the fluid age. A similarity scaling is determined for the residence time in the continuous jet case. It is then shown that the radial gradients of the age of injected fluid are greater in the continuous jet suggesting that, in continuous fuel injection, entrainment of older more-reacted fluid provides a mechanism to promote ignition further upstream compared to pulsed jets. The implications of scalar dissipation and entrainment rate transients for combustion are discussed.