Numerical Study on the Multiple Injection Strategy in Diesel Engines using a Modified 2-D Flamelet Model 2015-24-2406
The flamelet model is a widely used combustion model that demonstrates a good prediction of non-premixed combustion. In this model, the chemical time scales are considered to be smaller compared to those of the turbulence, which allows the heat and mass transfer equation to be decoupled from the flow equation. However, the model's dependency on the mixture fraction limits the combustion analysis to a single injection. To overcome this limitation, a two dimensional flamelet model, which uses two mixture fraction variables, was introduced to represent the non-premixed combustion of multiple injections. However, the model's computational time drastically increased due to the expansion of the solution domain. Thus, a modified 2-D flamelet model was introduced to reduce the computational time of the two dimensional flamelet model. In this model, the 2-D flamelet equations were only solved near the stoichiometric region; the other regions were changed to those of the steady-state 1-D flamelet solution if they were ignited. The objective of this study is to extend the modified 2-D flamelet model to three or more injection strategies without increasing the computational costs. A few multiple injection strategies (pilot-pilot-main/pilot-pilot-main-post) were applied to a multi-cylinder engine. Two operating conditions (1500/4, 1500/6, [rpm/bar]) were tested, and the simulation results of the pressure and the HRR using the modified 2-D flamelet model were compared with the experimental data. Lastly, the combustion and emission characteristics of each injection strategy were determined, and the model demonstrated a good agreement with the experimental results.