Development of a crevice model in CONVERGE CFD and analysis of its effects on combustion and HC emissions

The purpose of this thesis project is to analyse how combustion and HC emissions of a SI engine are affected by an ODE-based phenomenological model for crevice flow simulation implemented in CONVERGE CFD software by means of a user-defined function (UDF) coded in C language. This blowby model has been provided by Politecnico di Torino through MATLAB. CFD (Computational Fluid Dynamics) is a tool for the study of fluid phenomena by means of fluid mechanics, numerical analysis and computer-based simulations. Nowadays, thanks to the thorough developments in informatics of the last decades, the usage of CFD is in every area. There are several computational fluid dynamics simulation software, both commercial and non-commercial. CONVERGE is a commercial CFD software for simulating three-dimensional fluid flow and it is provided by Convergent Science GmbH, company located in Austria where this thesis work has been carried out. The spark-ignition engine analyzed in this work is the FIAT FIRE 1.4 16V Turbo and the experimental data of the engine have been provided by Politecnico di Torino, in particular by the DENERG laboratory. The engine is fueled with methane and the combustion is modelled through SAGE detailed chemical kinetics solver. This thesis project is subdivided into four main sections. The first is dedicated to briefly describe Navier-Stokes equations, numerical methods, how the software works and how it handles the autonomous meshing. The second part is a rapid review of the engine characteristics and experimental data. The third one is focused on the engine case setup: geometry, boundary conditions, grid settings and then turbulence, combustion and emissions models adopted. The fourth and last section is dedicated to the crevice model and to the analysis of the results obtained. The blowby phenomenon varies the amount of mass inside the engine cylinder: this mass variation influences combustion process and engine-out HC emissions. Fundamental equations and geometrical considerations necessary for the blowby model development are described; then, the UDF development and validation is dealt with. A user-defined function is essentially a function that allows to include in CONVERGE a capability that it is not yet provided with. Finally, experimental data and numerical results, both without and with UDF, are compared and the effects of the crevice model are shown.