Bio-diesel Spray Combustion Modeling Based on a Detailed Chemistry Approach

Licentiatavhandling, 2010

The increased use of bio-diesel fuel is expected to reduce emissions of carbon-containing greenhouse gases. To optimize the application of bio-diesel in internal combustion engines, a theoretical model has been developed. The bio-diesel fuels used in the modeling are Palm Methyl Ether (PME, C17H34O2) and Rapeseed Methy Ether (RME, C19H36O2), and blends of these methyl ethers with standard diesel oil. The thermo-physical properties of these bio-diesel fuels have been estimated using empirically determined thermodynamic, gas phase formulae. The description of their combustion processes has been based on the assumption that long chain methyl esters (C17~C19) decompose into short-chain methyl esters (C5}~ C11), for which combustion mechanisms are available. A comprehensive integrated chemical mechanism (with 98 species involving 395 reactions) was constructed to model the combustion of diesel oil surrogate, DOS, PME and RME. The bio-diesel fuels' physical properties and combustion mechanisms were validated using measured auto-ignition delays and laminar flame speeds. The bio-diesels physical properties and combustion mechanism were implemented into KIVA3V, rel.2 code. The KIVA3V fuel model was also extended to include mixing rules approach to calculate properties of blends of the fuel, e.g. RME50. Models of sprays of the fuels were validated against data acquired using observations of sprays in the Chalmers High Pressure /High Temperature (HP/HT) spray chamber. The liquid penetration, and average Sauter Mean Radius, SMR, for sprays of the different fuels were calculated and compared. To study the effect of bio-diesel fuels on the performance of engines, 3D CFD simulations of both a heavy duty engine (Volvo D12C) and a light duty engine (Volvo NED5) were carried our under various operating conditions. The combustion processes of the DOS and PME fuels in the NED5 engine were studied, and those of DOS, RME and RME50 in the Volvo D12C engine. Reasonable agreement between modelled and empirical in-cylinder parameters has been achieved.