Study On Oscillating Flow And Heat Transfer Characteristics Of The Cooling Gallery Inside Pistons Of A Diesel Engine

With the development of high strength,compactness and light weight of the internal combustion engine,the piston is one of the worst important parts of the internal combustion engine working condition,heat load of the piston is getting higher and higher.Reliability problems become more and more significant that is caused by thermal fatigue.The heat dissipation mechanism of piston cooling gallery is a very efficient heat transfer method,which can effectively cope with the increasing heat load problem of the piston inside the internal combustion engine.However,the heat transfer mechanism of the gallery is not completely clear,the existing empirical formula can not meet the needs of the development of modern internal combustion engine,and the heat transfer performance of the gallery is restricted.In this paper,the oscillating flow and heat transfer of the oil-air two-phase flow inside the gallery are systematically studied by theoretical analysis,numerical simulation and experimental study.It could be provide some indication to meet the demand of oscillating heat transfer performance of the gallery in the future,and to reduce the heat load on the piston and realize the optimal design of the gallery.The main research contents are as follows:(1)Research on oscillating flow and heat transfer simulation models of diesel engine piston galleryTo simplify the oscillating flow and heat transfer simulation models of piston gallery.A jet flow virtual model is built to describe the two-phase flow of jet flow section.A relative displacement method for numerical modeling is put forward.Comparison with simulation method existed,this method can not only guarantee the same simulation accuracy as the complete simulation model,but also sharply improve the calculation efficiency.Likewise,this method shortens a lot of calculation time and provides a better accuracy.(2)Development of simulated experimental equipments of piston gallery oscillating flow and heat transfer performance and experimental researchIn order to deeply research the oscillating flow and heat transfer of the gas-liquid two-phase flow of piston gallery,its process is divided into pure oscillating flow process and heat transfer process to design the experimental platform method separately.Similarity models of oscillating flow and heat transfer are built separately according to the principle of similitude.Experimental equipment of piston gallery oscillating flow and heat transfer is developed independently.It is found that,the oil temperature has the greatest impact on the heat transfer performance of 65.3%.A method of conjugate heat transfer is used to prove the high prediction error of the relative displacement simulation model was less than 20 percent off,so that it can be used to study the piston gallery oscillating flow and heat transfer mechanism.(3)Research on piston gallery oscillating flow and heat transfer mechanismNumerical simulation method is employed to research the influence relationship and change rule of cyclical impulsive load,engine speed and oil injection parameters on the oscillating flow and heat transfer performance of the gas-liquid two-phase flow of piston gallery.The results reveal that the secondary motion influence the transient flow and transient heat transfer performance of two-phase flow a lot to the maximal amplitude of 24.9%.However,its influence on steady state performance is less than 1%.In addition,the injection angle,injection pressure and nozzle diameter influence the oscillating flow and heat transfer performance of piston gallery in a certain degree.(4)Research on the multi-objective optimization of the gallery structureSet the maximum thermal stress and temperature as the optimization objectives as well as the boundary of piston as the constrained boundary of gallery geometric structure optimization.Support vector machine and NSGA-Ⅱ multi-objective optimization algorithm are used to build the multi-objective optimization numerical model of gallery structure based on the gallery oscillating flow and heat transfer performance.The Pareto optimal solution is gained afterwards.Three design plans are chosen by clustering algorithm to make contrastive analysis with the origin gallery.The results show that the maximum thermal stress of the piston can be reduced,and the maximum thermal stress can be reduced by 8.7MPa,when the gallery is far away from the grooves and the sectional area of the inner cooling oil cavity is reduced.The gallery is close to the piston combustion chamber bottom and the piston chamber top,the oscillation heat transfer performance can be enhanced because of increasing corss section area of the gallery,reduces the piston the highest temperature by 10.2 ℃.