Numerical Simulation On Combustion Process Of Diesel/Natural Gas Dual Fuel

In the face of the increasingly prominent energy shortage and environmental pollution,the application of new alternative fuels on engine has become an important direction of energy saving and emission reduction.Because of its high efficiency,large storage capacity and good economy,natural gas has become the priority alternative fuel.The diesel/natural gas dual-fuel engine is one of the most important ways for the engine to use natural gas because of its high efficiency and low emission.Because the dual fuel engine combustion process is very complicated,which burning rate is influence by the turbulence,chemical kinetics,temperature,pressure and other factors.Therefore,this paper uses diesel/natural gas dual fuel combustion mechanism to research the factors of the combustion process in cylinder.The main contents and conclusions of this paper are as follows:1)Based on the method of direct relation diagram with error propagation and sensitivity analysis(DRGEPSA),the reduced mechanism of natural gas consisting of 70 species and 393 reactions was constructed from the detailed mechanism.2)A reduced n-heptane-toluene-PAH mechanism was used to characterize diesel components,and the final n-heptane-toluene-PAH mechanism contains 85 species and 468 reactions by adding a reduced PAH mechanism.3)A reduced diesel/natural gas dual-fuel mechanism with 125 species and 718 reactions by combining above the reduced natural gas mechanism and the n-heptane-toluene-PAH mechanism.The 5 important reactions were adjusted based on sensitivity analysis.The optimized mechanism has been widely validated with the ignition delay,laminar flame speed and HCCI engine data,and the results show good prediction with experimental data.4)The three-dimensional fluid analysis software coupled with diesel/natural gas combustion mechanism which was validated was used to analysis the effect of different pilot spray parameters on combustion and emission.The main conclusions can be summarized as follows:(1)With the advance of pilot injection time,cylinder equivalent ratio decreases,the stratification of mass fraction of free radicals decreased,OH increased,the main distribution area and high temperature area is similar,the cylinder average temperature increased significantly after ignition,average pressure,heat release rate increased,CH4 and CO oxidation enhanced,NO emissions increase,mainly distributed in the high temperature zone.(2)With the increase of the pilot injection,the pressure,temperature and heat release rate in the cylinder first decreased and then increased.The cylinder equivalence ratio firstly increases and then decreases,and the area which mass fraction of OH is high shifts toward the bottom of the combustion chamber.When pilot injection quantity is small,the pressure,temperature and heat release rate decrease with the decrease of the injection quantity.When pilot injection quantity is large,multiple fire cores appear in the cylinder,average temperature,pressure and heat release rate in the cylinder decreases first and then increased.NO emission decreased first and then increased.