【作者】 解方喜；

【导师】 洪伟；

【作者基本信息】 吉林大学 ， 动力机械及工程， 2010， 博士

【摘要】 对于柴油机而言,燃油是缸内工作过程中主要的工质之一,燃油的射流运动与柴油机的动力性、经济性及排放性能等表观指标存在着极为密切的联系。同时,在柴油机实际工作过程中燃油射流的运动行为是极为复杂的,涉及到破碎、雾化、蒸发、混合、扩散及燃烧等多种物理及化学行为,深入了解燃油的这些运动行为,为我们深入了解缸内工作过程及降低有害排放物的生成具有重要的意义。据此,文中针对燃油射流运动行为的机理和影响因素进行了深入剖析,并且考察了喷射控制条件及喷油器结构与布置等条件对改善燃油射流运动及优化燃烧过程的应用潜力。为深入了解缸内燃油射流的破碎和雾化行为机理,文中首先总结并分析了过去研究人员的研究成果,对喷孔内部流动行为进行了深入而全面的了解。喷孔内的流动形态可划分为单相流、部分空化流、超空化流及柱塞流等四种基本流态,这四种不同的流动形态对应不同的气/液两相分布及流动结构,进而影响着孔外的射流雾化过程。为进一步了解喷射过程中喷孔内部流动的细节特征,通过FIRE软件中的双流体模型对单孔喷嘴进行了模拟研究。研究发现,喷射过程中随针阀升程和喷射时间的变化,孔内的流动状态发生显著变化,由单相流逐渐演变为空化流,流动结构也逐渐呈两区分布。对于轴对称喷嘴,在喷孔中心区域存在密度较高、速度较大且湍动较弱的液相区;而在壁面附近则为密度较低、流速较小且空穴气泡含量较高的空穴区。对于非轴对称结构喷嘴,喷孔内的空穴气泡分布及其流动参数也呈现非对称分布,空穴区主要产生于喷孔的上壁面附近。同时,文中还在一款实际的7孔喷油器基础上,针对喷射压力、背压、燃料性质等喷射条件,以及喷孔入口圆角半径比R/D、长径比L/D、进/出孔径系数K等几何结构参数的影响进行了研究,结果发现这些因素对孔内流动特性均具有较大影响,合理选用这些参数可以增强诱导射流破碎雾化的空穴、湍动及速度等因素。在已有的各种雾化假说和模型的基础上,结合FIRE软件自身的特点,以及孔内流动行为的研究,提出了一个能准确描述射流雾化过程的组合模型。模型中认为湍流、空穴及气动力等共同作用是诱导射流雾化的主要动因,并根据各因素作用区域的不同,相应地将射流雾化过程分为初次雾化与二次雾化两个阶段。初次雾化发生在近孔区域,喷孔内部形成的空穴和湍动是诱发雾化过程的主导因素。在构建初次雾化模型过程中,利用一个辅助网格将喷孔出口流动特性参数与初次雾化模型联系起来,通过质量守恒构建出射流液核的基本结构尺寸,然后破碎液滴在相应的液核表面网格上释放出来。模型中通过湍动长度尺度来确定雾化长度尺度和液滴直径,将空穴气泡的影响处理为湍动方程的一个源相;雾化时间尺度为湍流和气动破碎时间的加权平均值。该模型能被赋予准确详细的初始喷射条件,并可自动计算出SMD及喷雾锥角等特征参数。初次雾化产生的小液滴进入油/气混合区域后,由于液滴与周围气体之间具有较高的速度差,从而在气动力的作用下会进一步发生分裂破碎。文中采用KH-RT模型对液滴的二次雾化过程进行模拟,该模型中耦合KH与RT两种不稳定破碎模式,适用于低以及高Weber数条件下的多种破碎类型。利用轴对称结构与非轴对称结构的两种单孔喷嘴研究了孔内流动特性的变化对孔外射流雾化过程的影响,研究结果表明:对于轴对称结构喷嘴,喷雾体形态仍呈轴对称分布;而对于非轴对称结构喷嘴,由于孔内的空穴气泡呈现极强的不均匀分布,在空穴现象较为富集的上壁面,空穴气泡溃灭后,对射流体的扰动作用显著加强,射流破碎加速,并且破碎液滴获得更大的径向动能,从而具有较大的扩散锥角,而空穴气泡较少的一侧则与之相反,故而孔外射流体也呈现出非轴对称分布。利用所建雾化模型及可视喷雾形貌的实验装置进一步考察了不同喷射条件和喷嘴结构对多孔喷嘴射流雾化过程的影响。研究结果表明:耦合详细的初始边界,可使射流体形貌、贯穿距离和锥角等与实验结果更加吻合。同时,合理改进喷射条件和喷嘴结构能有效的改变喷孔出口的流动状态,从而改善射流雾化效果,扩大液滴的分布区域,故而从喷射条件和喷嘴结构两方面进行优化,是改善柴油机燃烧过程和降低有害排放物生成的有效手段。为对更为复杂的缸内工作环境下的燃油射流运动行为和燃烧过程进行深入了解和认识,文中将研究工作进一步拓展到一台实际的柴油机中,针对一台YD4A75-C3型柴油机建立了柴油机缸内工作过程的数值模拟和实验研究平台。同时,为对缸内燃油分布、燃烧过程及有害排放物生成过程进行深入和定量地分析,文中还引入了一种基于统计意义的微观信息“量”化分析法。研究结果表明:柴油机缸内工作过程中,燃烧前期和中期放热速率较快,缸内有较多的燃料参与燃烧反应,单位时间内放出的热量较多,并且局部存在放热速度更为快速的区域,促进了局部高温区的产生。同时,燃油燃烧过程中,整体上燃油分布区域较为集中,燃油扩散不足,空气利用率不高,燃油蒸汽边扩散边进行燃烧,燃烧释放的热量主要被未燃燃油和燃烧产物吸收,这也是促使局部高温产生、及有害排放物的生成的主要因素。鉴于柴油机工作过程中燃油射流运动与燃烧过程、高温区的产生及有害排放物的生成之间的密切关系,文中首先结合高压共轨燃油喷射系统灵活控制喷射过程的特点,针对喷射压力、主喷定时、预喷射及后喷射等喷射条件进行了研究。随后文中又研究了喷孔数、喷孔直径、喷孔布置形式和喷孔结构等喷嘴几何结构参数的影响。研究结过表明:合理地调整喷射条件及喷嘴结构能有效地改善燃油射流运动,燃油的扩散运动加强,分布区域增大,空气的利用率提高,从而有效降低有害排放物的生成。更多还原

【Abstract】 Fuel is the major working media during the operation of the diesel engine in cylinder. Jet flow is closely related with the diesel engine power, economy and emission performance. Meanwhile, the jet flow behavior is very complicated under the practical operation of the diesel engine, which includes fuel broken, atomization, evaporation, mixing and others’physical and chemical process. A deep knowledge of these behaviors is helpful for us to understand the working process in cylinder and the reduction of harmful emissions. The mechanism and influencing factors of fuel atomization is deeply analyzed in this paper. And the application potential of the injection control condition and injector structure and layout are investigated in order to improve fuel jet behavior and optimize combustion process.In order to deeply understand the principle of the breakup and atomization of fuel jet in cylinder, the research results of others were analyzed and summarized, the flow behavior inside the orifice was deeply and roundly studied in this paper. The flow