Thermal And Stress Analyses Of A Novel Coated Dual Pipe System With Microstructure Configuration

In recent years,the ultra-supercritical coal-fired power generation technology in China has been developing towards the design standard of large capacity,high parameter,high efficiency and low pollution.The application of advanced ultra-supercritical coal-fired power generation technology to raise the steam water temperature and pressure of ultra-supercritical unit to reach 630-700℃industry standard as soon as possible is a continuous goal sought by experts and scholars at home and abroad.But at present,the advanced heat-resistant material technology of ultra-supercritical units is still the main bottleneck restricting the design and construction of630-700℃industry standard ultra-supercritical fuel power plant.To overcome this bottleneck,advanced Thermal barrier coating（TBC）technology can be used,covering the surface of the heat-resistant material of the ultra-supercritical unit with the heat-protective material with lower thermal conductivity and higher stability,the industrial standard of ultra-supercritical units can be effectively improved by optimizing the design of multi-component materials and structures to explore the potential of materials and structures,this would be an ideal solution.In this paper,a new type of high-efficiency coated double-tube system used in 700℃advanced ultra-supercritical power plant is studied,based on the failure mechanism of thermal barrier coatings,the theory of materials,physics and mechanics is applied to analyze the failure process of thermal barrier coatings from two aspects:Theoretical Analysis and finite element simulation.The effects of thermal growth oxide（TGO）and Lanthanum zirconate/yttrium oxide partially stabilized Zirconia（LZO/YSZ）thermal barrier coatings on the temperature and stress distribution in a novel coating double-tube system were investigated.The main research contents and main research findings are summarized as follows:（1）Based on the steady heat conduction equation of the multi-layer cylinder model,the elastic stress and the elastic thermal stress equation of the thick-wall cylinder,the heat transfer model and the Thermal Stress Analytical Model of the double-tube system with five-layer coating are derived.With commercial ABAQUS software,the finite element model of a two-tube system with a five-layer structure and an ideal smooth interface coating was established,the correctness of the established finite element model is verified by the analytical model.The results show that the inner surface temperature of the main steam pipeline is about 590℃,and there is an obvious temperature gradient on both sides of the bi-ceramic thermal barrier coating,it shows that the thermal barrier coating with double ceramic structure can provide enough thermal insulation protection for the main steam pipeline.The radial stress is much less than the circumferential stress of the whole structure,and the maximum circumferential stress is mainly distributed in the region near TGO,therefore,the thermal growth oxide（TGO）may be the key factor for the spalling failure of thermal barrier coatings.（2）The effects of the ceramic layer thickness ratio and the key characteristic parameters on the temperature and stress distribution of the double-tube coating system with double-ceramic layer structure were studied.The results show that the thermal barrier coating with double ceramic layers provides sufficient thermal insulation protection for the Metal Matrix material of the new coating double tube system,and the influence of circumferential stress(S₃₃)on the structural integrity of the system is much greater than that of radial stress(S₁₁).When the LZO/YSZ thickness ratio is 300μm:500μm,the Mises stress of the coating system is lower,and the structural stability of the double-ceramic thermal barrier coating is improved.Appropriate cooling steam temperature,pressure,thermal conductivity of ceramic layer and Coefficient of thermal expansion can effectively improve the service life of dual-ceramic coating double-pipe system.（3）The effect of TGO thickness and interface morphology on the stress evolution of the double-tube system was studied.The results show that the thickness of TGO influences the distribution of Mises stress in the microstructure of TGO.With the increase of the thickness of TGO,the peak stress of TGO/BC interface increases continuously,and the peak stress at the trough decreases continuously,the stress at the crest is more complicated.With the increasing of the morphology period of TGO,the effect of the thickness of the outer ceramic layer on the Mises stress decreases or even disappears,the influence factors of the maximum Mises stress of the coating double-tube system from the strong to the weak are the morphology period of the TGO layer,the thickness of the outer ceramic layer and the interface period of the LZO/YSZ ceramic layer.At the TGO/BC interface,the increase of the amplitude makes the curvature of the interface wave become larger,which leads to the stress concentration at the TGO/BC interface,thus resulting in a greater peak stress of Mises.The Mises stress generated at these locations can easily lead to premature separation of the coating system,cause system failure.The maximum Mises stress of the coated double-tube system is dominated by the thickness of the outer ceramic layer under the condition that the two interfaces are far apart,and the maximum Mises stress of the coated double-tube system increases with the thickness of the outer ceramic layer,however,it does not change with the change of LZO/YSZ interface morphology.The ceramic layer（TC2）is too thin when the interface is close enough.The phase change of LZO/YSZ interface will affect the maximum Mises stress of the coating,which may be the interference of the two interfaces.