双燃料船机天然气喷射规律对燃烧特性的影响

Effect of Natural Gas Injection Regulation on Combustion Characteristics of Dual-Fuel Marine Engine

基于高压直喷(high pressure direct injection,HPDI)天然气船用低速机,采用计算流体力学(CFD)软件Converge开展三维数值模拟研究,探究了天然气喷射规律对缸内着火及燃烧特性的影响.结果表明,随着天然气喷射速率的降低,缸压、放热率和平均温度峰值显著降低.当喷射速率从1.30 kg/s降低到0.78 kg/s时,放热率峰值持续时间延长了约15°CA,缸压降低了约2MPa,导致燃烧相位延后,发动机做功能力降低.降低喷射速率使得缸内火焰发展速度变慢且火焰浮升长度更晚到达稳定值.进一步分析燃烧过程的高温反应和低温反应发现,天然气喷射速率降低使得燃烧初期的低温反应速率降低;2.5°CA前总体高温反应无明显变化,但随后高温反应随着天然气喷射速率的降低而减弱.天然气喷射速率越低,燃烧反应进行至约1.5°CA后富燃区低温产物CH_(2)O和H_(2)O_(2)浓度峰值越高,且峰值持续时间更久,其原因为富燃区低温产物消耗较慢且火焰对富燃区的影响相对滞后,使低温产物快速消耗的时刻推迟.

Based on the high pressure direct injection(HPDI)mode low-speed marine engine in which natural gas is injected directly in cylinder with high pressure,a three-dimensional numerical study is carried out by using the computational fluid dynamics(CFD)software Converge to explore the effect of natural gas injection regulation on in-cylinder ignition and combustion characteristics.The calculation results indicated that the peak values of incylinder pressure,heat release rate and average temperature are significantly reduced with the decrease of natural gas injection rate.When the injection rate decreases from 1.30 kg/s to 0.78kg/s,the peak heat release duration is extended by about 15°CA,and the in-cylinder pressure is reduced by about 2MPa,which leads to the delay of combustion phase and the decrease of engine work capacity.Decreasing the injection rate slows down the flame propagation and causes the flame lift-off length to reach the stable value later.According to the further study of high and low temperature reaction in the combustion process,it is found that decreasing the injection rate reduces the low temperature reaction rate at the initial stage of combustion.The overall high temperature reaction shows no obvious change before 2.5°CA,but then it weakens with the decrease of the injection rate.As the injection rate decreases,the peak concentration of low temperature products(CH_(2)O and H_(2)O_(2))in the fuel rich regions increases after about 1.5°CA,and the peak duration gets longer.The reason is that the consumption of low temperature products in the fuel rich regions is slow,and the time when the flame affects the fuel rich regions is relatively late,which delays the rapid consumption of low temperature products.