Study On Mechanism Reduction And Numerical Simulation Of Alternative Fuel For Marine Diesel Engines

The detailed chemical kinetics mechanism of internal combustion engine fuel includes large-scale species and reactions,which leads to a large amount of computational resources to be consumed in numerical simulation.It is a highly effective method to construct a reduced mechanism model of alternative fuels by systematically and efficiently reducing the detailed mechanism model,deleting redundant components and unimportant elementary reactions in the mechanism,and characterizing some important components of the fuel.CFD simulation has the advantages of low development cost,short design cycle,and can provide information that can’t be obtained in experimental research.By establishing the reduced chemical kinetics mechanism of fuel combustion reaction and coupling the three-dimensional fluid calculation model of internal combustion engine,the research results have certain theoretical significance and application value for in-depth analysis of heavy oil combustion and emissions of marine diesel engines.On the basis of analyzing the main chemical components of heavy oil,and considering the characteristics of marine heavy oil that is mostly emulsified,the fuel combustion will have some oxygen components.Therefore,a reduced mechanism model of three heavy oil species including n-heptane,α-methylnaphthalene and nbutanol was established.First,the detailed mechanisms of n-heptane and n-butanol were analyzed by sensitivity analysis and oxidation reaction path analysis,respectively,and reduced by DRGEP,DRGEPSA,FSSA mechanism reduction methods in a wide range of experimental conditions.A reduced mechanism of n-heptane containing 132 species and 585 reactions,and a reduced mechanism of n-butanol containing 82 species and 383 reactions were constructed,The predicted results of the reduced mechanism were compared with the data of the shock tube and jet stirred reactor.The results show that the predicted results of the reduced mechanism are basically consistent with the ignition delay and the concentration of the main components.Select and arrange 77 species and 287 reactions reduced mechanism of α-methylnaphthalene,coupled with the above reduced mechanism,finally constructed a heavy oil reduced mechanism containing 205 species and 927 reactions,and calculated the HCCI ignition time of the developed heavy oil reduced mechanism model.The results show that the reduced mechanism model of heavy oil has high accuracy in predicting the ignition and combustion time of HCCI under different temperature conditions,and can accurately describe the HCCI combustion process of fuel.Finally,this paper uses CONVERGE software to build a three-dimensional fluid calculation model of marine two-stroke diesel engine,which is coupled with the reduced mechanism of heavy oil.The model is verified by comparing the measured data.The effects of exhaust valve closing angle and EGR rate on combustion process and emissions were studied using a coupled model.The results show that with the increase of the exhaust valve closing angle,the exhaust phenomenon is obvious,the effective compression ratio decreases,the ignition and combustion speed decreases,the average combustion temperature and pressure in the cylinder decreases,and the NOx emission decreases.With the increase of EGR rate,the oxygen concentration in the intake air decreases,the specific heat capacity of the intake air mixture increases,and the degree of low-temperature combustion in the cylinder increases,reducing NOx emissions.