【作者】 刘静；

【导师】 徐旭；

【作者基本信息】 北京航空航天大学 ， 航空宇航推进理论与工程， 2010， 博士

【摘要】 使用液体碳氢燃料的超燃冲压发动机中的雾化是其工作中的一个重要环节,它对提高发动机的性能起着至关重要的作用。由于超声速气流中燃料射流从喷出到完全雾化的时间为毫秒量级,气流速度很快且液雾浓密,所以对观察和测量都带来一定的困难,到目前为止雾化的具体过程和雾化机理还不清楚。传统的对雾化过程的数值模拟方法主要是拉格朗日粒子追踪方法,这种方法借助于雾化模型对雾化过程进行模拟。现有的雾化模拟存在以下一些问题:1、大多是半经验模型,存在经验常数,实验数据大多来源于低速气流中的雾化实验,不一定完全适用于超声速气流中的射流雾化。2、由于对雾化机理的认识欠缺,一次雾化模型缺乏理论基础,属于简化计算。本文从数值模拟和实验两方面对超声速气流中横向射流雾化过程进行了研究。对于超声速气流中横向射流雾化过程的数值模拟,本文采用了两种截然不同的方法进行研究,分别是界面追踪方法和拉格朗日粒子追踪方法。界面追踪方法对射流雾化的具体演化过程进行研究,对液柱和液滴的界面变形破碎过程进行了数值模拟。目前大部分的界面追踪方法仅限于不可压流动。由于本研究中涉及的气流可压缩而雾化燃料近似不可压,所以为在整个流场中对二者进行统一求解,本文采用了高精度PPM方法结合体积分数模型来模拟多介质可压缩流动。对一次雾化过程研究了不同来流速度和不同喷孔直径对射流形状和穿透深度的影响,发现来流速度减小后,射流穿透深度增高,展向宽度基本不变。喷孔直径增大将导致射流展向宽度变大,穿透深度增大。而对二次雾化过程的研究发现,来流速度增大后,液滴四周被剥离的程度增大,破碎后的产生的液块更小、更多。当液滴直径增大后,在相同来流条件下,液滴在相同计算时间内保持完整性,更不易破碎。拉格朗日粒子追踪方法中,首先研究了通用的一次雾化模型如Blob模型,和二次雾化模型如TAB、K-H波以及改进后的混合雾化模型在超声速气流条件下对雾化过程模拟的适用程度,并与实验测量结果进行了对比。拉格朗日粒子追踪计算中,主要研究了以下几个方面的内容:1、不同雾化模型的经验参数对计算结果的影响规律;2、针对超声速气流的特点,对混合雾化模型的时间判断准则结合实验结果进行了改进。通过对几种雾化模型的研究对比发现,改进后的混合雾化模型更适用于超声速气流中横向射流的雾化计算。3、研究了雾化过程中气相湍流度、动压比以及蒸发模型对计算结果的影响。4、进行了三维拉格朗日雾化计算,研究了液相场和气相场的特点。鉴于界面追踪方法不适合对细小的液滴进行追踪,而拉格朗日方法无法真实模拟射流未充分雾化部分,本文尝试将界面追踪和拉格朗日粒子追踪方法结合起来,通过一个简单的方法转化条件,实现了两种方法耦合求解。应用耦合求解方法对整个雾化过程进行了计算。采用耦合方法对一次雾化和二次雾化进行了初步研究,通过与实验结果对比发现,耦合方法结合了两种方法的优点,能对真实的雾化过程进行更加全面的数值模拟,单独使用一种方法很难模拟的问题,采用耦合方法能更好的解决。实验研究方面,本文采用纹影法对超声速气流中射流雾化过程进行了初步实验研究,得到了穿透深度的拟合公式,并与计算结果进行了对比。另外,作为界面追踪的尝试计算,本文还采用Level Set方法对不可压流动中的液滴的二次雾化进行了详细研究。计算得到了亚声速气流中几种典型的二次雾化模态,并分析了韦伯数(Weg)、液体雷诺数(Rel)、气体雷诺数(Reg)和密度比(γ)等无量纲数对破碎过程的影响。更多还原

【Abstract】 The liquid fuel atomization processes play an important role on scramjet performance. The time for liquid jet atomization in supersonic cross flow is only several milliseconds. Because of the fast gas flow velocity and the dense liquid droplets clouds, it’s difficult to observe and measure the atomization processes, the atomization mechanism is still unclear now. Traditional atomization calculation method is Lagrangian particle trace method. This method use semi empirical models to calculate the atomization processes. The problems are: firstly, most of the models are semi empirical model. The empirical constants used in these models were obtained from low speed flow experiments. Whether these models are applicable in supersonic atomization calculation is still unknown. Secondly, the primary atomization model is too simple, because there is little atomization mechanism known for this process. This thesis researched the atomization processes in supersonic cross flow using numerical simulation and experimental methods.This thesis used two distinct methods to numerical simulate the atomization processes, they were interface trace methods and Lagrangian particle trace method. The interface trace methods calculated the liquid breakup interface processes, most of which were applicable for uncompressible flow only. Since the gas flow in this research is compressible and liquid is uncompressible flow, high precision PPM(Piecewise Parabolic Method) method and volume of fraction model were used to calculate the multiphase compressible flow. The primary atomization and secondary atomization processes were simulated using interface trace method. For the primary atomization, the influence of different gas flow velocities and injector diameters were analyzed. The penetration depth was increased and span width kept constant when the gas flow velocity decreased. Both the penetration depth and the span width were increased when the injector diameter increased. For the secondary atomization, liquid drop was pealed more quickly and the breakup liquid patches were smaller when the gas flow velocity increased. When the liquid diameter increased, it was much more difficult to breakup during the same duration of time.The primary atomization model Blob model and second atomization model such as TAB model, K-H wave model and hybrid model were tested in supersonic cross flow conditions, the results were compared with experimental data. Lagragian calculation research includes some aspects as follows: 1 the influence of empirical constants in atomization model; 2 the hybrid atomization model was much more suitable for the supersonic atomization calculation. The breakup time criteria was changed according to experimental data in order to get a good simulation results; 3 The influences of the turbulence intensity, aerodynamic pressure ratio and evaporation model were researched in breakup process; 4 the three dimensional atomization processes were calculated, both the gas and liquid phase flow field characters were analyzed.For the interface trace method is not fit for small liquid particle simulation, and Lagrangian method is not good for liquid breakup in primary atomization. The interface trace method and Lagrangian method were coupled together to calculate the whole atomization processes in this thesis. The coupled method can take the advantages of the two methods, it is much better for the whole atomization processes simulation compared to each of the individual methods. Schlieren method was adopted to observe the jet atomization phenomena in experiments. The empirical relations of penetration depth were obtained from experimental pictures , which was in good agreement with the numerical results.Level Set method was also used to simulate the liquid drop atomization in uncompressible flow. Several typical secondary atomization phenomena were obtained in interface trace calculation. The influence of some nondimentional numbers such as Weber number (Weg)、liquid Reynolds number(Rel)、gas Reynolds number(Reg)and density ratio(γ)on atomization were analyzed .更多还原