Structure Optimization Of Cooling Gallery Based On Enhanced Heat Transfer

It is well known that the piston plays a vital role in the diesel engine,which directly connect the combustion chamber.It needs to endure the thermal loads caused by the high temperature and great thermal gradient in combustion,and also needs to withhold the mechanical loads caused by the gas pressure and reciprocating motion.Therefore,the reliability problem caused by the thermal fatigue damage of the piston has become an increasingly prominent problem.Cooling gallery is one of the piston cooling methods widely used,which can effectively reduce the thermal fatigue of the piston.However,the heat transfer performance of the cooling gallery is affected by the structure,position,speed and other factors.Therefore,the cooling design of the piston cooling gallery should be paid attention.Whether the design meets the cooling needs,the cooling gallery should be deeply explored.In this paper,the accuracy of the simulation analysis is verified by the experimental results by using the visualization experimental bench of oscillating flow of the cooling gallery.The difference of the oscillating flow characteristics of the cooling medium in the rectangular transparent cavity under different engine speeds and different filling rates is analyzed.Then,according to the cooling needs of different characteristic zones of the piston,the multi-objective controllable thermal state optimization is carried out.Finally,considering the influence of intake and exhaust on the temperature field distribution of the piston,based on the optimization results of the crosssection shape of the cooling gallery,three kinds of variable cross section cooling galley are designed.The difference of heat transfer effect between variable cross-section cooling galley and common shape cooling gallery is compared and analyzed.The main research contents are as follows:(1)visual experimental analysis and numerical analysis were carried out on the oscillating flow characteristics of the two-phase flow in the rectangular transparent cavity by utilizing the rectangular transparent cavity model instead of the internal cooling oil cavity model.The oscillating flow pattern of the two-phase flow was obtained.The influence of engine speed and filling rate on the oscillating flow pattern of two-phase flow is studied in the rectangular transparent cavity.The results show that the flow pattern of two-phase flow oscillation of the simulation results and the experimental results in rectangular transparent cavity is basically the same,which indicates that the numerical simulation results are credible.At low engine speeds,the two-phase flow has a poor oscillation effect,which affects the heat transfer effect;while at high speeds,the gas-liquid two-phase flow directly,continuously and quickly impacts the top and bottom surfaces of the cavity,increasing heat exchange.Too high and too low filling rate will affect the oscillating heat transfer performance of two-phase flow.A filling rate of50% can obtain the best oscillating heat transfer performance.(2)A controllable thermal state design method is proposed according to the cooling needs of local zones of the piston.Local thermal state of piston cooling gallery is described by multi-objective optimization technique.The research results show that the regression model determination coefficients of piston temperature,temperature gradient and the temperature of the first ring zone are all greater than 0.95,indicating that the prediction results of the SVR(Support Vector Machine for Regression)model are in good agreement with the simulation results.The SVR model can quickly obtain the cooling performance of each sample point to save calculation cost.The temperature of the first ring zone is effectively controlled,the maximum relative reduction is 4.48% compared to original model.In three distinctive solutions,Solution B is a compromise between the solution A and the solution C,which is a design solution at which three optimization objectives are sufficiently satisfied.(3)Simulation models of the variable cross-section cooling gallery are established by using the relative displacement method.The differences of the oscillating flow characteristics and heat transfer characteristics of the common cooling gallery and the three variable cross-section cooling galleries are studied.The research results show that: comparing the wall instantaneous average heat transfer coefficients of the cooling galleries,the heat transfer performance of the three variable cross-section cooling galleries is better than the common cooling gallery.And it can meet the more stringent cooling requirements of the piston high temperature exhaust side.For the different areas cooling needs of the piston,a new design idea of the cooling gallery is provided.