| High temperature superconducting(HTS)flux-pinning magnetic levitation(maglev)mainly relies on the magnetic flux pinning effect of the high temperature superconducting bulk material carried in the train system to provide the levitation force and guiding force.It has the advantages of stable running process,low noise,environment protection and no active control,so it has great potential in the application of high-speed maglev train.The core of engineering application of HTS maglev train lies in the size of levitation force produced by HTS bulk material.It is directly related to the load capacity and economy of maglev.In the previous studies,most studies on the relationship between levitation force and the external magnetic field are limited to the permanent magnet guideway(PMG).However,due to the magnetic field is relatively fixed and the intensity of it is small in the PMG,it is difficult to comprehensively study the influence relationship between the levitation force of HTS bulks and external magnetic field.In addition,when the HTS maglev train constantly pushed into the process of engineering,the HTS bulks run at high speed in the complex and changeable magnetic field environment.AC loss will be produced and internal temperature rise due to the irregularity of the PMG magnetic field.This phenomenon of heat loss and temperature rise will be represented in the change of the levitation force of the bulk,and ultimately affect stable running of HTS maglev system.In this thesis,it was based on the original superconducting magnet platform in the group.The internal magnetic field of the superconducting magnet is adjustable and strong.A mixed external magnetic field was combined with the constant or fluctuating magnetic field generated by the external coil.An adjusting platform for B_rwas set up.Under mixed external magnetic field conditions,the magnetic characteristics and AC heat loss characteristics of single-grain cylindrical HTS YBa Cu O(YBCO)bulks which were fabricated by top-seeded melt-growth technique were systematically studied.The magnetic condition in the experiment includes different intensity,transverse/vertical magnetic field variations(B_rand B_z)and frequency and amplitude wave forms in different wave directions.Firstly,a dynamic simulation measurement platform for AC loss of HTS with mixed external magnetic field was built,and it was based on the test platform for levitation force of superconducting magnet.The levitation force variation trend of HTS bulk with different B_rin the background magnetic field(magnetic field inside the superconducting magnet cavity)were measured on the test platform.According to the previous research on affecting the levitation force of magnetic field characteristic factors(B_zand B_r),the levitation force of bulk will appear saturated in the variation of B_z.Therefore,the levitation force gain and relaxation attenuation of the bulk were measured by increasing B_r.They were also studied in the vertical magnetic field variation(ΔB_z)before saturation and after saturation under the range of 2 T of the superconducting background magnetic field condition.So as to study the magnetic force characteristics of the bulk under the mixed external magnetic field.The results show that it can increase the peak value of the levitation force by increasing the intensity of B_r.At the same time,the B_ris increased,the greater the background magnetic field intensity,the more the levitation force of the bulk increases.Compared with before saturation ofΔB_z,the increase of B_rafter saturation(0.72 T)can obtain greater levitation force gain.For the relaxation attenuation of levitation force of bulk,it can be reduced by increasing B_r.And the greater B_ris,the less the relaxation of levitation force attenuates.In order to study the influence of magnetic field fluctuation on the AC loss characteristics of bulks under the complex external magnetic field condition,the influence of different background magnetic fields and different B_rfluctuation conditions(wave frequency,wave amplitude,wave direction)on the internal heat loss and levitation force fluctuation of bulks were further studied.At the same time,the influences of growth inhomogeneity of the bulk and theΔB_zon the levitation force and temperature rise were also studied.Eventually it was found that the internal temperature of bulk increased sharply at first and then gradually remained stable.After removing the fluctuation,the temperature decreased sharply until it was stable.The smaller the background field,the larger the fluctuation frequency,the larger the fluctuation amplitude and the smallerΔB_zwere,the temperature rise phenomenon was more obvious and the attenuation of levitation force was greater.It was found that the temperature rise in the growth region of the bulk is more obvious than that in the growth boundary.In addition,the difference of magnetic field waveform will not affect the difference of levitation force fluctuation,but it will cause the difference of temperature rise phenomenon inside the bulk.And the temperature rise phenomenon of square wave was the most obvious among the three kinds of waveform.The COMSOL Multiphysics simulation software was used to build a two-dimensional rotary dynamic electromagnetic-thermal-force multi-field coupling model of bulk in the superconducting magnet.In this thesis,the experimental process of the research on the AC loss characteristics of the bulk in the mixed field is firstly reproduced with the simulation analysis method.The comparative experimental results verify the real reliability of the simulation method and its applicability in the extreme conditions.By using the established electromagnetic-thermal-force multi-field coupling model of HTS pinning maglev,the simulation study of B_zfluctuation was carried out,including different background magnetic field intensity,magnetic field fluctuation frequency and magnetic field fluctuation amplitude.It combined with the experimental result and was found that compared with the fluctuation of B_z,the fluctuation of B_rwill cause greater heat loss and levitation force attenuation of the bulk,and the process of the levitation force fluctuation is also different.Through the analysis of the simulation results,it was found that the AC loss does not increase when the magnetic field fluctuation frequency exceeded 200 Hz.So it further demonstrated that the HTS flux-pinning maglev has the potential of high speed and stable running in the extreme magnetic field.Based on the above research results,they can provide theoretical guidance for the improvement of levitation performance and stability performance about HTS maglev system in the future. |
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