Modeling Engine Turbulent Auto-Ignition Using Tabulated Detailed Chemistry 2007-01-0150
In Homogeneous Charge Compression Ignition (HCCI) as well as in conventional Diesel engine, fuel oxidation chemistry determines the ignition timing and the subsequent heat release. Auto-ignition is characterized by the production of large active intermediate radicals during the initial stage of oxidation. This makes the modeling task more complex, as it demands high computing resources to solve several hundreds of species transport equations involved in the detailed chemical mechanism. Therefore, introduction of complex chemistry details into a CFD code in a simple way is necessary. A new 3D auto-ignition model Tabulated Kinetics for Ignition with Probability Density Function (TKI-PDF) is presented. The objective is to include detailed chemical kinetics and the turbulence/chemistry interactions during auto-ignition. The model development and the validation against experiments are described in two stages. First step concerns introducing the detailed chemical kinetics and the second step concerns the mixing description and its interactions with auto-ignition chemistry.
The detailed chemistry in TKI-PDF is described using a progress variable reaction rate lookup table constructed with the results obtained from complex chemistry simulations at constant volume. The results are then casted as a lookup table with entry parameters temperature (T0), pressure (p0), mixture fraction (Z), dilutant residual gas fraction (Xres). Instantaneous local reaction rates inside the CFD computational cell are then calculated by linear interpolation inside the lookup table, depending on the local thermodynamic conditions. This chemistry tabulation has been validated against complex chemistry simulations with homogeneous mixtures at constant and variable volume adiabatic combustion. Good agreement has been found between the model and the detailed chemistry results.
As for the interactions between chemistry and turbulence, a refined mixing model based on a presumed PDF is addressed in the context of ECFM3Z combustion model proposed by Colin and Benkenida . A balance equation for the mixture fraction and a modeled transport equation for its variance are solved in ECFM3Z. The above detailed tabulated chemistry is then coupled with this mixing model following the same presumed pdf approach to calculate the average progress variable reaction rate, For this, reaction rates are integrated with a beta pdf over mixture fraction space, resulting in a final lookup table with a segregation factor S as an additional turbulence ingredient into the data base. Model validation tests are performed in a high pressure combustion cell with optical access. Engine simulations were carried out using the TKI-PDF model in conventional Diesel and a HCCI engine (NADI™) configurations. Engine simulation results are compared with experimental data and show a clear improvement due to the proposed model.