Model of Soot Formation: Coupling of Turbulence and Soot Chemistry

Abstract

The aim of our study is to model soot formation in the extreme conditions of Diesel engines. The first concept has been to use a formulation for soot formation deduced from the most recent experimental work. However, in order to take into account the effects of turbulence and pressure, associated to soot production, we had to use very simplified assumptions. This ensemble of hypotheses is justified in the case of the Diesel engine only. The first step of this study consisted in obtaining a model coupling the turbulence of the flow and soot formation, by computing turbulent jet flames at atmospheric pressure. This is what we are presenting here. To model soot formation in Diesel engines, it will be necessary to distinguish the following three different processes, formation, coagulation and oxidation, then to take into account the thermal radiation of soot particles. Though the mechanisms for oxidation and coagulation have not been tested for Diesel conditions, they are reasonably understood. On the contrary, the formation mechanism must be analyzed to derive a very simplified relationship to fulfill the requirements of numerical fluid modelling. We propose to model soot formation by a method of an “extended flamelets approach” which includes high temperatures zones where pyrolysis and soot oxidation reactions may occur as well as thermal radiation. The second step will consist to introduce the expression for soot production at high pressures, in order to simulate soot inside the Diesel engine. This is in progress within the european research program CEC-JRC/IDEA. In this paper we will present first the characteristic properties of Diesel soot and the different time scales produced by an engine cycle. Then, we will see how these characteristics impose the expression of the coupling between turbulence and soot production. We will describe the semi-global model for soot production that has been tested in a laminar diffusion flame. After a detailed description of the turbulent model, a comparison between experiments and calculations will be presented in a turbulent ethylene-air jet. The inclusion of our model in a large computer code for simulating Diesel engine is actually under testing, and will not be presented here.