柴油机喷孔内空化特性的研究
摘要
在不同针阀落座速度下，针阀关闭过程中压力室及喷孔区域的平均压力都呈现出先减小，后趋于外部环境压力的趋势，整个计算区域内平均压力的大小分布，从大到小依次为外部环境、喷孔区域以及压力室。针阀落座速度越高，压力室及喷孔区域内的平均压力出现的负压值越大。针阀关闭时，针阀落座速度越高，喷孔内负压越大，流速越大，在喷孔出口截面上柴油的喷出速度和空气的倒流速度都越大。针阀关闭后，针阀落座速度越高，最终在孔内形成的气泡就越大，柴油残余率越小。因此本文采用数值模拟的手段对针阀关闭过程柴油机喷孔内的瞬态流场进行研究。首先，基于开源软件 OpenFOAM，本文提出通过设计三套连续的网格，将针阀关闭过程定义成两种连续的网格运动使针阀完全关闭，同时采用 VOF 方法，并引入接触角模型，建立三维两相流动模型，分三步模拟针阀关闭过程柴油机喷孔内的瞬态流场。计算结果与 S.Jollet 实验对比，验证了所建立计算模型的准确性。随后对比针阀关闭后两种喷孔内气液分布的变化过程，分析了气泡形成的原因。在此基础上，进一步分析了喷孔直径、长度以及锥度（K>0）等三种结构参数对针阀关闭过程喷孔内瞬态流场的影响。结果表明，针阀关闭过程中喷嘴压力室及喷孔区域内的平均压力都具有先减小后趋于环境压力的变化趋势；针阀落座时，喷孔出口截面上，外部倒流空气的速度随着孔径增大、长度缩短及锥度减小而增大；针阀关闭后，在喷孔内形成气泡的大小同样随着孔径增大、长度缩短及锥度减小而增大。最后，分析了喷油压力、背压以及针阀落座速度等三种喷射条件对孔内瞬态流场的影响。结果表明，针阀关闭过程中喷孔压力室及喷孔区域内的平均压力也是先减小后趋于环境压力；针阀关闭时，喷孔出口截面上，外部倒流空气的速度随着喷油压力增大、背压减小及针阀落座速度增大而增大；针阀关闭后，在喷孔内形成气泡的大小也随着喷油压力增大、背压减小及针阀落座速度增大而增大。
关键词：柴油机；喷孔；针阀关闭过程；瞬态流场；气泡；数值模拟
I
Abstract
ABSTRACT
Under different needle valve seating speed, the average pressure in the pressure chamber and nozzle area decreases first, then tends to the external environment pressure. The average pressure distribution in the whole calculation area is in the order of external environment, nozzle area and pressure chamber from large to small. The higher the needle valve seating speed, the greater the negative pressure value of the average pressure in the pressure chamber and the nozzle area. When the needle valve is closed, the higher the needle valve seating speed is, the greater the negative pressure in the nozzle and the flow velocity are, the higher the diesel injection speed and the air reverse flow speed are at the exit section of the nozzle. When the needle valve is closed, the higher the needle valve seating speed is, the larger the bubbles formed in the hole and the smaller the diesel residual rate. In this paper, numerical simulation is used to study the transient flow field in the nozzle of diesel engine during the needle valve closure process. Firstly, based on open source software OpenFOAM, this paper proposes to design three sets of continuous grids to define the needle valve closing process as two kinds of continuous gridding motion to make the needle valve completely closed. At the same time, VOF method and contact angle model are introduced to establish a three-dimensional two-phase flow model, which can simulate the transient flow field in the nozzle of diesel engine in three steps during the needle valve closing process. the shortening of the length and the decrease of the taper of the nozzle outlet section; when the needle valve is closed, the size of the bubbles formed in the nozzle also increases with the increase of the aperture. Length and taper decrease and increase. Finally, the effects of injection pressure, back pressure and needle valve seat speed on the transient flow field in the hole are analyzed. The results show that the average pressure in the nozzle chamber and the nozzle area decreases first and then tends to the ambient pressure during the closing process of the needle valve; when the needle valve closes, the velocity of the external backflow air increases with the increase of the injection pressure, the decrease of the backpressure and the increase of the needle valve seat speed at the outlet section of the nozzle; when the needle valve closes, the size of the bubbles formed in the nozzle also increases with the increase of the injection pressure. The back pressure decreases and the needle valve seat speed increases.
Key words: diesel engine; nozzle; needle valve closure process; transient flow field; bubble; numerical simulation
目录
目录
第 1 章 绪论 ....................................................................................................... 1
1.1 选题背景及课题来源............................................................................ 1
1.2 柴油机喷嘴孔内流动的研究概况........................................................ 3
1.2.1 柴油机喷嘴孔内流动的实验研究概况..................................... 3
1.2.2 柴油机喷嘴孔内流动的数值模拟研究概况............................. 4
1.3 本文研究内容........................................................................................ 7
第 2 章 针阀关闭过程运动网格的处理........................................................... 9
2.1 三套连续网格的生成............................................................................ 9
2.2 运动网格的处理.................................................................................. 13
2.2.1 两种连续网格运动的定义....................................................... 13
2.2.2 三步计算过程的概述............................................................... 16
第 3 章 针阀关闭过程柴油机喷嘴内两相流动模型..................................... 17
3.1 VOF 模型 .............................................................................................. 17
3.2 接触角模型........................................................................................... 18
3.3 foam-extend 中数值计算条件的设定.................................................. 21
3.3.1 多相流求解器 interDyMFoam 的算例文件结构 .................... 21
3.3.2 离散格式的设置....................................................................... 22
3.3.3 代数方程求解器及压力修正算法的设定............................... 23
3.4 计算模型的验证及分析...................................................................... 24
3.4.1 初始计算条件的设定............................................................... 24
3.4.2 网格依赖性分析....................................................................... 26
3.4.3 与实验结果的对比及分析....................................................... 27
第 4 章 喷孔结构对针阀关闭过程孔内瞬态流场的影响研究..................... 30
4.1 喷孔直径对针阀关闭过程孔内瞬态流场的影响.............................. 30
4.2 喷孔长度对针阀关闭过程孔内瞬态流场的影响.............................. 35
4.3 K 系数对针阀关闭过程孔内瞬态流场的影响 ................................... 39
4.4 本章小结.............................................................................................. 43
参考文献............................................................................................................. 45
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