inside the orifice was subdivided into single-phase flow, partly cavitation flow, fully cavitation flow and plug flow. The four different flow regimes correspond to different gas/liquid two-phase flow distribution and structure, and had influences on the atomization process of jet out of the hole. For the further understanding of detail features about the flow process inside the orifice during the injecting process, single hole nozzle was numerically studied by dual fluid model in FIRE software. With the needle valve lift and fuel injection time changed, the flow state inside the hole changes apparently from single-phase flow gradually evolved into cavitation flow and the flow structure appears bi-region distribution gradually. Toward the axisymmetric nozzle, in the center of nozzle there is a higher density, higher velocity and weaker turbulence liquid area, and near the orifice wall there is a less density, lower velocity void area with more air bubble. When it comes to asymmetric orifice, the distribution of cavity bubble and flow parameter present asymmetric distribution, the void area exists around the upper wall. At the same time, based on a real injector with seven holes, the injection pressure, the backpressure, fuel characteristics and inlet rounded corner diameter ratio R/D, length rate L/D, inlet /outlet aperture ratio K and other structural geometries was studied .The results showed that these factors made high impact on the flow characteristics. The proper choice of these factors can enhance the cavity, turbulence and velocity of the inducing jet-flow that improves the atomization.Based on the study of the various atomization hypothesis and models, combining with the characteristics of FIRE software and the study of flow characteristics inside the hole, a more accurate and detailed description of the jet flow atomization procedure was put forward. In the model, turbulence, cavitation and aerodynamic were all important factors on the effect of jet flow atomization procedure. Cavitation and turbulence in the nozzle are one of the dominant factors to induce jet flow primary atomization. Turbulent kinetic energy and cavitation bubble motion in the nozzle greatly strengthen initial perturbations on the jet surface. Then the initial disturbance surface wave further developed under the influence of aerodynamic and finally broken isolated droplets. In the process of establishing the model, spray atomization processes was divided into primary breakup and secondary breakup to be simulated. Primary breakup near the nozzle hole, the flowing characteristics of nozzle and spray model contacted by additional grid and the basic measure of liquid can be calculated by Conservation of Mass. Then broken droplet was accordingly released in liquid core surface mesh. The length and diameter of droplets were obtained by length of turbulent in this model. The impact of the bubble cavity was treated as a source of turbulent phase equation. Atomization time scale is the weighted average of turbulence and aerodynamic crushing.The accurate condition of original injection was given to this model. SMD and spray cone angle can be calculated by this model. At the same time, KH-RT model was used to simulate the droplet atomization process of secondary breakup. The unstable breakup models of KH and RT were coupled in this model which applied to many breakup models