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排序方式: 共有493条查询结果,搜索用时 156 毫秒
111.
我国企业债券信用评级存在的问题与对策 总被引:2,自引:0,他引:2
由于我国企业债券发展时间短,在发展过程中出现了许多问题.其中,企业债券信用评级的不完善是一个重要方面.文章分析了我国企业债券信用评级的现状和存在的问题,并借鉴美国发展企业债券信用评级的经验,进而提出规范发展我国企业债券信用评级问题的五点对策. 相似文献
112.
电容式传感器在飞机燃油测量系统中的应用 总被引:2,自引:0,他引:2
叙述了电容式传感器在飞机燃油测量系统中应用。首先,分析了电容式传感器在飞机燃油测量系统中工作的基本原理:因燃油液面位置变化而使电容量产生变化,将其转化为实时电信号,可以实现对飞机油箱油量变化的实时测量。其次,分析了测量误差产生的原因,并提出了改进措施。 相似文献
113.
Progress on Microgravity Sciences in China 总被引:1,自引:1,他引:1
The main progress of the research activities on microgravity fluid physics, combustion, biotechnology research and fundamental Physics in China are briefly summarized in the present paper. The major space missions and experimental results obtained on board the Chinese recoverable/nonrecoverable satellites and the Chinese manned spaceship named "Shen Zhou" are presented summarily. The recent main activities of the ground-based studies in China are introduced in brief. 相似文献
114.
Experimental investigation on aero-heating of rudder shaft within laminar/turbulent hypersonic boundary layers 总被引:1,自引:0,他引:1
The aero-heating of the rudder shaft region of a hypersonic vehicle is very harsh, as the peak heat flux in this region can be even higher than that at the stagnation point. Therefore, studying the aero-heating of the rudder shaft is of great significance for designing the thermal protection system of the hypersonic vehicle. In the wind tunnel test of the aero-heating effect, we find that with the increase of the angle of attack of the lifting body model, the increasement of the heat flux of the rudder shaft is larger under laminar flow conditions than that under turbulent flow conditions. To understand this, we design a wind tunnel experiment to study the effect of laminar/turbulent hypersonic boundary layers on the heat flux of the rudder shaft under the same wind tunnel freestream conditions. The experiment is carried out in the ?2 m shock tunnel(FD-14 A) affiliated to the China Aerodynamics Research and Development Center(CARDC). The laminar boundary layer on the model is triggered to a turbulent one by using vortex generators, which are 2 mm-high diamonds. The aero-heating of the rudder shaft(with the rudder) and the protuberance(without the rudder) are studied in both hypersonic laminar and turbulent boundary layers under the same freestream condition. The nominal Mach numbers are 10 and 12, and the unit Reynolds numbers are2.4 × 10~6 m~(-1) and 2.1 × 10~6 m-1. The angle of attack of the model is 20°, and the deflection angle of the rudder and the protuberance is 10°. The heat flux on the model surface is measured by thin film heat flux sensors, and the heat flux distribution along the center line of the lifting body model suggests that forced transition is achieved in the upstream of the rudder. The test results of the rudder shaft and the protuberance show that the heat flux of the rudder shaft is lower in the turbulent flow than that in the laminar flow, but the heat flux of the protuberance is the other way around,i.e., lower in the laminar flow than in the turbulent flow. The wind tunnel test results is also validated by numerical simulations. Our analysis suggests that this phenomenon is due to the difference of boundary layer velocities caused by different thickness of boundary layer between laminar and turbulent flows, as well as the restricted flow within the rudder gap. When the turbulent boundary layer is more than three times thicker than that of the laminar boundary layer, the heat flux of the rudder shaft under the laminar flow condition is higher than that under the turbulent flow condition. Discovery of this phenomenon has great importance for guiding the design of the thermal protection system for the rudder shaft of hypersonic vehicles. 相似文献
115.
为了研究外加磁场对PPT性能的影响,建立了PPT带外加磁场的机电模型,并用三种不同放电能量水平的PPT验证了该模型的可靠性。利用该模型研究了外加磁场强度、模式、位置以及长度对PPT性能的影响。结果表明,对于尾部馈送型PPT,当外加磁场增加时,PPT性能先增加后减小,存在最优的外加磁场强度。对于LES-6 PPT和LES-8/9 PPT,外加磁场从极板的最左端开始施加效果最好;当外加磁场强度分别为0.25T,0.50T,0.75T,1.00T时,这两种PPT对应的最适合施加磁场长度分别是1.3mm,1.8mm,2.1mm,2.3mm和1.6mm,2.8mm,3.4mm,3.7mm。对于TMU PPT,外加磁场从极板的最右端开始施加效果最好,但是施加磁场长度应该根据具体实验结果并结合仿真计算来决定。 相似文献
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文章研究了A1GaN/GaN高电子迁移率晶体管(HEMT)的质子辐照效应。在3MeV质子辐照下,采用三种不同辐照剂量6×1014,4×1014和1×1015protons/cm2。在最高辐照剂量下,漏极饱和电流下降了20%,最大跨导降低了5%。随着剂量增加,阈值电压向正向漂移,栅泄露电流增加。AlGaN/GaN HEMT电学特性的退化主要是由辐照引入的位移损伤引起的。从SRIM软件计算出空位密度,将Ga空位对应的能级引入Silvaco器件仿真软件中,仿真结果与实验结果相匹配。Hall测试结果显示二维电子气(2DEG)浓度和迁移率在辐照后有所降低。 相似文献
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120.
为改善端面滚切法加工的锥齿轮齿面接触质量,基于奥利康锥齿轮全展成加工方法,对直线刀廓圆弧修形及齿面啮合接触分析进行了研究.首先对圆弧刀廓进行了几何设计,推导出了刀齿切削刃方程.在建立锥齿轮端面滚切加工数学模型的基础上,推导出了被加工齿轮理论齿面方程.研究了刀廓圆弧修形对齿面形状的影响,利用数值方法计算出了齿面修形量.建立了考虑安装误差的齿轮副滚检数学模型,推导出了齿面接触分析简化算法.最后对采用圆弧刀廓加工的一对奥利康锥齿轮进行了啮合分析,结果表明,选取合理的圆弧刀廓半径对齿面修形可以降低边缘接触风险,降低对安装误差的敏感性,改善内对角接触,此外还可以实现对两齿面接触区进行独立修正. 相似文献