Direct Numerical Simulation of Multi-Injection Mixing and Combustion in Diesel Environments

Multi-injection mixture formation strategies are known to improve diesel engine operation in terms of pollutant emissions, noise and controllability. A pilot injection, where a small amount of fuel is injected prior to the main injection, shows high levels of premixing and leaner mixtures at time of ignition. The main injection has a longer duration, and its ignition is promoted by the presence of the pilot, leading to less premixing at the time of ignition compared to the pilot injection. The dwell time between the two injections controls the amount of time the pilot injection has to ignite and the degree of mixing before interaction with the main injection. While diesel multi-injection has been investigated experimentally, a fundamental understanding of the mixture preparation, ignition and and flame stabilization is still elusive. We will present two- and preliminary three-dimensional Direct Numerical Simulations (DNS) of multi-injection of a vaporized fuel-rich n-dodecane/air mixture under reacting and non-reacting conditions relevant for engine operation. We investigate how mixture formation, ignition, flame structure and flame stabilization of the second injection are influenced by the first injection, and how this is affected by a variation in dwell time.