高压喷射GDI喷孔几何结构对喷孔内流及喷雾特性的影响
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摘要
采用大涡湍流模拟结合多相流耦合喷雾的方法对GDI喷孔内流和喷雾特性进行了数值研究,着重分析了喷射压力及喷孔结构形状对喷孔出流特性和液滴粒径的影响。结果表明:提高喷射压力有利于增加喷孔出口流速及湍动能,增强燃油破碎;当喷射压力提高到30 MPa之后,进一步提高喷射压力时索特平均直径(SMD)变化不明显,但小粒径占比显著增加;对于变截面喷孔,变截面双曲线喷孔出口处速度和湍动能最大,其SMD最小,小粒径占比最多,有利于喷雾质量提高;与渐缩形喷孔相比,渐扩形喷孔出口处湍动能较大,有利于喷雾初次破碎,然而较多的空泡堵塞喷孔,喷孔出口处流速较低,不利于燃油二次破碎。
The internal flow and spray characteristics for GDI nozzle were investigated by the large eddy simulation(LES) combined with multi-phase fluid and spray method. The effects of injection pressure and orifice geometry on the fluid characteristics and droplet size were mainly analyzed. The results show that the increase of injection pressure is beneficial to increase the flow velocity and turbulent kinetic energy at the nozzle outlet and enhance the fuel breakage. The SMD changes little, but the proportion of small droplet increases significantly when the injection pressure is beyond 30 MPa. The flow velocity and turbulent kinetic energy are the largest, the SMD is the smallest and the small droplet ratio is the most at the exit of hyperbolic orifice with the variable section, which is beneficial to improve the spray quality. Compared with the converging nozzle, the turbulent kinetic energy of expanding nozzle is larger at the outlet, which is beneficial to the initial breakup of spray. However, the more cavities block the orifice so as to the low flow velocity, which is not helpful to the fuel secondary breakup.
The internal flow and spray characteristics for GDI nozzle were investigated by the large eddy simulation(LES) combined with multi-phase fluid and spray method. The effects of injection pressure and orifice geometry on the fluid characteristics and droplet size were mainly analyzed. The results show that the increase of injection pressure is beneficial to increase the flow velocity and turbulent kinetic energy at the nozzle outlet and enhance the fuel breakage. The SMD changes little, but the proportion of small droplet increases significantly when the injection pressure is beyond 30 MPa. The flow velocity and turbulent kinetic energy are the largest, the SMD is the smallest and the small droplet ratio is the most at the exit of hyperbolic orifice with the variable section, which is beneficial to improve the spray quality. Compared with the converging nozzle, the turbulent kinetic energy of expanding nozzle is larger at the outlet, which is beneficial to the initial breakup of spray. However, the more cavities block the orifice so as to the low flow velocity, which is not helpful to the fuel secondary breakup.
引文
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[2] 李梦仁,胡敏,吴宇声,等.缸内直喷汽油机排放颗粒有机物特征及影响因素分析[J].中国电机工程学报,2016,36(16):4443-4451,4532.
[3] 高永强,魏明锐,FAN L,等.喷孔空化特性和近孔初始射流结构研究[J].农业机械学报,2017,48(9):369-376.
[4] 张美娟,宋睿智,居钰生,等.空化对GDI喷嘴内部流动及喷雾特性的影响[J].车用发动机,2015(6):79-84.
[5] 程强,张振东,谢乃流,等.喷孔结构对多孔GDI喷油器喷雾特性的影响[J].内燃机学报,2014,32(1):45-51.
[6] Shost M A,Lai M C,Befrui B,et al.GDI Nozzle Parameter Studies Using LES and Spray Imaging Methods[C].SAE Paper 2014-01-1434.
[7] Lee S,Park S.Experimental study on spray break-up and atomization processes from GDI injector using high injection pressure up to 30 MPa[J].International Journal of Heat and Fluid Flow,2014,45:14-22.
[8] Hoffmann G,Befrui B,Berndorfer A,et al.Fuel System Pressure Increase for Enhanced Performance of GDI Multi-Hole Injection Systems[J].SAE International Journal of Engines,2014,7(1):519-527.
[9] Befrui B,Berndorfer A,Breuer S,et al.Effect of Fuel Pressure on GDI Multi-Hole Injector Particulate Emissions and Tip Coking Robustness[C]//Haus de Technik 12 International Congress "Engine Combustion".Ludwigsburg:[s.n.],2015.
[10] Karra P K,Kong S C.Experimental Study on Effects of Nozzle Hole Geometry on Achieving Low Diesel Engine Emissions[J].Journal of Engineering for Gas Turbines and Power,2009,132(2):022802-022810.
[11] 卢国权,虞钢,何秀丽,等.喷孔几何特征对变截面喷油孔空穴流动状态的影响[J].内燃机学报,2012,30(3):254-259.
[12] Saha K,Quan S,Battistoni M,et al.Coupled Eulerian Internal Nozzle Flow and Lagrangian Spray Simulations for GDI Systems[C].SAE Paper 2017-01-0834.
[13] Yoshizawa A,Horiuti K.A Statistically-Derived Subgrid-Scale Kinetic Energy Model for the Large-Eddy Simulation of Turbulent Flows[J].Journal of the Physical Society of Japan,1985,54(8):2834-2839.
[14] Wang B,Jiang Y,Hutchins P,et al.Numerical analysis of deposit effect on nozzle flow and spray characteristics of GDI injectors[J].Applied Energy,2017,204:1215-1224.
[15] Winklhofer E,Kull E,Kelz E,et al.Comprehensive hydraulic and flow field documentation in model throttle experiments under cavitation conditions[C]//Proceedings of the ILASS-Europe Conference.Zurich:ILASS,2001:574-579.