Abstract:

Three-dimensional direct numerical simulation is conducted to simulate the auto-ignition of the highoctane fuel PRF70 under partially premixed combustion (PPC) engine conditions. A skeletal primary reference fuel (PRF) chemical kinetic mechanism is adopted, including 33 species and 38 elementary reactions. Compression/expansion effects caused by piston motion, the real engine geometry, and the working conditions are considered. The simulation includes two injections, the first being used to form a relatively uniform base mixture and the second to forma stratified mixture and trigger the ignition. It is found that the combustion process in PPC engines is a rather complex combination of homogeneous combustion, rich premixed and diffusioncontrolled combustion. The region between the two injections is near stoichiometry, resulting in the formation of NO_x, while abundant CO is retained in the region with equivalence ratio (φ) > 2, which needs to diffuse to meet the oxidizer and burn in a diffusion flame. The marching cube method is used to extract the 3D flame surface and show the temporal evolution of the reaction front. Finally, the joint PDF of the Gaussian curvature (k_g) and principle mean curvature (k_m) and temporal evolution of the probability density function (PDF) in terms of km show that km plays a more important role and becomes negative as time evolves because of the consumption of rich premixed flame in the center.

Key words: Direct numerical simulation, Partially premixed combustion, PRF70, Combustion characteristics, PPC engine