柴油机进气-压缩过程的三维瞬态数值模拟研究

3D Transient Numerical Simulation for Intake-compression Processes of Diesel Engine

应用动网格技术,对某柴油机在进气和压缩过程中气道-气门-气缸内的瞬态流场进行了数值模拟。模拟结果表明:在进气过程初期,缸内气流运动是紊乱的;随后,逐渐形成多个小涡流;到进气过程中后期,单一大尺度进气涡流形成并逐步得到发展、稳定和加强。而压缩过程初期,在进气涡流继续保持的同时,压缩涡流形成;到压缩过程后期,挤压涡流出现,气流以螺旋方式从气缸进入燃烧室,同时缸内的滚流逐渐加强。压缩过程中缸内气体的平均湍动能随曲轴转角的增大呈先减小后增大再减小的趋势。压缩上止点附近气缸边缘的湍动能很弱,而气缸中部挤流区域的湍动能则相对很强。水平方向的涡流、轴线方向的滚流以及湍动能的这种分布特性都有利于燃油的雾化以及与空气的混合。

Dynamic simulation is carried out to study the transient intake and in-cylinder flow generated during intakecompression processes using CFD code Fire. Well convergent results show formation and development of swirl, squish, tumble and turbulence. In-cylinder flow generated during intake process is disorganized at the beginning, and then it develops into a large number of small-scale swirls, which develop into a single stable large-scale swift at the later half of intake process. At the beginning of compression process, intake swirl is maintained and compression swift is generated, while at the end squish comes into being when helical flow moves from cylinder to combustion chamber. At the same time, in-cylinder tumble is strengthened gradually. With the increase of crank angle, the mean kinetic energy of in-cylinder turbulence during compression process decreases at first, then increases and finally decreases again. At the end of compression process, the turbulent kinetic energy of flow around the edge of cylinder is weak, while that of squish in the middle is relatively strong. Horizontal swirl, axial tumble and the distribution of turbulent kinetic energy are very important for fuel atomization and mixture formation.