进气加湿耦合涡流比对船用柴油机燃烧和排放的影响

Effects of Intake Air Humidification Coupled with Swirl Ratio on Combustion and Emissions in a Marine Diesel Engine

针对某一款四冲程船用柴油机,利用商业模拟软件AVL-Fire建立柴油机三维数值模型,研究进气加湿耦合涡流比对燃烧的影响以及改善NOx和碳烟折中关系的潜力,给出优化方案,得到满足TierⅢ排放法规的技术路线并保证指示燃油消耗率增长在2%以下.结果表明,加湿率增大,缸内温度和压力的峰值均降低,且最大降幅都约为2%;滞燃期延长,燃烧重心后移,对燃烧热效率不利.缸内温度和进气氧浓度的下降有效降低了NOx排放,最大降低幅度约为56.7%.一定的涡流比可以促进油气混合,而太大的涡流比会降低油束贯穿距,使喷雾前端偏转角度增大,影响油气混合.涡流比的增大对NOx和碳烟的排放形成先增后降的影响,在涡流比为0.5时碳烟排放最低.此外,对各种进气加湿率和初始涡流比获得的结果进行优化,其中有9种算例同时降低了NOx和碳烟排放,并且在初始涡流比为0.50时,加湿率100%和80%的两种方案满足TierⅢ的排放标准.

A three-dimensional numerical model of a four-stroke marine diesel engine was established by a commercial simulation software AVL-Fire.Numerical investigations were conducted to explore the potential of intake air humidification coupled with swirl ratio to improve the trade-off relationship of NOx-soot.This paper also provided a solution to meet TierⅢemission regulations with a loss of indicated specific fuel consumption of no more than 2%.The simulation results indicate that both in-cylinder peak temperature and pressure decrease by about 2%with the increase of humidification rate.The ignition delay gets longer,and CA50 is delayed,resulting in reduced combustion thermal efficiency.The NOx emissions can be significantly reduced due to the decrease in temperature and intake oxygen concentration,with the greatest decrease of about 56.7%.An appropriate swirl ratio is conducive to fuel/air mixing.However,with the increase of swirl ratio,jet penetration becomes shorter due to jet deflection,which will influence fuel/air mixing quality.The NOx and soot emissions first increase and then decrease as the swirl ratio increases,and the soot emission is the lowest when the swirl ratio is 0.5.Furthermore,optimization is carried out on the results obtained for various humidification rates and initial swirl ratios.There are nine cases that can simultaneously reduce NOx and soot emissions.When the initial swirl ratio is 0.5,the two cases with 100%and 80%humidification rate are found to have achieved the optimum emission and meet TierⅢemission regulations.