Chemical kinetic mechanism and a skeletal model for oxidation of n-heptane/methanol fuel blends

Abstract

It has been found in previous experimental studies that the ignition of diesel or n-heptane is delayed by methanol addition. However, detailed analysis of the interaction between the two fuels especially under low temperature oxidation has not been reported previously. In order to discover the chemical mechanism and create a skeletal model which can be coupled with 3-D simulation software efficiently, following works were conducted: Firstly, by using a detailed n-heptane oxidation mechanism which was slightly revised, the reaction paths analysis was conducted to seek the coupled relationship between n-heptane and methanol; Secondly, based on the reaction paths and sensitive analysis, a skeletal kinetic model including only 38 reactions related to 30 kinds of species was created to predict the ignition delay and the combustion of methanol/n-heptane blends as well as each pure fuel. The detailed mechanism analysis results show that when two fuels are blended, the radical pool is the only bridge between them. OH· is converted into H2O2 when affected by methanol, and H2O2 then frozen due to its high decomposition activation energy in the temperature below about 1000K, thus the reaction activity of the whole system deceases; The skeletal model test result shows that the ignition delay of the n-heptane, methanol and the blends is well predicted under various equivalence ratios, initial temperatures and pressures compared with the results of experiments or detailed mechanism. HCCI engine test also show that the ignition, in-cylinder temperature as well as the major species all have satisfactory predictions.