发动机冷却水套穴蚀机理分析与试验

Analysis and experiment on cavitation mechanism in cooling water jacket of diesel engine

为研究柴油机冷却水套内空化现象的产生机理,该文使用计算流体力学(CFD)方法研究施加壁面振动的不同入口流速、不同流场温度时冷却水套内部流体的流动特性及空化特性,同时设计并搭建可施加壁面振动的可视化空化试验台,对计算结果进行了验证。研究结果表明:空化现象主要发生在圆弧壁最小间隙位置,并在下游区域发展壮大;冷却水入口流速的增加会使得空化现象略有加强,但并不明显;当冷却水温为50℃时,空化现象较强,当水温逐渐升高时,空化现象反而减弱;当圆弧壁面有振动时,空穴现象明显加强,并且其产生的空化效果明显强于冷却水温及入口流速等因素的变化所产生的空化波动。此研究的结果将有助于控制发动机冷却水套中空化现象的发生,并降低冷却水套穴蚀的发生风险。

Light weight and high power become the trend of the development of diesel engine.At the same time,the liner cavitation erosion of engine cylinder becomes one of the important restrictions of engine reliability and life.A lot of research has been made and the results show that cooling water jacket cavitation erosion theories widely accepted are：A high frequency vibration of cooling water jacket leads to the cavitation and the cavitation bubbles breaking produces a shock wave and micro-jet,which have the mechanical action to cooling water jacket and lead to the occurrence of cavitation erosion.Therefore,the study of the flow characteristics of the cooling water jacket near the vibrating wall is an effective way to understand the spatial and temporal distribution of cavitation.In this paper,the computational fluid dynamics（CFD）simulation method was used to investigate the forming mechanism of cavitation in engine cooling jacket.By analyzing the cooling water jacket of the diesel engine,taking the part of cooling water jacket in the minimum space of the flow channel near the vibration wall as the research object,a three-dimentional model of diesel cooling water jacket was built.Then according to the calculation and analysis of the vibration wall components’single knock experiment,the moving mesh was set.Then the characteristics of flow and cavitation under various inlet velocity and fluid temperature with and without wall vibration were simulated.The cavitation characteristics were compared and analyzed under the conditions of different inlet velocity and different flow field temperature with the influence of cylinder wall vibration,and the numerical theory of cooling water jacket cavitation erosion was improved,which also guided the cooling water jacket cavitation erosion experiment of the vibration.According to the simulation results,a wall vibration cavitation erosion visualization bench was designed and built,the related experimental verification was proceeded in the visual experiment platform,and the credibility of the simulation calculation was confirmed.The simulation results were validated by the experiments on this optical rig.The outcome of the study indicated that cavitation occurred in the minimum space of the flow channel,and grew up in the downstream.Higher velocity of inlet flow（from 2 000to 3 000 liters per hour）caused stronger cavitation near the vibration wall of the cooling water flow field,but it was not significant,so the significant change was not observed in the cavitation images under the condition of different inlet flow velocity.On the contrary,higher temperature of inlet flow（above 50℃）caused weaker cavitation,and the strongest cavitation occurred at the temperature of 50℃.An obvious cavitation phenomenon appeared when the wall which fluid flowed by was vibrated in a very high frequency,and this change of cavitation was much stronger than those caused by various inlet flow velocity and fluid temperature.So the author thought that cooling water jacket vibration was the more important factor than cooling water flow velocity and temperature that caused cavitation.This study will be helpful in controlling the occurrence of cavitation in cooling water jacket and lowering the cavitation erosion risk of cooling water jacket.