Study On Influencing Factors Of Thermal Fatigue Strength Of Engine Exhaust

With the continuous strengthening of the engine,the heat transfer problem of engines become more and more prominent. Exhaust manifold is one of the key components of engine after-treatment system,and the working condition of this manifold is directly in relation to the reliability and durability of the engines and the thermodynamic properties and gas flow performance of it directly affects the engine power performance, fuel economy and emissions.The structure and the working environment of exhaust manifold are of extremely complication.In working condition, the high temperature exhaust gas makes the teperature of the manifold very high, and the temperature distribution is very uneven,resulting in generation of thermal stress and thermal deformation of the manifold.Owing to the periodic changes of working condition of the manifold, thermal fatigue crack emerges. And due to a great drop of the material performance in high temperature, at this time, the vibration response is also a potential research direction, this paper only studies on the free modal and constrained modal characteristics of the exhaust manifold and the results are compared and analyzed. In order to reduce the influences of the exhaust manifold thermal load and thermal fatigue failure as much as possible, study on heat transfer of engine exhaust manifold has already become an important research aspect of thermal load of engine parts.Therefore, To study the influence of each given factor on gas flow and the flow field characteristics and the mianifold heat transfer process and the calculation of fatigue safety coefficient of the exhaust manifold is of great significance in improving the structure of the exhaust mianifold and extend its employing life, and strengthening its working reliability.This paper firstly reviewed the domestic and foreign research status of the heat transfer process of engine exhaust manifolds, and established the transient thermal-structure coupling heat transfer model of the exhaust manifold, and then the constrained modal and free modal analysis was calculated by ABAQUS;Secondly, AVL-BOOST is applied to build the 1-D dynamics simulation model,implemented 1D-3D coupling with AVL-FIRE, obtained transient flow boundary conditions of the flow field, and analyzed the transient flow characteristics of the flow field. Thirdly,Temperature field and thermal stress distribution and the thermal deformation of the exhaust manifold are analyzed by using AB AQUS; Then influences of the different structure factors on the thermal behaviour of the exhaust manifold were studied and the results were analyzed contrastively;Finally, the analysis of safety factor of thermal fatigue Under different structure influencing factors were carried by using FE-SAFE software.Results show that:the maximum temperature of the exhaust manifold changed from 805.2℃to820.8℃ as the outlet length increased, the maximum temperature of the exhaust manifold changed from805.2℃ to 819.4℃ as the outlet diameter increasing,and the maximum temperature of the exhaust manifold dropped to 784.0℃ as the outlet transferred to the body symmetry position. Structure factors such as exhaust outlet position, outlet length and outlet diameter had a great influence on the exhaust manifold thermal load.The maximum thermal stress value was 326.21 MPa of the case of 21 mm outlet length and 5 mm outlet diameter outlet with the non-symmetry outlet position; the minimum thermal stress value was 174.1 MPa of the case of 19mm outlet length and 5mm outlet diameter with the non-symmetry outlet position;.The maximum safety factor value was 1.825 in the case of 19 mm outlet length and 5 mm outlet diameter outlet with the non-symmetry outlet position; The minimum safety factor value was 1.497 in the case of 21 mm outlet length and 5 mm outlet diameter outlet with the non-symmetry outlet position.