Application Of Fluorescent Imaging Technique In Monitoring Thermal Characteristics Of Superconducting Tapes

The high temperature superconductor tapes represented by YBa₂Cu₃O_7-x(YBCO)have wide application prospects in the fields of power transmission,information communication and military defense,due to their advantages of high transition temperature,high current carrying capacity,high irreversible field and wide range of applicable temperature.However,superconducting devices usually run in extreme environments such as extremely low temperature,high current,strong magnetic field,high vacuum,etc.,which are easy to cause the local temperature to rise suddenly due to the disturbance of environmental changes,and cause the superconducting materials to change from the superconducting state to the normal state(short for quench phenomenon)through heat propagation,resulting in the shutdown of superconducting devices or catastrophic accidents.For example,in 2008,the giant superconducting magnet of the European Hadron Collider exploded due to quench,resulting in a shutdown of 14 months and an economic losses of$60 million.Therefore,it is of great scientific significance and engineering application prospect to study the quench(i.e.,heat transfer)characteristics of superconducting materials.The existing research methods for the quench of superconducting materials mostly adopt the method of pasting a thermometer on the surface of superconducting materials.This kind of measurement faces the limitations of slow response,single point measurement and electromagnetic interference.At the same time,the superconducting material will inevitably bear the electromagnetic force and thermal stress in the process of operation,which will cause the deformation of the superconducting material.On the one hand,it will cause the electromagnetic field around the superconducting material to change,which will make the initial calibration result difficult to use.On the other hand,it will cause the pasted thermometer to fall off,which will cause the failure of the experiment.In addition,YBCO superconducting tape is a typical layered composite material,and researchers have established four theoretical models for the calculation of its equivalent thermal conductivity,but so far there is no experimental verification.In order to solve above-mentioned problems,a low-temperature fluorescence thermography test system based on temperature-sensitive paint EuTFC is established in this dissertation.This method has the advantages of full field measurement,fast response and no electromagnetic interference.The temperature distribution characteristics of YBCO superconducting tapes were systematically tested on the basis of temperature calibration.The differences between the existing theoretical model and experimental results were compared,and it was found that the theoretical results of Maxwell's model were in good agreement with the experimental results.Subsequently,the temperature propagation characteristics of YBCO superconducting tape during tension were studied by combining the low temperature fluorescence test system with the low temperature loading system of Lanzhou University.The normal zone propagation velocity and the minimum quench energy under different tensile strains were obtained,which laid a foundation for the further engineering application of this kind of superconducting material.The main work of this dissertation is summarized as follows:1.The low-temperature fluorescence thermal imaging test platform based on temperature-sensitive paint Eu TFC was established,and an adaptive data processing method was proposed for fluorescence brightness-temperature calibration,which successfully solved the problem of temperature calibration under non-uniform ultraviolet excitation.In order to ensure the accuracy and reliability of the fluorescence test system,the fluorescence performance of EuTFC polymer film was characterized by static and dynamic calibration experiments.The experimental results showed that the fluorescence brightness of the EuTFC film remained basically unchanged under continuous irradiation,which proves that its photoluminescence characteristics have good stability;the monotonicity and repeatability of temperature dependence of photoluminescence was proved by measuring the fluorescence brightness-temperature calibration curve of the same sample at different times and during different heating and cooling processes,and the temperature measurement accuracy was calibrated to 0.1K;then the response time of fluorescent film was no hysteresis in temperature changes when comparing with the resistance thermometer measurements.2.It is proved for the first time that the Maxwell model can effectively predict the equivalent thermal conductivity of YBCO superconducting tapes.Based on the above-mentioned low-temperature fluorescence thermal imaging test system,the brightness change of the surface fluorescent film of YBCO superconducting tape during heat conduction was recorded,and the temperature change curve of the studied area was extracted by post-processing data,and even the two-dimensional temperature distribution image of the sample surface was re-constructed.Then,the transient heat conduction process of YBCO coated conductors was reproduced on the basis of different equivalent thermal conductivity theoretical models under the help of ABAQUS finite element simulation software.By comparing the experimental results with the simulation results,it is found that only the result of Maxwell model can be used to predict the equivalent thermal conductivity of YBCO superconducting tapes.3.The temperature propagation characteristics of YBCO superconducting tapes under different pulse heat sources and tensile strains were studied by using low temperature fluorescence thermography system,and the minimum quench energy and normal zone propagation velocity under different conditions were obtained.The results shown that the minimum quench energy is 0.3675J under experimental conditions,the normal zone propagation velocity increases with the increase of the perturbation energy,and the closer to the heat source,the faster the normal zone propagation velocity,and the diffusion velocity of the normal region to the left and right sides is symmetrical.When different tensile strains are applied to the tape,the normal zone propagation velocity increases with the increase of tensile strain,and the defect caused by tensile strain will affect the normal zone propagation law of YBCO superconducting tapes,which makes the symmetry of the normal region diffusing to the left and right disappear.