柴油近场喷雾初期形成过程的研究

Study on the Initial Formation Processes of Near-Field Diesel Spray

为了更好地理解喷油初期柴油喷雾的形成过程,利用长工作距离显微镜头和高速摄影相结合的方法,试验研究了不同喷油压力、环境压力以及喷孔长径比下喷油初期单孔柴油喷油器喷孔下方3.5 mm内的近场喷雾特性.结果表明,在喷油初期阶段,喷雾贯穿度存在一个加速发展过程,而油束的破碎区域由头部破碎区和液柱破碎区组成.燃油喷出后,油束头部先产生破碎,形成了一个随时间发展而逐渐增大的伞状头部,并伴随油丝剥离现象的产生,同时喷油压力越大,相同喷射时间下的头部破碎区域越大,油滴群分布范围越广;液柱区初始破碎的时间要晚于头部破碎区.随着空化流发展至孔外,液柱区开始产生破碎,油束宽度明显变大,柱状形态逐渐消失,导致近场喷雾锥角迅速增大;当环境压力为4.5 MPa、喷油压力从80 MPa增加至140 MPa时,喷雾液柱区开始破碎的时间由72μs提前至36μs;当喷油压力为120 MPa、环境压力从2.5 MPa提高至4.5 MPa时,喷雾液柱区的破碎时间也相对推迟,同时喷雾贯穿度和喷雾面积明显减小;当喷油压差平方根呈线性增加时,相同喷油时间下喷雾前端速度的增加幅度明显变大.当喷孔直径不变时,减小喷孔长度,喷雾贯穿度增加,喷雾液柱区的破碎时间提前,近场喷雾锥角相对增加,破碎程度随之增大,油束破碎区域也更加对称.

To understand the initial formation processes of the near-field diesel spray,the characteristics of the nearfield spray within 3.5 mm below the hole exit of a single-hole diesel injector under different injection pressures,ambient pressures,and length-to-diameter ratios were experimentally investigated;this was carried out by combining a long-distance microscope and a high-speed digital camera.The results show that an acceleration process occurs during the spray development,and the breakup zones of the spray are composed of the spray head and the liquid column zones.The breakup first occurs in the spray head,then an increased umbrella-shaped head is formed,and stripped fuel filaments are also formed during injection.The larger the injection pressure is,the larger the area of fuel droplets distributed at the same injection timing.The breakup time of the liquid column area is later than that in the head.With the outflow of the cavitation area in the nozzle hole,the liquid column begins to break up,the width of the spray significantly increases,the columnar shape gradually disappears,and the near-field spray cone angle rapidly increases.At the ambient pressure of 4.5MPa,the time of the liquid column breakup is advanced from 72μs to 36μs when the injection pressure is increased from 80 MPa to 140 MPa.At the injection pressure of 120MPa,the time of the liquid column breakup is delayed,and the penetration and spray areas are significantly reduced when the ambient pressure is increased from 2.5MPa to 4.5MPa.Moreover,after fuel injection,the spray front speed increases sharply with the increase of the square root of the injection pressure differential. At a fixed diameter of thenozzle hole,the reduction of the nozzle hole length results in increased spray penetration,advanced breakup time ofspray liquid columns,relatively increased spray cone angle,increased degree of spray breakup,and a more symmetricaldistribution of fuel droplets relative to the center line of the spray.