of low and high Weber number.Based on the non-axisymmetric and axisymmetric structure, the effect Liquid-Gas Distribution on the process of jet puverization was studied. The results showed that the jet flow of the axisymmetric nozzle is axisymmetrical distribution and the non-axisymmetric nozzle jet flow was non-axisymmetric. The reason is that the bubble of cavity is non-uniformity distribution in the hole of the non-axisymmetric nozzle. The disturbance to jet flow was strengthened in the thicker side of the cavity area after the cavity collapsed. The body of the jet disturbance was significantly enhanced speed jets and fragmentation solution greater radial kinetic energy drops, which have large diffusion cone angle, while the hole on one side of the bubble is less contrast.Utilizing atomization model and the structure of jet atomization in the visual experiment, the influence of different injection conditions and spray hole structure on the jet atomization of porous nozzle holes were further studied. The results showed that the jet structure, spray penetration and cone angle were more consistent with the experimental results by the detailed coupled original boundary. Reasonable injection conditions and improved jet nozzle structure can effectively change the nozzle exit flow state. The jet atomization is improved , which expand the geographic distribution of droplets and promote fuel and gas mixture. Therefore the optimization on the two aspects of the spray conditions and the nozzle structure is important technical means to improve the diesel combustion process and reduce harmful emissions.In order to further understand the combustion process and the jet flow behavior in the complicated surroundings in cylinder, the research platform of the in-cylinder working process of the numerical simulation and experiment was established based on the YD4A75-C3 diesel engine. To deeply and quantificationally analyze the fuel distribution characteristics, combustion and the generation process of harmful emissions NO and Soot, the paper introduced the microscopic information quantity analysis method. The results showed that rate of heat release of prophase and intermediate combustion was higher. Because much fuel burned and heat rejection is much, the temperature is high at the area of high exothermic rate in this stage. The distribution of fuel is concentrated totally and the fuel spread inadequately, so the utilization of air is low at the process of combustion. The combustion is diffusive combustion. The heat was mainly absorbed by unburned fuel and combustion products which was the main factor of creating high temperature at some places and harmful emissions.Combined with the high-pressure common rail system of high degree of flexibility the injection pressure, main injection timing, pre-injection and post injection were studied in this paper based on the close relationships of jet flow, combustion process, high temperature zone and pollutant generation. The factors of nozzle hole number, diameter of nozzle hole, arrangement of nozzle hole and post injection were also studied. The research results showed that the jet flow, the diffusion motion of fuel, the distribution areas of fuel and the utilization of air was improved better and the harmful emissions was reduced when the injection condition and nozzle structure were reasonably improved.更多还原