A two mixture fraction flamelet model applied to split injections in a DI Diesel engine

摘要

The laminar flamelet equations, which were originally derived for a two-feed system with one fuel and one oxidizer stream, are extended for a three-feed system with two fuel and one oxidizer streams. Both fuel streams are associated with one separate mixture fraction. Using a three-scale asymptotic analysis, two-dimensional flamelet equations are derived, which can describe the transfer of heat and mass between two mixture fields in flamelet space. The representative interactive flamelet model which was previously used successfully for the simulation of Diesel engine combustion cases, is extended to accommodate the two-dimensional flamelet equations and multiple mixture fractions. This new model is applied to a split injection case with a pilot and a main injection representing the two fuel streams. The three-dimensional mixture field in the engine is analyzed using a multi-dimensional β-PDF, and an interaction coefficient is defined to describe the degree of merging of the mixture fields. Depending on the coefficient, different phases of combustion and interaction between the mixture fields resulting from the different injections are identified. Results using the two-dimensional flamelet model are compared to experimental data for a modern direct-injection Diesel engine equipped with a Common-Rail injection system. Possible simplified models are introduced, and their performance is compared to the full model.