Turbulence-Chemistry-Interaction Modelling in 3D-CFD for Study of Auto Ignition Phenomena

Christof Benz; Jürgen Nocke; Egon Hassel; Henrik Hoffmeyer; Karsten Michels

doi:10.4271/2012-01-0158

From the point of view of the customer purchasing a car the ecological as well as the price aspect is in the main focus today and in the years that come. This will increase due to global warming, the accelerated depletion of raw materials and significant price increases. Downsizing of spark ignition engines is an opportunity to lessen these shortcomings by decreasing the displacement volume of the engine and for a constant power increasing the load. In the case of extreme downsizing, especially in the case of low engine speed, auto ignition occurs in the air/fuel mixture. As a consequence cylinder pressure tends to exhibit high amplitudes and frequencies, which can lead to engine damage.

This paper presents a model which allows linking 3D-CFD with a detailed chemical reaction system. Therefore a three-dimensional numerical model in OpenFOAM is formulated that includes all physical characteristics of a direct-injected, highly charged spark ignition engine. The conservation equation for mass, momentum and energy form the mathematical basis for the modeling approach. Additional equations for spray modeling and for turbulence-chemistry interaction are required whereat the latter bases on a partially stirred-reactor approach. This model was developed and implemented in OpenFOAM at the Chalmers University of Technology in Göteborg, Sweden.

A detailed chemical reaction system that describes the combustion of iso-octane/n-heptane and air is used. This mechanism could be reduced by means of sensitivity and 0D-reactor studies. Thus it is adopted to the cold-flame range at which auto ignition occurs. The reduction of the chemical mechanism has great importance especially for the coupling with the CFD-code.

Several hypotheses for causes of autoignition already exist. The aim of this paper is to verify such causes with CFD methods, particular the sensitivity with respect to engine parameters. An operating state at low engine speed and high load is chosen as a reference point. Measured pressure history can be applied to validate the complete numerical model. Finally it is used for a systematic parameter study to gain insight into the character of autoignitions.

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Turbulence-Chemistry-Interaction Modelling in 3D-CFD for Study of Auto Ignition Phenomena 2012-01-0158

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