1993-10-01

Soot Formation and Oxidation in a DI Diesel Engine: A Comparison Between Measurements and Three Dimensional Computations 932658

ABSTRACT

Three dimensional computations of Diesel combustion were performed using a modified version of Kiva II code.

The autoignition and combustion model were tuned on a set of experimental conditions, changing the engine design, the operating conditions and the fuel characteristics. The sensitivity of the model to the different test cases is acceptable and the experimental trends are well reproduced.

In addition the peak of pressure and temperature computed by the code are quite close to the experimental values, as well as the pressure derivatives.

Once tuned the combustion model constants, different but simple formulations for the soot formation and oxidation processes were implemented in the code and compared with the experimental measurements obtained both with fast sampling technique and two colors method. These formulations were found unable to give good prediction in a large range of engine operating conditions, even if the model tuning may be very good for each test point.

Therefore a new multistep model for soot nucleation and surface growth processes, starting from a major crucial pyrolitic specie (acetylene), was implemented. The first test of this model seem to be promising for the future research work.

SOOT EMISSIONS in diesel engines are controlled by the interaction among different and complex phenomena like:

spray atomization and dispersion,

turbulent heat and mass transfer,

complex chemistry at low and high temperature.

The formation and oxidation processes of soot particles are strongly influenced also by the engine operating conditions and by the combustion system design; clearly the particulate emissions of a diesel engine depends on a number of other factors, as the piston and the seals design, the injection apparatus and also the fuel quality. In modern low emissions engines the particulate is mainly composed by the insoluble fraction; in addition, in the next future, the sulphur content of the fuel, that has strong influence on the insoluble fraction, will be reduced under 500ppm in U.S.A as well as in E.E.C. countries. So the soot contribution to particulate will become dominant.

Therefore multidimensional modeling of combustion processes, and a soot formation and oxidation model, sensitive to the engine geometry and to some characteristics of the fuel, as the cetane number, may be considered as a tool useful to provide guidelines for prototype development.

Multidimensional computations of soot formation and oxidation processes are today present in the literature.

In particular Zellat and others [1] perform computations of a i.d.i. diesel engine using Kiva II code [2]. They modified the original code, implementing a multistep ignition delay model and a combustion model based on the Magnussen work [3]. The soot formation modeling has been derived from Tessner [26], while the soot oxidation model was developed as the combustion model under Magnussen hypothesis. The computations, performed for four different operating conditions of the engine, show that the code reproduces the trends observed on the engine.

Nishida and Hiroyasu [4] present three-dimensional computations of an engine equipped with different cylindrical bowls. They use the Kono model [5] for soot formation and oxidation processes: results show a quite acceptable fit with smoke measurements at the engine exhaust.

Nagakita and others [6] applied Tessner and Farmer [17] models for soot formation, while the soot oxidation process was modeled using Nagle [27] and Magnussen theory. The switch criterion between the two different models during the formation and oxidation processes was based on the smaller formation and oxidation rate at each computational time. The agreement between the numerical results and the experimental ones, obtained by means of back illuminated photography, is quite good.

More recently Gorokhovski and Borghi [7] have developed for Chonchas Spray codea linked ignition - soot formation global kinetic model. The Shell model [8] has been used to represent the ignition global kinetics. Starting from a stable hydrocarbon intermediate for ignition, the soot surface growth was determined using experimental correlations of final soot volume fraction, obtained from literature. Finally soot oxidation rate was computed using Lee and others formula [9]. Just qualitative agreement was found with the experimental data.

From the previous picture a lack of detailed experimental information in comparison with 3D computations is quite evident. In a previous work [10] comparisons were reported between numerical data of in-cylinder pollutants, computed with Kiva II code, and experimental results, carried out with a fast sampling technique.

The soot formation and oxidation processes were simulated by the simple Kono model and acceptable agreement with experimental data was found. However it can be noted that the comparisons were established just for one fuel (pure tetradecane) and for one engine operating condition.

The present paper is aimed to enlarge the experimental data base, in order to test the accuracy of the numerical results and to improve some physical submodels.

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