A Detailed Modeling Study for Primary Reference Fuels and Fuel Mixtures and Their Use in Engineering Applications

Sammanfattning: The aim of this work is to generate detailed and simplified kinetic models for the oxidation of the Primary Reference Fuels (PRF) n-heptane, iso-octane and their mixtures, with low numbers of species and reactions. These mechanisms are consistent in terms of the choice of kinetic parameters for the different reaction classes. The further aim is to validate the kinetic models for a wide range of different combustor operating conditions, such as engines, shock tube, jet-stirred reactors, flow reactors, laminar flames and burners, to cover the full range of temperatures, pressures and air-fuel equivalence ratios. In the validation procedure the focus is on both extensive tests for the low temperature ignition and a validation against flame experiments. This development is motivated by the fact that the broadly validated hydrocarbon fuel oxidation mechanisms that have been developed previously are large and thus consumed more CPU-time than is acceptable by commercially available CFD-tools, because of their complexity, which prohibits the direct incorporation in simulations of complex reactor models, e.g. with turbulent flow. For the simplification and the reduction of mechanisms a stepwise efficient and simple lumping strategy for different reaction types has been developed and subsequently combined with a necessity analysis .The lumping of species with the same functional groups helps to reduce the different complex pathways into one lumped reaction pathway, which reduces the mechanism in terms of both numbers of species and reactions, causing only a negligible loss of information of the detailed mechanism. This is controlled by comparing predicted concentration profiles of lumped species with the added profiles of the isomeric species. Then further reduction of the mechanism is achieved by using necessity analysis (reaction flow combined with a sensitivity analyses) which eliminated less necessary species and their corresponding reactions. The developed mechanisms are validated against ignition delay times measured in shock tube experiments from Fieweger et al., Ciezki et al. and Davidson et al. for temperatures from 600?1300 K, for fuel air equivalence ratios in the range of 0.5 to 3.0 and pressures from 3.2?40 bar. It is also validated against shock tube data from Horning et al. and Smith et al. and Vermeer et al. for high temperatures from 1250?1800 K, for fuel air equivalence ratios in the range of 0.5 to 2.0 and pressures from 1 4 bar. The detailed, lumped and skeleton mechanisms are further validated against the laminar flame speed experimental data of Davis and Law and for flame structure data of El Bakali et al.. Predictions for the low and medium temperature range are also tested against species and heat release profiles obtained in plug flow and jet stirred reactors at pressures between 3 and 12.5 bar against the experimental data of Callahan et al., Held et al., and Dagaut et al.. The mechanisms are further validated and applied in HCCI (Homogeneous Charge Compression Ignition) engine simulations using a zero dimensional ignition model. Mechanism show good agreement in cylinder pressure, temperature and also heat release rate with the experiments of Tsurushima et al.. Finally detailed and lumped mechanisms for the oxidation of PRF fuel blends n-heptane/iso-octane and the alternative fuel blend n-heptane/toluene are presented and validated under conditions relevant for HCCI engines. For mixtures of n-heptane/toluene the mechanism was validated against shock tube experimental data from Burcat et al. and for mixtures of iso-octane/n-heptane the mechanism has been validated against the shock tube experimental data of Fieweger et al. for different octane numbers. In addition, the mechanism has been tested and applied using a zero dimensional engine model for HCCI. Results were compared against the experiments from Kalghatgi et al. and Christtensen et al. under a range of different initial operating conditions and varying fuel blends.