A new combustion model is presented that can be used to simulate the diesel combustion process. This combustion process is broken into three phases: low temperature ignition kinetics, premixed burn and high temperature diffusion burn. The low temperature ignition kinetics are modeled using the Shell model. For combustion limited by diffusion, a probability density function (PDF) combustion model is utilized. In this model, the turbulent reacting flow is assumed to be an ensemble of locally laminar flamelets. With this methodology, species mass fractions obtained from the solution of laminar flamelet equations can be conditioned to generate a flamelet library. For kinetically limited (premixed) combustion, an Arrhenius rate is used. To transition between the premixed and diffusion burning modes, a transport equation for premixed fuel was implemented. The ratio of fuel in a computational cell that is premixed is used to determine the contribution of each combustion mode. Results show that this combustion model accurately simulates the diesel combustion process. Furthermore, the simulated results are in agreement with the recent conceptual picture of diesel combustion based upon experimental observations.
Large eddy simulation (LES) models for momentum exchange and scalar flux were incorporated into the KIVA solver. In this formulation, the turbulent viscosity, ?t, is determined as a function of the sub-grid turbulent kinetic energy, which is in turn determined from a one equation model. The formulation for the scalar transfer coefficient, ?s, is similar to that of the turbulent viscosity, yet is made to be consistent with scalar transport. Test cases were run verifying that both momentum and scalar flux can be accurately predicted using LES. Once verified, these LES models were used to simulate the diesel combustion process for a Caterpillar 3400 series engine. Results for the engine simulations were in good agreement with experimental data.