Numerical Simulation Of Cooling-water Cavitation Flow Of Cylinder Liner For A Diesel Engine

Cavitation erosion is one of major failure forms of diesel cylinder liners. Recently, cavitation erosion of diesel cylinder liners has become more serious with enhancing power, raising speed and decreasing engine weight. Cavitation erosion of cylinder liners is a key problem affecting reliability of diesels and lifetime of cylinder liners. The flow-field characteristics of cooling-water cavitation flow are closely related to cavitation erosion of cylinder liners. In this thesis, numerical simulation of cooling-water cavitation flow of cylinder liner for a diesel engine was carried to reveal the flow-field characteristics of water flow and their evolution process. The major factors affecting cavitation of water flow and cavitation erosion of cylinder liners were analyzed. The research is significant for understanding cavitation-erosion mechanism and improving cavitation-erosion resistance of cylinder liners.A mixture two-phase flow model and a full cavitation model by Singhal with dynamic mesh method were applied to establish a mathematical model of cooling-water cavitation flow of cylinder liners. The 2D and 3D geometry models of cooling-water channel for TC231101-87 diesel were established. The unsteady flow-field characteristics of cooling-water cavitation flow were numerically simulated. The effects of liner vibration on formation, development and collapse of bubbles were analyzed by cavitaion, micro-jet and bubble collapse theories. Evolution of the flow-field characteristics of cooling-water cavitation flow was revealed. The cavitation-erosion mechanism of cylinder liners was analyzed. The simulated results show that the flow-field characteristics of cooling-water cavitation flow, proximity to the liner, vary periodically in consistence with liner vibration. The flow-field characteristics of water flow, proximity to the liner, non-uniformly distribute along the liner. The pressure on the upper-middle wall zone seriously fluctuates, forming impulse pressure. The peak value of impulse pressure reaches 335kPa, which concludes that serious cavitation erosion occurs on this wall zone. The considerable pressure fluctuation of this local flow, proximity to this wall zone, leads to variation of vapor fraction between 5.4% and 0.4%, which shows that cavitation of this local flow is serious. The relation of liner pressure and liner-vibration velocity was quantitatively analyzed. It is revealed that liner pressure and liner-vibration velocity have a linear relation as liner-vibration velocity is small. For the formation process of impulse pressure, they have an exponential relation.Compared with the cylinder-liner cavitation-erosion statistics results and the experimental results, the simulated results are validated. The simulated results can reveal the evolution process of the flow-field characteristics of cooling-water cavitation flow.The effects of working condition parameters of cylinder liner and cooling system on the flow-field characteristics of water flow and cylinder-liner cavitation erosion were quantitatively studied. As the liner-vibration frequency and amplitude is increased by 30%, the linear relation between liner pressure and liner-vibration velocity is not varied. But the effects of impulse pressure of water flow on the liner are dramatically enhanced. The mass fraction of non-condensed gas has significant effects on cavitation of water flow and pressure fluctuation on the liner. As the mass fraction of non-condensed gas reaches to 1.5x10-4, cavitation of water flow is enhanced and a large volume fraction of gas evidently restrains pressure fluctuation on the liner. As the mass fraction of non-condensed gas further rises, the pressure fluctuation is prone to be stable on the liner and the vapor fraction of water flow is remarkably increased. The peak value of impulse pressure on the maximum deformed wall zone is high at water temperature 50℃-70℃. At water temperature of 90℃, the pressure fluctuation is obviously weakened and the peak value of vapor fraction reaches to 9% on the same wall zone. The inlet-flow velocity and direction have a little influence on the flow-field characteristics.The effects of different turbulent models and near-wall functions on simulation of cooling-water cavitation flow were studied. The pressure and vapor fraction on the maximum deformed wall zone were compared. The cavity location, shape and size of water flow, proximity to the liner, were analyzed.