扫气口角度对双燃料船机混合及燃烧的影响

Effect of Scavenging Port Angle on Mixing and Combustion in the Cylinder of a Dual Fuel Marine Engine

针对一台低压喷射二冲程天然气/柴油双燃料发动机进行三维全尺寸模拟,研究了扫气口角度对天然气混合过程以及功率和排放的影响.结果表明:引燃油喷射前,天然气分布呈现出分层,其中扫气口角度θ<20°时,天然气集中分布在活塞顶附近,呈"上下"分层;扫气口角度θ≥20°时,混合气浓区集中分布在气缸壁附近,呈"左右"分层,同时会出现天然气逃逸现象;适中的扫气口角度(θ=25°)有利于降低天然气消耗率,低于此值时预燃室附近天然气稀薄,火焰传播速度低,燃烧持续期长;高于此值时,燃烧相位前移,压缩负功增加.此外,当扫气口角度过小时,由于活塞顶部天然气过浓,会使NOx排放急剧增加,甚至超过TierⅢ排放法规的限值.

A full-scale low-injection-pressure two-stroke natural gas/diesel dual-fuel engine was studied using threedimensional computational fluid dynamics simulation,with particular focus on the effect of scavenging port angle on the natural gas-air mixing,the indicated power and emissions.The results show that,prior to pilot fuel injection,the methane is stratified in the cylinder.Specifically,when the scavenging port angle is small(θ<20°),the methane is mostly distributed on the top of the piston and thereby exhibits a vertical stratification pattern.When the scavenging port angle is large(θ≥20°),the methane is mostly distributed on the lateral of the cylinder and hence forms a horizontal stratification pattern,meanwhile natural gas escape will occur.Regarding the indicated specific gas consumption of the engine,an intermediate scavenging port angle ofθ=25°is found to yield best performance,as a smaller value leads to lean mixture in the pre-combustion chamber and consequently slow flame propagation,while a larger value advances combustion phase and thereby causes overlarge compression work.Moreover,when the scavenging port angle is excessively small,the large natural gas concentration nonuniformity distributed at the top of the piston could lead to substantial increase in NOx emissions and may even exceed the TierⅢemission limit.