Modeling of NOx and Soot Formation in Diesel Combustion

A S Dederichs,F Mauss,M Balthasar

doi:10.2516/ogst:1999021

Abstract

N Modeling of NO x and Soot Formation in Diesel Combustion N An approach to model the formation and oxidation or reduction of soot and NO in turbulent diffusion flames is presented. The model is based on the flamelet library approach and extended to account for radiative heat losses in the flame. Due to the rather slow processes leading to soot and NO a modified flamelet library approach is used. Instead of taking the mass fractions directly from flamelet libraries the different source terms for soot and NO formation are calculated and a transport equation for the mean mass fractions is solved in the CFD calculation. The source terms are obtained from laminar counterflow-flame calculations using a detailed chemistry model for the gas phase species and the formation and oxidation of soot. Transport equations for the mean mixture fraction and the mixture fraction variance are solved and the chemical source term is closed by presuming a beta-function like distribution of mixture fraction and a log-normal distribution of the scalar dissipation rate. The model was first tested in laminar and turbulent jet flames. By applying a reduction strategy for the flamelet libraries of the source terms it was made applicable to the simulation of soot formation in a Diesel spray taking different oxidizer temperatures and pressures into account. Additionally, different formulations of the flamelet equations have been tested and their accuracy has been evaluated by comparing them to turbulent flame experiments.

Highlights

There are numerous model approaches used for the calculations of soot formation and oxidation in Diesel engines
The most recent approach is based on the laminar flamelet concept [1], solving so-called Representative Interactive Flamelets (RIF) on line to the CFD code [2,3]
The flamelet library approach for the sources of soot formation and oxidation has been tested in laminar C2H2/air jet flame first [6]

Summary

INTRODUCTION

There are numerous model approaches used for the calculations of soot formation and oxidation in Diesel engines. The most recent approach is based on the laminar flamelet concept [1], solving so-called Representative Interactive Flamelets (RIF) on line to the CFD code [2,3]. A simpler approach based on stationary flamelet libraries of the sources of soot and NO formation and destruction was formulated [6]. This model represents, combined with a reduction strategy of the calculated flamelet libraries giving the flamelet libraries as algebraic functions, an attractive alternative for the simulation of pollutant formation in Diesel engines in terms of CPU time and computer storage requirement [7]

Concept

The Detailed Chemical Soot Model

Flamelet Calculations

RESULTS

CFD CALCULATION