BEPCII超导螺线管磁体及低温系统热负荷分析

采用三维有限元和有限体积软件对北京正负电子对撞机重大改造项目(BEPCII)超导螺线管磁体(SSM)及低温冷却系统的主要设备进行了热负荷计算及分析.计算结果表明,在现有设计结构下,SSM超导磁体工作温度在4.5 K左右,满足性能要求;磁体及低温系统的总热负荷约120 W,同时还需要0.4 g/s的冷氦流冷却电流引线.从热负荷分布分析,由于磁体结构尺寸大,热辐射漏热为主要漏热;传输管线漏热占系统总漏热的10 %左右.计算结果为超导磁体低温系统的设计和进一步优化提供了依据.     

磁共振三维动态增强技术参数对血管成像质量的影响

目的:探讨三维动态增强血管成像(3D dynamic contrast enhancement magnetic resonance angiography,3D DCE MRA)技术参数对图像质量的影响.方法:106例行三维动态增强血管造影检查的病例,分为三组分别采用以下方法成像:①智能(smart)跟踪法;②test小剂量测试法;③经验法.同时将可变参数进行对比,采用SPSS 10.0统计软件,卡方检验和秩和检验对数据进行处理.结果:三组间图像质量差异具有显著性(P=0.038).测试法成像质量优率最高. K空间中心充填和K空间顺序充填对图像质量的影响差异具有显著性(P=0.015),K空间中心充填图像质量好于K空间顺序充填.结论:3种3D DCE MRA成像方法中,test小剂量团注测试法是3D-DCE MRA成像最佳的延时时间计算方法,其成像质量最佳.高质量的3D DCE MRA图像依赖于患者的配合、合理的扫描序列及参数选择、最佳扫描延迟时间的把握、合理的造影剂剂量和注射速度.     

冷中子源逆布雷顿循环氦制冷机性能分析和优化

运用能量守恒和(火用)分析方法,对冷中子源氦制冷逆布雷顿循环过程进行热力分析和(火用)分析.找出了系统(火用)效率和各部件(火用)损失随着压缩机压比、膨胀机等熵效率、跑冷量、换热器冷热流体平均温差变化的规律,并提出减小循环跑冷量、换热器内冷热流体温差,以及提高压缩机压比、膨胀机等熵效率、物料分配均匀度以提高循环性能和系统(火用)效率的措施.基于换热器内部冷热流体温差分布对循环性能影响的分析,设计了膨胀机预冷循环方案,该方案的(火用)效率相对于基本循环提高了24 %.     

A fractal model for the effective thermal conductivity of nanoparticle suspensions

Extending the effective medium approximation with the fractal theory for describing nanoparticle clusters and their radius distribution, a predictive model is proposed for the effective thermal conductivity of nanoparticle suspension, combined with the consideration of size effect and surface adsorption effect of nanoparticles.The predicted effective thermal conductivity of nanoparticle suspension is consistent with experimental data in the dilute limit.     

Adjustable character of microwave attenuation in BaTiO3 electrorheological fluids

The behavior of microwave attenuation in BaTiO3 electrorheological fluids is studied when considering microwave propagation in the directions perpendicular and parallel to the particle chains, respectively. In the former case, the microwave attenuation increases with field strength when the particle concentration is low, and the increase of the particle concentration can also increase the microwave attenuation. However, when the particle concentration exceeds a critical value, the attenuation will first increase then decrease with field strength. At the same time, the higher the field strength, the greater the change of microwave attenuation. Moreover, there is a saturation field strength. When the field strength is lower than the saturated one, the change of microwave attenuation is fast. On the other hand, in the case of microwave propagation parallel to the particle chains, the microwave attenuation increases with the field strength monotonously. In addition, the variation of microwave attenuation with the field strength shows relaxation effect. The adjustable character of microwave attenuation in BaTiO3 ER fluids can be attributed to the dielectric changes resulting from the field-induced-structural transformation and the polarization of BaTiO3.     

基于等效视数的ISAR干扰效果评估方法

由于逆合成孔径雷达(ISAR)是一种新体制雷达,一些用于评估常规体制雷达干扰效果的方法和手段对于ISAR来说并不适用.综合考虑雷达干扰的原理和图像处理的理论,从等效视数的角度出发,提出了一种新的对ISAR压制干扰效果的评估方法,即采用干扰前后两幅图像的等效视数差来评估干扰效果,该方法能清楚地反映受干扰目标从临界干扰到严重干扰整个过程中目标图像的失真程度,仿真结果表明了该评估方法的正确性和有效性.     

Intelligent Control on Hot Strip Coiling Temperature

A new intelligent control scheme for hot strip coiling temperature is presented. In this scheme, the prediction model of finishing temperature and the presetting model of main cooling zone are establish based on BP neural network, the feed-forward open-loop control model of main cooling zone is constructed based on T-S fuzzy neural network, a new improved structure of T-S fuzzy neural network is developed, and the feedback close-loop control model of precision cooling zone is obtained based on fuzzy control. The effectiveness of the proposed scheme has been demonstrated by computer simulation with a satisfactory result.     

Numerical simulation of oasis-desert interaction

Based on a simplified dynamic model and the entropy balance equation, by describing the energy and moisture exchange between oasis and desert, the characteristics of the oasis-desert interaction are studied dynamicallv and thermodynamically. The dependence of the oasis evaporation rate on the oasis wetness and the oasis scale is revealed. The results show that the oasis scale and oasis wetness are two key dominant factors controlling the cold island effect. A smallest critical oasis scale for maintaining and developing oasis is obtained.     

Mathematical model for the thermal process of controlled cooling of wires and its numerical simulation

The mathematical model for the thermal process of billets rolling has been established, including transporting in air and temperature-holding cover, descaling with high-pressure water, and the process of rolling and cooling in water box. The calculated data by the model have been compared with the measured data and the results show that the model is right and creditable. Based on the model, the main thermal characters of rolling line have been simulated and the influence of all the parameters on the temperature of rolling has been analyzed.     

Formation mechanism of internal cracks in continuously cast slab

In order to make clear the formation mechanism of centerline cracks in continuously cast slabs, the form, distribution and other characteristics of the cracks were analyzed. The final solidification point, surface temperature of the slabs and strain in solidifying shell were investigated. The results were that: (1) Five relatively low temperature zones exist on slab surface below the three water spraying nozzles and near the two edges, respectively, which corresponds to the places of centerline cracks and triangle-zone cracks. (2) Centerline cracks and triangle-zone cracks occur because of weak secondary cooling, uneven cooling along slab width,and large variation of roll gap. (3) After minimizing the variation of roll gap and applying the new secondary cooling pattern, the occurring frequency of centerline and triangle-zone cracks minimizes to zero.