可变滚流GDI发动机气流循环变动的大涡模拟

Multi-Cycle Large Eddy Simulation of Cyclic Variations in a GDI Engine with Variable Tumble Ratios

对一台可变滚流比直喷汽油机(GDI)缸内冷态湍流流场采用大涡模拟方法进行了多周期数值模拟,并通过PIV结果进行试验验证.在此基础上,引入了本征正交分解(POD)方法,基于相关系数(RI)将瞬时流场分解为平均流场、拟序流场及湍流流场,并对不同滚流强度下各分流场的循环变动特性进行了研究.结果表明:滚流翻板关闭将显著提高缸内滚流强度;提升转速及滚流比会使缸内气体质量明显增加,同时使缸内充量循环变动显著降低;流场分解所获得的分流场可分别反映流场不同尺度流动的特性.各分流场所占能比例不同,且随曲轴转角变化,反映出各流场之间能量的相互传递,其中平均流场占能比最高,在70%以上.POD分解所得到的各分流场的时间系数均方差能够量化其循环变动,拟序流场的循环变动要远远大于其他分流场的循环变动,提升发动机转速使循环变动略有降低,但增大滚流比能够明显降低循环变动.

Multi-cycle large-eddy simulation(LES)was performed on a GDI engine to investigate the incylinder turbulent flow,and the simulation results were experimentally verified through the PIV results.The method of proper orthogonal decomposition(POD)was introduced to analyze the in-cylinder flow.Based on the relation coefficient(RI),the instantaneous flow fields were decomposed into the mean,coherent and turbulent parts. And the cyclic variations of each part were studied under different tumble intensity conditions. Results show that closing the inlet flap significantly increases the in-cylinder tumble intensity,yet the effect of increased engine speed on tumble intensity is not obvious. In addition,the enhancement of speed and tumble ratio not only improves the intake charge remarkably,but also decreases the CCV of intake charge. Each part of the flow field obtained by the decomposition respectively reflects the characteristics of the flow field in different scales,and takes different percentages of total kinetic energy which varies with crank angle. The above process reflects the mutual transfer of energy between each partial flow field,and the average part owns the highest energy which is more than 70% of the total kinetic energy. Variance of the time coefficients generated by POD quantifies the cyclic variation of each flow field part. In detail,the cyclic variations of the coherent part is far greater than other parts,and it is slightly lower with increasing engine speed,but obviously lower with the increased tumble ratio.