Study On Thermal Load And Vibration Performance Of Exhaust Manifold Of A Heavy Diesel Engine

With the exposures of the consumption of non-renewable energy such as petroleum and environmental pollution,the requirements of national emission regulations have become increasingly stringent.For traditional fuel vehicles,the advantages and disadvantages of the exhaust system have become the key to solving the above problem,the exhaust manifold has always been the core of the exhaust system,it is not only subjected to the impact of high-temperature exhaust gas,but also affected by engine vibration,under the action of this alternating load,it is easy to cause the exhaust manifold becoming failure,which affects the performance of the engine exhaust system.Therefore,it is especially important to optimize the design of the exhaust manifold with superior performance and reasonable structure by studying the thermal load and vibration performance of the exhaust manifold,which also has certain theoretical and practical significance for improving the power,economy and emissions of the engine.This paper first introduces the properties of engine exhaust manifold and common failure modes,including the structure and processing flow of exhaust manifold,and introduces the current research status of exhaust manifolds by experts and scholars at home and abroad.Secondly,the paper takes a heavy-duty 3.2L displacement diesel engine exhaust manifold model as the initial research object,according to the experimentally measured boundary conditions,the flow state of the internal flow field of the exhaust manifold is studied.Based on the theory of heat-flow-solid multi-field coupling,the temperature and heat flux of the inner and outer surfaces of the exhaust manifold,the temperature field,thermal stress strain and deformation of different parts are calculated.Then,the fluid model of exhaust manifold is optimized by central composite experiment design based on response surface optimization method,and the solid model of exhaust manifold is optimized based on literature experience,the optimized exhaust manifold is compared with the original model before optimization.Finally,the vibration characteristics of exhaust manifold before and after optimization design are analyzed,and the resonance between the natural frequency of modal analysis and the excitation frequency of engine is judged.The results show that in the analysis of internal flow field,there are some phenomena such as excessive gas velocity and eddy current gathering in the manifold manifold manifold branch 1,3 and 4,which result in too low flow uniformity index and too high pressure loss non-uniformity in the manifold,it is necessary to optimize the bending radius of the manifoldfluid model branch 1,3 and 4.When the Thermal-Fluid-solid coupling is applied to the exhaust manifold,it is found that the temperature field distribution of the initial model is too high in the local area,the thermal stress and thermal deformation are too large at the bending parts of branch pipes 1,3 and 4,and at the intersection of exhaust outlet and exhaust pipe,it is easy to damage the exhaust manifold,so the exhaust manifold needs to be optimized.Compared with the initial model,the flow uniformity index of the optimized exhaust manifold model is increased by 6 percentage points,and the flow field is more uniform,the results of temperature field,heat flux and thermal stress are obviously improved,which proves the rationality of the optimization improvement measures.At the same time,the cold and hot modal analysis results show that the optimized exhaust manifold model's frequencies are much higher than those calculated by the formula before optimization,which will not cause resonance phenomenon,and the rule that temperature will reduce the modal analysis frequency of exhaust manifold.