Extending the Flamelet Generated Manifold for Soot and NOx Modeling in Diesel Spray Combustion

Abstract

Diesel spray combustion is prone to lead to high soot and nitrogen oxides (NO_x) emissions. Regulations impose lowering the emission limits drastically because of the severe impact on health and environment. Apart from health and environmental issues, soot production also results in an efficiency loss due to the heat loss through radiation and late oxidation of the soot particles. Thermal NO_x and soot modeling for practical engine applications is challenging. In this study, a two-equation multi-step phenomenological (MSP) soot model is fully coupled with the gas phase chemistry to solve the 1D detailed flame (flamelet) equations for the so-called Spray-A conditions from the Engine Combustion Network (https://ecn.sandia.gov/). Two different soot kinetics models are used that either utilize C₂H₂ or PAH as the soot-precursor. For the gas phase, a chemical reaction mechanism of n-dodecane comprising of 253 species and 1437 reactions is utilized. The resulting flamelet database includes emission information, NO_x and soot carbon concentrations, and the soot particle number density. Detailed flamelet results are inspected in the mixture fraction (Z) domain to characterize the soot formation and oxidation, and NO_x formation. The other important controlling variable, the progress variable (PV), is used for the tabulation of the flamelet results. The definition of PV is optimized for key parameters such as soot and NO_x. Furthermore, the detailed solutions are stored in a low-dimensional manifold, i.e. a Flamelet Generated Manifold (FGM), with respect to the controlling variables (Z and PV). Here, the FGM tabulation is extended with soot and NO_x emissions for non-premixed combustion. Finally, FGM is coupled with CFD and validated by studying an igniting diesel spray and comparing the simulation results with the Spray-A experiments.