Applications Of The Laminar Flamelet Model To Turbulent Combustion In Diesel Engines

The combustion process in diesel engine is a typical turbulent diffusion combustion, which is an extremely complicated flow phenomenon coupled with chemical reactions. To simulate the diesel engine working process, the most used models decouple the turbulent flow and the chemical reaction into two parts. Firstly, the two parts are solved separately, and the results are then coupled together. Because the thickness of the combustion reaction zone at high Re and Da numbers is less than the turbulent diffusion scale in diesel engines, the flame is extremely thin, its micro-structure keeps the laminar structure, so the turbulent flame can be regard as the ensemble of locally one dimensional, thin laminar flamelet embedded within the turbulent flow field. This builds up the fundamental of the flamelet model, and makes it possible to use the laminar flamelet model to simulate the complicated turbulent combustion process in diesel engines.The laminar flamelet model has been widely used in the recent years, but its application to the internal combustion engine is still scarce. The domestic investigation in this aspect is almost blank. As the first attempt, this paper applies the laminar flamelet model to the typical diffusion combustion in diesel engines. A database of the laminar flamelet for a DI engine is built up in the phase space of the mixture fraction with the scalar dissipation rate as parameter. Based on the KIVA-3 code, deactivating its original chemical reaction module, the in-cylinder multidimensional turbulent flow field is numerically computed; to which two transport equations for the mean mixture fraction and its variance are solved additionally. By utilizing an assumed Beta function formed probability density function (PDF), the results obtained from the laminar flamelet database and the KIVA-3 code are coupled and integrated, the time history and space distribution of parameters, such as the species mass fraction and temperature through the engine working processes can be then determined.Comparing the results of the DI engine to the experimental and computational results for a similar engine from the literature, the temperature curve and the heat release rate curve are in accordance with each other in general. This indicates the feasibility of applying the laminar flamelet model to the engine simulation, and the results of this paper can be a reference for further investigation.