A Numerical Investigation On Combustion Characteristics Of High-pressure Direct Injection Dual-fuel Marine Engine

With the increasingly stringent emission regulations for marine diesel engines and the shortage of oil resources,diesel ignited natural gas engine has been paid more and more attention because of its advantages such as good economy and good emission reduction effect.Among them,high-pressure direct injection dual-fuel low-speed marine engine has the advantages of high thermal efficiency,low emission and high inflation efficiency,so it is gradually becoming the first choice of ship power.However,at present,the research on the combustion mechanism of natural gas ignited by diesel is mainly based on the mode of injecting diesel into natural gas / air mixture,and there is a lack of research on the direct injection mode of dual fuel.As a result,the research on the distribution characteristics of the rich premixed core and outer diffusion flame of the natural gas jet flame in the high pressure direct injection dual fuel marine engine is not deep enough,and the research on the change mechanism of the high and low temperature reaction of natural gas and diesel is not comprehensive.Under this background,a three-dimensional numerical simulation model of high pressure direct injection dual fuel marine engine is established in CFD numerical simulation software CONVERGE,and then the numerical simulation of combustion characteristics with different parameters is carried out based on this model.The combustion characteristics studied in this paper mainly include the distribution characteristics of rich premixed core and outer diffusion flame in natural gas jet flame and the change mechanism of high and low temperature reaction.Firstly,the effect of natural gas injection parameters on the combustion characteristics of high pressure direct injection dual fuel marine engine is studied.The results show that:(1)reducing the natural gas injection rate makes the rich premixed core form in a larger equivalence ratio region.The higher the injection rate of natural gas,the quicker the high temperature reaction at the front of the flame,while the decrease of the injection rate leads to a greater proportion of low temperature combustion.(2)advancing the injection time of natural gas accelerates both the high temperature reaction and the low temperature reaction in the cylinder at the initial stage of combustion,and spontaneous combustion and ignition occur at the same time.The earlier the injection time of natural gas,the less the diffusion flame produced by the diesel,which in turn reduces the amount of high temperature methane in the cylinder at the initial stage of combustion.In this paper,the effect of diesel injection parameters on combustion characteristics is studied.The results show that:(1)reducing the amount of diesel makes the flame develop more slowly,but the flame deflects more along the swirl direction.With the decrease of the amount of diesel,there are some very slow low temperature reaction zones in the fuel rich area,and the peak value of high temperature reaction at the front of the flame decreases.(2)when the injection time of the diesel is delayed for a long time,the spontaneous combustion and ignition occur at the same time at the initial stage of combustion.Natural gas spontaneous combustion occurs in the front area of the fuel with an equivalent ratio of 1,and then the flame develops to the area with high equivalence ratio at the nozzle.When natural gas spontaneous combustion occurs,the low temperature reaction first occurs in the region where the equivalence ratio is 1,and the high temperature reaction is concentrated in the region where the equivalence ratio is 0.5.Finally,this paper explores the effects of scavenging temperature and turbulence on combustion characteristics.The results show that:(1)increasing scavenging temperature can make the combustion of n-heptane faster,which further makes the combustion reaction of methane faster.With the increase of scavenging temperature,the formation rate of rich premixed core is slower,and the critical equivalent ratio of diffusion flame to rich premixed core is higher.In addition,the higher the scavenging temperature,the faster the development of the flame in the cylinder,and the quicker the high temperature reaction of the flame front.(2)with the decrease of turbulence intensity,the later the formation time of rich premixed core is,and the more low temperature fuel is.The higher the turbulence intensity is,the greater the deflection of the flame development is,the quicker the high temperature reaction is and the higher the temperature of the low temperature product is.In summary,this paper studies the effects of different parameters on the combustion characteristics of high-pressure direct injection dual-fuel marine engine,which provides theoretical guidance for the optimization of combustion process of high-pressure direct injection dual-fuel low-speed marine engine.