Modellierung der Multikomponenten-Verdampfung im homogenisierten dieselmotorischen Brennverfahren (English shop)

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The demands placed on combustion engines are mainly ever lower fuel consumption and ultimately zero pollutant emissions. One focus here lies in the development of new combustion processes in order to prevent the formation of pollutants already during combustion. In this context, the present work examines the homogenised diesel combustion process (HCCI). In addition, the most recent research work is concerned with assessing the potential of using alternative fuels.

Within the scope of this development work, CFD simulation offers the possibility of carrying out extensive fuel and parameter variations as a preliminary design step for experimental investigations. In this way, the necessary cost-intensive experimental investigations can be greatly reduced. A prerequisite for this, however, is the most accurate possible modelling of the fuels used, in order to be able to reproduce the processes occurring during evaporation and mixture formation. Furthermore, for modelling ignition, combustion and pollutant formation, the corresponding input variables must be available from the evaporation model in order to be able to carry out the calculation of these processes.

Within the scope of this work, a multicomponent evaporation model was therefore developed which describes real fuel mixtures by means of two statistical distribution functions. The chosen approach thus makes it possible to take into account fuel constituents from two different groups of homologous components (alkanes and aromatics). This model is implemented in the CFD code KIVA-3V and validated with calculations of the evaporation of single droplets as well as by Mie scattering investigations of simple injection processes in a rapid compression machine. The work concludes with the calculation of the mixture formation of various fuels by means of an injection strategy designed for the HCCI diesel combustion process.

Keywords:
Evaporation – Multicomponent – CFD – Continuous thermodynamics -
Homogenised diesel combustion

Abstract

Two important requirements of combustion engines are minimal values of pollutant emissions and low fuel consumption. The main focus being the development of new combustion processes like the Homogeneous Charge Compression Ignition (HCCI) method to minimize the amount of pollutant emissions already during the combustion process. Additionally, in this context the potential of using synthetic fuels is also investigated. In this case the CFD-simulation offers the possibility of carrying out fuel and parameter variations, reducing cost intensive experimental studies. But an assumption for these calculations is a realistic modelling of the fuel mixtures being used. The fuel behaviour during evaporation and carburation must be described exactly. Furthermore, for modelling the following processes like ignition, combustion and emission formation, the right input parameters have to be present. Only under these boundary conditions can reasonable CFD-investigations be realised.

Therefore, within this project a multicomponent evaporation model was developed, which uses the statistically based approach of continuous thermodynamics to describe several fuel mixtures with distribution functions instead of describing them with a single component surrogate fuel. In the present model formulation the composition and the properties of fuels can be described with two different distribution functions in dependence of the molecular weight. This approach offers the possibility of considering two different homologous groups of hydrocarbons (alkanes and aromatics) contained in real fuels. The model was implemented into the CFD-code KIVA-3V and validated with single droplet calculations, Mie-Scatter- and Schlieren-investigations in a rapid compression machine. Finally, CFD-simulations with a multistage fuel injection strategie for the HCCI-combustion process are made to analyse the carburation behaviour of different fuel mixtures.

Keywords:
Evaporation – Multicomponent – CFD – continuous thermodynamics -
Homogeneous Charge Compression Ignition (HCCI)