柴油机喷嘴结构对喷雾特性影响的耦合模拟研究

Coupled Simulation of the Effect of Diesel Nozzle Structure on Spray Characteristics

高压共轨系统能有效地改善柴油机的喷雾质量,但随着燃油喷射压力的增加,燃油雾化过程变得更加复杂。而喷嘴内部湍流和空穴现象对喷雾雾化有重要的影响,特别是空穴现象。利用同步辐射X射线同轴相衬成像技术准确获得喷嘴几何结构尺寸,建立精确的喷嘴内部流动模型,建立起耦合喷嘴内空穴流动的耦合喷雾模型,分析喷嘴内空穴流动对燃油喷射雾化的影响。利用在高压共轨喷雾试验台架上的高压喷雾试验,验证该喷雾模型的准确性。利用该验证后的喷雾模型直接得到喷嘴结构参数,包括喷孔长径比、喷孔入口圆角半径及喷嘴压力室结构对喷雾特性的影响,得出喷孔入口圆角半径和压力室结构对喷雾的索特平均直径有较大的影响。研究结果为柴油机燃油喷射系统的优化提供了理论依据。

Fuel atomization and penetration are known to significantly affect the combustion and emission processes in diesel engines. High-pressure common rail system can supply better diesel spray to enhance combustion efficiency and reduce emission. With the fuel pressure increasing, spray process becomes more and more complex which makes the study on atomization mechanism more challenging. Under the super high pressure, the internal turbulent flow and cavitaing flow of the nozzle are important to fuel spray characteristics, especially the cavitaion has been the key to relating the internal flow of nozzle and atomization behavior. So it is necessary to simulate fuel spray coupling with internal cavitaing flow of the nozzle. The internal geometry of injector nozzle is measured with the X-ray phase contrast imaging. The more exact CFD model of nozzle is established using geometry parameters from the measurement, which is used for the three-dimensional numerical simulation of the cavitating flow in nozzle. Moreover, analysis of the influence of cavitaing flow in the nozzle hole on spray is made and a coupling spray simulation is carried out with the output data of cavitaing flow in nozzles using FIRE v2010. The results of this work show that the characteristics of cavitating flow in orifice have an effect on both macroscopic properties and microscopic properties. Compared with simulation result, a spray experiment under high pressure is tested on a high-pressure common rail injection system, which has verified the spray model. On base of the verified spray model coupled with the cavitating flow in nozzle, multi-schemes of cavitating flow in nozzle and subsequent spray are simulated with various geometry parameters of nozzle. These simulation results supply a theoretical basis for the optimization design of the high-pressure common rail injection system.