柴油机活塞二阶运动对内冷油腔机油振荡流动与传热的影响

柴油机活塞的二阶运动不仅影响活塞侧击力、摩擦磨损、机油耗和漏气量,而且还对活塞内冷油腔内机油的振荡流动与传热性能产生影响。在活塞动力学与运动学分析的基础上,结合活塞内冷油腔内的振荡传热性能模拟试验结果,采用计算流体力学仿真方法,建立了包含往复运动与二阶运动的计算流体力学仿真模型,研究了活塞二阶运动对内冷油腔内机油的振荡流动与传热性能的影响规律。研究结果发现,二阶运动的径向运动主要影响内冷油腔中机油的振荡流动,偏摆运动主要影响内冷油腔的瞬时换热性能。二阶运动使内冷油腔的瞬时充油率降低,循环平均降低4.6%。对油腔壁面的瞬时换热性能影响很大,最大的变化幅值为24.9%。对于整个换热过程,虽然充油率降低,但平均换热系数变化不大。因此,二阶运动对内冷油腔综合换热性能的影响可以忽略不计。该研究可为耐高温高强度铝合金活塞的设计提供理论和技术参考。

Impact of piston secondary motion on oscillating flow and heat transfer of oil inside piston cooling gallery of diesel engine

Abstract: Because of the stringent emission and fuel economy standards, automotive engineers are forced to develop engines with much higher power densities. Pressure and temperature levels within a modern internal combustion engine cylinder have been pushing to the limits of traditional materials and design. Piston cooling is a critical measure for achieving designed engine performance especially for heavy-duty internal combustion engines. The various piston cooling gallery structures have been widely applied in piston design to provide high cooling efficiency. In previous research of achieving high cooling efficiency of the piston gallery, only the reciprocating motion of piston has been considered and investigated fully. However, the secondary motion is another important quantity due to the inevitable gap between piston and cylinder liner. For its tiny displacement, the impact of piston secondary motion on oscillating flow and heat transfer of cooling oil inside the piston gallery has not been investigated or recognized. In order to obtain the secondary motion, a piston dynamics model was established in this study. And then, a simulation model named model-B was established with the computational fluid dynamics simulation method and a relative displacement method with a consideration of the reciprocating motion as well as the secondary motion. The piston secondary motion was directly applying on the boundary of the piston gallery. The relative displacement method allows the cooling gallery to be treated as a rigid body, and the original constant boundary conditions could be translated into varying conditions that change as a function of engine crank angle. As a contrasting model, another model named model-A was established without the secondary motion in order to find out the degree of impact. In order to validate the accuracy of the computational fluid dynamics simulation model-A, a recognized test of a cube cavity was borrowed to contrast with the simulation results. The simulation results were difference with the experiment values by ±15 %. In other words, the computational fluid dynamics simulation model has certain ability for predicting the rules of the oscillating flow and the heat transfer processes. The result of this study showed that the secondary motion could be regard as an impact load for the gallery with a radial acceleration of 2,450 m/s2. The interface between the oil phase and the air phase was more unstable due to radial displacement. The oil flow inside the fluid mixing zone was more disorder. The results showed that the radial displacement of the secondary motion had the main influence on the oscillating flow of the cooling oil inside the gallery. Two dimensionless numbers were used to characterize the oscillating flow and heat transfer performance of the cooling oil inside the piston gallery. The Reynolds number indicated the flow state of fluid, and the Nusselt number indicated the convective heat transfer performance. The tilting angle of the secondary motion had the main influence on the instantaneous convention heat transfer performance of the piston gallery. The instantaneous oil charge rate of the gallery was reduced by the secondary motion, and the cycled averaged oil charge rate was reduced by 4.6%. The instantaneous convention heat transfer performance was affected by the secondary motion, and the biggest change ratio was 24.9%, which appeared in exterior region. For the whole heat transfer process, although the oil charge rate was reduced by the effect of the secondary motion, the cycle averaged heat transfer coefficient changed a little. The heat transfer efficiency of the cooling oil inside the gallery was improved, but the impact of the secondary motion on the comprehensive heat transfer performance of the gallery can be neglected.