
In diesel engines, fuel sprayed into the cylinder evaporates, mixes with the surrounding gases, and then autoignites as compression raises the temperature and pressure. The diesel unsteady laminar flamelet model, based on the work of Pitsch et al. and Barths et al. [ 285, 22], models the chemistry in a single, onedimensional laminar flamelet. By reducing the costly chemical kinetic calculation to 1D, substantial savings in runtime can be achieved over the laminarfiniterate, EDC or PDF Transport models.
The flamelet species and energy equations (Equations 15.36 and 15.37) are solved simultaneously with the flow. The flamelet equations are advanced for a fractional step using properties from the flow, and then the flow is advanced for the same fractional timestep using properties from the flamelet.
The initial flamelet condition at the start of the diesel simulation is a mixedbutunburnt distribution. For the flamelet fractional timestep, the volumeaveraged scalar dissipation and pressure, as well as the fuel and oxidizer temperatures, are passed from the flow solver to the flamelet solver. To account for temperature rise during compression, the flamelet energy equation (Equation 15.37) has an additional term on the righthand side as
where is a the specific heat and is the volumeaveraged pressure in the cylinder. This rise in flamelet temperature due to compression eventually leads to ignition of the flamelet.
After the flamelet equations have been advanced for the fractional timestep, the PDF Table is created as a NonAdiabatic Steady Flamelet table (see Section 15.4.3). Using the properties of this table, the CFD flow field is then advanced for the same fractional timestep.
The diesel unsteady flamelet approach can model ignition as well as formation of product, intermediate and pollutant species. Enabling the Diesel Unsteady Flamelet model is described in Section 15.8.6