Numerical Study On Spray Combustion And Emission Of Heavy Fuel Oil In A Marine Low-speed Two-stroke Diesel Engine

Marine low-speed two-stroke diesel engines are now the main propulsion system of large vessels.In the Tier III emission regulations implemented by IMO in 2016,an important measurable indicator is the nitric oxide?NO_x?emission load,which reduced by 76%compared to the Tier II regulation,leading to an urgent demand for study on reducing NO_x emission.Spray combustion process in the marine diesel engine has direct impact on the production of NO_x,but the fuel used in the marine engine is mainly the heavy fuel oil which has complex physical and chemical properties leading to the lack of study on its spray combustion process.Meanwhile,as the high cost of experiment of marine engines compared to the high accuracy and low cost of numerical simulation method,this paper studied the spray combustion characteristic of heavy fuel oil as well as the influence on the combustion and emission of marine low-speed two-stroke diesel engines by different factors using numerical simulation method.The difference in spray characteristics of heavy fuel oil and diesel depends on the difference of their physical properties.Therefore,for better understanding of spray characteristics of heavy fuel oil,this paper investigated the spray model sensitivity of diesel and heavy fuel oil and validated the model based on the experimental data of a Constant Volume Combustion Chamber?CVCC?.The influence on spray of diesel and heavy fuel oil by fuel temperature and ambient temperature was further investigated.It is found that compared to diesel,the liquid spray penetration length of heavy fuel oil is harder to be stable while the gas phase is harder to evaporate,and the evaporated mass and Sauter Mean Diameter?SMD?are more temperature sensitive.Lastly the influence on spray of heavy fuel oil by swirl was studied,and it is found that swirl has great influence on the penetration length evolution and evaporated effect of spray.Based on the experimental data in a Fuel Ignitability Analyzer?FIA?and CVCC,this paper investigated the combustion and emission of heavy fuel oil by numerical simulation method.By doing sensitivity analysis on combustion model parameters of heavy fuel oil in the FIA,the model was calibrated by rate of heat release and validated by flame length evolution in the CVCC.The influence on combustion and emission of heavy fuel oil by swirl and oxygen concentration was studied,and results show that swirl and oxygen concentration will have great influence on the flame configuration and structure and then on the emission.This paper studied the influence on combustion and emission of marine diesel engine by swirl ratio,air fumigation,EGR and air fumigation coupled with EGR by numerical simulation method.Results show that increasing swirl ratio will promote spray evaporation leading to the increase of cylinder temperature,pressure and NO_x emission.And EGR and air fumigation can influence the combustion and NO_x emission by introducing CO₂ and H₂O and changing the heat capacity and oxygen concentration of the intake mixture respectively.Beyond that,30%EGR and 50%air fumigation has small effect on cylinder pressure while the NO_x emission reduces obviously.In conclusion,this paper investigated the spray combustion and emission process of heavy fuel oil in a marine low-speed two-stroke diesel engine,which provide a theoretical basis for optimizing the spray combustion process of marine engine and NO_x emission.