Study On The Quench Prediction And Thermoelastic Coupling Behavior Of Superconducting Materials And Structures

Due to the excellent superconducting properties such as zero resistance effect and Meissner effect,superconducting materials have extensive and important prospects in the application fields of modern energy,transportation,medical technology,communication and aerospace,etc.In the extreme and complex operating conditions of extra low temperature,high magnetic field and large carrying current,quenches would inevitably occur in superconducting materials and structures caused by various small perturbations(such as thermal disturbances,mechanical and electromagnetic,etc.),which are serious threats to the operating stability and security of superconducting electromagnetic systems.At present,the quench of superconductors is still one of the open and challenging scientific problems,which also relates to the function realization and operating security of Tokamak large superconducting magnets in International Thermonuclear Experimental Reactor(ITER)program.The normal zone propagation velocity(NZPV)of high temperature superconducting(HTS)materials is lower than their low temperature counterparts for 2?3 orders of magnitude due to the higher heat capacity and critical temperature,which causes great difficulties for the quench detection of HTS structures.Once the occurrence of quench could not be detected timely and effectively,thermal runaway will lead to electrical and mechanical failure of HTSs.Thus,quench and effective detection methods are key issues urgently need to be addressed for the application of HTS materials.The existing experiments have shown that the strains also occur abrupt changes during the quench of superconducting magnets besides the change of temperatures and voltages,which could be used to detect the quench of superconductors.However,the related quench mechanisms have not been well revealed.In this thesis,the thermoelastic coupling quench mechanisms are theoretically studied and explored for superconducting composite materials and structures.The quench behaviors as well as the transient thermoelastic responses are quantitatively simulated for LTS and HTS wires and HTS stacks and coils.The mechanisms of abrupt changes of strains and strain rates are revealed and the internal relations between mechanical changes and temperatures are investigated in depth during a quench.Firstly,a one dimensional thermoelastic coupling quench model is proposed based on the Lord-Shulman(L-S)generalized thermoelasticity and current shearing model.The quench evolutions and corresponding thermoelastic responses are obtained numerically and the influences of thermal wave effect on quench as well as its thermoelastic responses are analyzed.The thermoelastic strain and strain rate occur abrupt changes during quenching process and the peak points of strain rate precisely correspond to the critical value of temperature.The quantitative results reveal the internal mechanism of strain and strain rate jumping changes during a quench and give a reasonable explanation for existing experimental phenomena,which also provide a theoretical support to the strain based quench detecting method.According to the prediction of quench behaviors for Nb Ti/Cu LTS wires,the quench occurs earlier and the NZPV decreases with the increasing thermal relaxation time.For REBCO HTS tapes,the numerical results of temperature evolutions and NZPVs are in good agreement with the experimental ones.Obvious inflection points corresponding to the critical temperature can also be observed on the evolution curves of strain rate for a REBCO tape,which relate to the temperature dependence property of the electrical resistivity of cooper in HTS tapes.Thus,the electrical properties of metal matrix have significant influences on the quench characteristics.Furthermore,by considering the influence of strain on critical current of a HTS tape,a one dimensional thermoelastic quench model is developed for HTS tape with pre-strains.The influence of pre-tension strains on quench occurrence,propagation and corresponding thermoelastic responses are analyzed for a pre-tensioned Bi2223/Ag HTS composite tape.The quench occurs earlier and the normal zone propagation is enhanced under higher pre-strains,the influences of which get more obvious when the pre-tensioned strain is beyond the critical one,showing some control effects from prestrains on the quench of HTS materials.During a quench,the strains and strain rates occur observable decreasing behaviors before the quenches arrive at location points far away the heater,and obvious extreme points appear on both the strain and stain rate evolution curves which could be used to detect a quench earlier and give a quench prediction with early warning.By analyzing the thermoelastic behaviors during a quench protection process,the feasibilities and effective intervals of quench detecting criteria based on the extreme points are discussed further.The studies that controlling and detecting the quench by applying pre-strains may provide a new solution to the quench detection of HTS in engineering applications.Thirdly,two dimensional layered and homogenized thermoelastic coupling quench models are built to analyze the quench and thermoelastic behaviors of HTS composite stacks,which are widely used in large superconducting magnets.The decreasing behaviors and feature points are also observed on evolution curves of strain and strain rate which means that the quench detecting criteria based on strain and strain rate could be also extended to the quench detection of HTS structures.The influence of physical properties of insulating and impregnating materials on the quench propagations and mechanical responses are investigated in depth.The quench propagation shows obvious anisotropy caused by the low thermal conductivity of insulation layer and the longitudinal NZPVs are faster than the transverse ones for an order of magnitude.Compared to the homogenized model,the layered model can give the discontinuities of stress between different layers caused by the transient temperature changes and more detail features of stress in HTS stacks during a quench.In addition,the local longitudinal strain could be decreased by increasing the elastic modulus and thickness of impregnating materials.Finally,in order to investigate the influence of magnetic fields on quenches as well as the mechanical behaviors in a HTS insulted coil,a two dimensional axisymmetric magneto-thermo-mechanical coupling quench model are developed.The quench and corresponding multi-field behaviors are analyzed under different background magnetic fields.Both the evolution curves of radial and axial strains exhibit decreasing behaviors before the local temperature starts to rising and rise with the arrive of normal zone at probe points far away the heater.Thus a quench might be detected by monitoring the strain variations and the extreme points on strain curves could be the detecting criterion for HTS insulted coil.The quench occurs more easily and the NZPV increases with increasing magnetic field intensity.Under the influence of Lorentz force and transient temperature rising,the axial and hoop stresses in normal zone will change from tension stresses to compressive ones during the quenching process.