Experiment And Numerical Study On The Failure Behavior Of Steam-side Oxide Scale On High Temperature Tubes

The steam-side oxide scale of high temperature components is one interdisciplinary problem.At present,the oxide studying focuses on the oxide growing behavior and the mechanics property,while the oxide failure behavior still needs more reseach attention.This paper studies on the superheater tubes of one 600 MW supercritical once-through boiler.The experimental study and numerical simulation is adopted to investigate the oxide failure behavior of high temperature components systematically,which includes the following five fields.(1)An experimental method for detecting the failure type of oxide scale is proposed,which can be used to measure the critical strain and calculate the fracture toughness.In this paper,the acoustic emission(AE)is adopted to identify the oxide failure types.Firstly,the AE count,peak frequency and AE energy is used to divide the failure process.The metalloscope observes the oxide failure type.The time-frequency information of AE signals is extracted by the wavelet analysis,and developes the connection between the AE signal and oxide failure.The results of signal analysis can be explained by the strain energy of oxide scale.The result shows that the through-thickness cracks are induced at first,then the horizontal cracks are formed,finally the oxide spallation appeares.(2)The previous signal processing method cannot be used to detecte the oxide failure behavior at high temperature.The high temperature oxide behavir experiment is proposed.Firstly,the high temperature experimental equipment is applied to conducting the room temperature experiment,and developes the identification method for the high temperature experimental equipment.Then,this method is used to identify the oxide failure types at high temperature.Through comparing the results of room temperature and high tmeprature test,the influence of temperature on oxide failure behavior is studied.At last,the ANSYS software is used to develop the numerical model of the tensile sample,and studied the influence of temperature on oxide spallation.The results show that material property of substrate is the key influence of oxide failure behavior at high temperature.(3)Considering the influence of creep and physical defect,the ANSYS software is applied to developing the elastic-plastic model.The crack load and critical parameter of oxide failure is calculated to analysis the oxide failure behavior during the shut down process.Based on the oxide stress distribution,the physical defect is implemented in the critical area,and assess the occurrence of oxide failure as a function of physical defect length,oxide thickness and cooling rate.Interestingly,the influence of physical defect length on the through-scale crack is larger than the interface crack,while the cooling rate and oxide thickness has a more significant effect on the interface crack.(4)Considering the thermal deviation,convection heat transfer and steam-side oxide scale,the creep rupture life of the whole superheater tubes is predicted.The 3-D CFD gas model is applied to calculating flow and heat transfer of outside high temperature gas,and the 1-D tube and steam analytical model is applied to calculate the temperature profiles and continuous growing steam-side oxide thickness.Based on the calculated temperature profile,the creep rupture life of tube is assessed in this paper.The results show that oxide growing rate decrease with time,and the Creep damage rate increases with time.(5)Combining the discretization equation and CFD model,the dynamical stress and temperature model of superheater tubes is developed.This model is applied to calculating the dynamical stress field of oxide scale as the changing inlet steam temperature.The oxide thickness is less than the tube length,and the response time of steam and gas temperature is greater than the tube temperature.The C#software is used to develop the dynamic temperature profiles of steam and tube panels,and the FLUENT software is applied to developing the 3-D CFD model of high temperature gas.The calculated results could be used to detect the critical area of oxide failure,the oxide scale of T23 tubes at the inlet of superheater tubes is susceptible to failure,while the oxide scale of T91 tubes is susceptible to failure at outlet of superheater tubes.