| With the increasing work intension of engines, the components of engine are bearing higherthermal and mechanical loads. The engine exhaust manifold works with alternating shock ofheat and cold, and also bears additional inertia force due to the engine vibration. Consider tothe bad working condition, if any local area of the exhaust manifold is seriously heated, or theheat is unable to be dissipated in time, local deformation or even break of the exhaustmanifold will be caused, as a result that the engine is unable to work normally. As the flowand thermodynamic properties of the exhaust system directly influence the engine power,economy and emission, a study on the exhaust system is necessary.The solution method to the thermal load and intensity is usually a key technology toimprove the exhaust system, and the thermal load and intensity have a directly effect on thereliability and durability of the engine’s exhaust system. The previous studies mainly focusedon the thermal stress calculation and preliminary optimization for the entire exhaust manifold,but the relationship between the exhaust manifold performance and a certain structurechanges was never analyzed detailed. This paper mainly focuses on the exhaust manifold of aturbocharged gasoline engine, and based on fluid-solid coupling, a thermal analysis method isused to solve the problem of fatigue failure about the exhaust manifold which happens oftenin the thermal shock test. As a result, the influence rules of different structures on the flowfield and thermal stress of the exhaust manifold is indicated, and an effective method forsimulation study on the exhaust manifold is proposed.Main research contents and conclusions in this paper：1. The fluid-solid coupling simulation model of exhaust manifold is established.Firstly,calculating the performance parameters of the engine using the one-dimensional simulationsoftware GT-POWER based on the design parameters provided by the manufacturer, gettingthe inlet mass flow and temperature and outlet mass flow and temperature; use these results as CFD boundary conditions, calculating the fluid temperature and heat exchange coefficient ofboth the internal and external surface of the manifold using STARCCM+, then use thetemperature and heat exchange coefficient as thermal boundary conditions of the manifold,mapping these conditions to the finite element model of the manifold. Calculating thetemperature field of the manifold by means of the finite element software ABAQUS, andfinally use the temperature field as thermal load of the manifold, calculating the heat stressdistribution and thermal deformation. compare the calculation results with the actual cracks ofthe manifold, found that cracks are more likely formed in the area where exists strong andconcentrated thermal stress, indicating that this model is accurate, and able to offer certainpredication of fatigue failure of the exhaust manifold.2. To verify the temperature field of fluid-Solid coupling simulation model throughexperimental. Exhaust manifold temperature platform is set up to help us to havecomprehensive view of temperature when exhaust manifold is working and to provide testbasis. We use thermal infrared imager to monitor the temperature of exhaust manifold whenmotor is full-load working (different speed). Full-load working, speed4500r/min, the highesttemperature is658℃.We get the temperature of gas and ektexine through thermocouplearrangement. The temperature variation trend is the same with thermal infrared imager’result.We compared the test result with the simulation result and found there was no difference intemperature range (biggest error is4.3%, which is acceptable), which can prove Fluid-SolidCoupling simulation temperature field test working well and we can use it in simulationresearch and manifold design.3. Improving the overall structure of the exhaust manifold in order to solve the problem offatigue fracture. Using Fluid-Solid coupling simulation model established to analyze the flowfield and thermal stress of the original manifold structure. It was found that the flowuniformity and liquidity are poor, leading fatigue fracture as a result of the uneven distributionof thermal stress. In such cases, the original structure was improved three times, and the threeimproved structure of the flow field and thermal stress were all analyzed. Although, theresearch results of improve the structure1and improve the structure2in some improvementon the flow field and the distribution of thermal stress, but the improvement is not apparent,there are still some cracks caused by the excess thermal stress; This requires to do further analysis of structural improvements which based on the basic structure of the exhaustmanifold. The flow uniformity through the flow field, pressure loss and pressure lossunevenness of the improve the structure3calculated are best among the original model andthe optimized structure1and2; thermal stress and deformation on the solid outer region arealso significantly better than the previous three models; and fatigue fracture problem does notoccur in the bench test, indicating that a scheme with four-to-two-to-one meets the needs ofthe engine’s normal operating, the effect of structural improvement is significant.4. Research on the influence laws of the local structure changed on the exhaust manifold.On basis of the research of integrated model,Analyse these4kinds of local structurerespectively which may impact flow field and thermal stress of the exhaust manifold.Theselocal structures are bending radiusof single branch pipe,intersecting angle of cross branchpipe,wall thickness of single branch pipe and groove depth. And then conclude the influencelaw of each single factor.(1) Bending radius of52mm,57mm,62mm,67mm and72mmare selected to researchand analyse about these five local branch pipes. It is concluded that pressure loss, temperatureand thermal stress increase with the increasing bending radius.(2) A research and analysis has been done about five local branch pipes of whichintersecting angles are respectively,57mm,62mm,67mm and72mm. Pressure loss of localbranch pipes increase with the increasing intersecting angles. However thermal stress does notchange along with variation of the intersecting angles. But it concludes that thermal stress isquite small when the intersecting angles are50.5°and57.4°.(3) Research on the local model with different thickness of exhaust manifold wall andgroove depth ofwall. The selected wall thickness are3mm、2.5mm、2mm、1.5mmand1mm.Meanwhile the selected groove depth are respectively,1mm、1.5mm、2mm、2.5mmand3mm.It draws a conclusion that maximum thermal stress and the maximum difference ofthermal stress are both gradually increased with the decreasing wall thickness or increasinggroove depth. Comparing the two situations.,it can be found thatthermal stress and itsmaximum difference’s value of fluted model is greater than the model thickness changedwhen the value of wall thickness is greater than groove depth’s. However the outer wallthermal stress’ maximum value of the model thickness changed is the only one which greater than the model thickness changed,and the rest of the parameters of fluted model are basicallygreater than the model thickness changed when the value of wall thickness is less than groovedepth’s.So it is much better to make wall thickness large or groove depth smaller as far aspossible when designing the exhaust manifold. Try to choose the form of smooth inner wall, iftwo shapes are both available.5. The multi-factor optimal design of exhaust manifold outline based on the responsesurface method. In view of pre-research on the hole and the part of exhaust manifold; basedon the optimal design method developing from response surface method; taking the radius ofintersecting hook face rolling ball of exit, branch pipe and pressure stabilizing cavity, asdesign parameter; applying steady state CFD flow field analysis and Design-Expert softwareto get the influence regularity of design parameter. Finally summering a preferable designscheme. After optimization, there are different level increase of two indexes and remarkableimprovement of liquidity and flow uniformity. |
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