【作者】 丁红元；

【导师】 黄荣华；

【作者基本信息】 华中科技大学 ， 动力机械及工程， 2013， 博士

【摘要】 燃用乙醇汽油的缸内直喷式(GDI)汽油机在燃油经济性以及降低CO2, NOX排放上有很大的优势,是未来乘用车发动机的主要型式,是我国节能减排需要发展的技术之一。GDI汽油机多使用高压共轨系统和多孔喷油器,在喷雾过程中会遇到空化和闪急沸腾现象。喷雾好坏直接影响燃烧过程进而决定发动机的性能。闪急沸腾和空化一方面会造成燃油计量不准确、喷雾结构发生变化,另一方面会增加扰动促进雾化。闪蒸沸腾喷雾具有雾化快、液滴小、蒸发快、贯穿距离短的优点。为了合理利用空化和闪急沸腾促进雾化的优点,需要研究空化和闪急沸腾的发生条件、发展规律及对雾化的影响。选用泡点作为燃油闪急沸腾的判断依据。建立了基于离散组分的乙醇汽油燃料模型。采用Peng-Robinson物性方程对乙醇汽油的气液平衡过程进行计算,研究了不同乙醇含量对燃料泡点/露点曲线的影响。结果表明：较低的乙醇含量明显拓宽了燃油的两相区域范围,在低乙醇含量条件下随着乙醇含量的增加两相区范围变大；而在高乙醇含量条件下,随着乙醇含量的增加,两相区变窄,P-T曲线更狭长；采用泡点曲线预测闪急沸腾现象更为准确合理,同时较低的燃烧室压力或者较高的燃油温度会促进闪急沸腾的发生。采用双流体法,对高压喷射条件下GDI多孔喷油器喷嘴内部流动进行三维数值模拟,分析和总结了GDI多孔喷油器发生空化流动的条件和发展规律。结果表明：在喷孔入口转角处以及针阀开启或关闭时针阀与阀座壁面缝隙处均有空化现象发生；喷孔入口转角处的空化气泡逐渐向下游发展,可发生部分空化及超空化流动；针阀缝隙处的空化气泡不向下游发展,在针阀完全打开或关闭时消失；喷嘴内部流动形态受燃油性质及喷嘴几何结构影响较大,在针阀保持最大升程时,喷孔内仍观察到不稳定的空化流动；空化流动造成喷嘴各孔流量、速度产生差异,影响喷雾雾化过程,是设计和喷雾模拟时必须要考虑的因素。在可视化定容室喷雾试验台上开展了GDI多孔喷油器使用乙醇汽油的喷雾特性试验研究。结果表明：环境压力对贯穿距的影响最大,喷射压力次之,环境温度对贯穿距的影响最小；环境温度与环境压力对油滴索特平均直径的影响较大；喷雾贯穿距随喷射压力增加而增加,随环境压力增加而减小,随环境温度变化不明显；喷雾锥角随环境压力升高而增大,但基本不受喷射压力及环境温度的影响；油滴索特平均直径随喷射压力及环境温度升高而降低,随环境压力升高而升高。在可视化定容弹喷雾试验台上,采用纹影法研究了不同燃油温度及不同环境压力条件下闪急沸腾喷雾发展过程。试验发现与传统喷雾相比,闪急沸腾喷雾贯穿距较短、喷雾锥角较大。在某些喷射条件下,喷雾的外缘发现了气泡聚集发展的迹象。在雾束的某些部位发现了泡状的亮斑。气泡的聚集发展会使相应区域的“膨胀”“隆起”或者“塌陷”。气泡的这种行为使得喷雾边缘更不规则,并促进了燃油的雾化。相同环境压力条件下喷雾贯穿距随温度增加先减小后增加,喷雾锥角随温度增加先增加后略有减小。闪急沸腾喷雾在高过热度条件下存在转变过程。为模拟闪急沸腾喷雾现象,提出了基于空隙率和过热度共同控制的闪急沸腾雾化新模型和基于双区法的蒸发模型,并编写相关程序在KIVA软件中实现了模型功能。模拟结果表明,闪急沸腾喷雾新模型较好的预测了喷雾贯穿距及喷雾锥角及其发展趋势,喷雾图像与实验结果吻合较好。该模型对中低过热度条件下空隙率控制的闪急沸腾喷雾以及高过热度闪急沸腾喷雾过渡过程有较好的模拟能力,同时可以在一定程度上预测更高过热度条件下的闪急沸腾转变过程。更多还原

【Abstract】 Gasoline direct injection (GDI) engines fueled with ethanol gasoline blends which is a main type of future gasoline engines, have a great advantage in fuel economy and reducing CO2, NOx emissions and will be an important technology support for our energy conservation policy. Since multi-hole injector with high-pressure common rail system being widely used in GDI engines, cavitation flow and flash boiling spray emerge more often during the convention injections. The spray quality of GDI engines determines the combustion process directly and therefore affects the engine performance. Flash boiling spray and cavitation flow will cause inaccurate fuel metering and spray structural changes on the one hand, and promote the atomization by increasing disturbance on the other hand. Comparing with convention injection flash injection, which has smaller droplets and shorter penetration distance, atomize and evaporate faster. In order to use these advantages, more understandings about the occurrence of conditions and the law of development of cavitation and flash boiling atomization is required.The bubble point is selected to predict flash boiling phenomenon of ethanol-gasoline. A new fuel model based on discrete components is created to study the component effects on the bubble point/dew point curves and the evolution of the two-phase region, using Peng-Robinson state equation to calculate the vapor-liquid equilibrium. It shows that the two-phase region is significantly broaden with low ethanol content and increases while ethanol content rises. However two-phase region and the PT curves become narrower when ethanol content increased in high content conditions. The prediction using bubble point curve is more accurate and reasonable, while the lower combustion chamber pressure or high fuel temperature will promote the occurrence of flash boiling.Three-dimensional simulation of internal nozzle flow in GDI multi-hole injector during high-pressure injection conditions is carried out using two-fluid method to analyze the details about cavitation flows in nozzle. The criterion and characteristic pattern of cavitation flow are summarized. The results show that cavitation often occurs at the inlet corner as well as the gap between needle and valve seat. Cavitation bubbles at inlet corner gradually developed towards the downstream, while partial cavitation or super-cavitation flow formed accordingly. Cavitation bubbles around needle valve gap won’t develop towards downstream and will disappear when the needle is fully open or close. Partial cavitation can still be observed even when at maximum needle lift. The internal flow patterns are influenced by nozzle structure and fuel properties. Differences of flow rate and velocity between each hole caused by cavitation will affect the spray atomization process, which must be considered during the design.Experiments on spray characteristics of GDI multi-hole injector fueled with ethanol gasoline are carried out on constant volume chamber (CVC) test rig. Results show that ambient pressure is the greatest impact on spray penetrations followed by injection pressure and ambient temperature. Suater mean diameter (SMD) of droplets is mainly influence by ambient pressure and temperature. Spray penetration increases with inject pressure increases and decreases with ambient pressure arise, but slight changes with ambient temperature. Spray cone angle increases with ambient pressure increases, but no changes with inject pressure or ambient temperature. SMD decreases with inject pressure or ambient temperature increase, while increases with ambient pressure increases.Schlieren shadowgraph method was adopted on CVC test rig. Experiments were carried out to study flash spray development under different fuel temperature and ambient pressure conditions. Results show that penetrations of flashing sprays were shorter and the spray angle was larger than traditional sprays. Bubble dynamic development, which will cause spay ’expansion’ or ’collapse’, were found on spray edge in some conditions. Bubble dynamic makes the spray edge more irregular, but promotes atomization. Under the same environment pressure, penetration first increased then decreased with the increasing of temperature, while the spray angle first increased then decreased slightly, implicating that flash boiling may have transition conditions.A new flash boiling spray model whose atomization criterion based on the void fraction and superheat as well as evaporation model based on the dual-zone method is established to simulate the flashing sprays. The model function is coded and implemented in KIVA program. The new flash boiling spray model predicts spray tip penetration and spray cone angle and its development trend, in good agreement with the experimental results. The model has a good capability in simulating flash sprays at low superheat conditions, which breakup is controlled by void fraction, as well as high superheat transition process. It can also predict flare flashing sprays to some extent at higher superheat conditions.更多还原