Mechanism of stall and surge in a centrifugal compressor with a variable vaned diffuser

To expand the stable operating range of compressors, understanding the mechanism of flow instability at low flow rates is necessary. In this paper, the mechanism of stall and surge in a centrifugal compressor with a variable vaned diffuser is experimentally investigated, where the diffuser blade setting angle can be adjusted. Many dynamic pressure transducers are mounted on the casing surface of the compressor. From the design condition to surge, dynamic pressure data is recorded throughout the gradual process. According to the signal developing status, the typical modes of compressor instability are defined in detail, such as stall, mild surge, and deep surge. A relatively high-frequency stall wave originates in the impeller and propagates to the diffuser, and finally stimulates a deep surge in the compressor. The compressor behavior during surge differs at different diffuser vane angles. When the diffuser vane angle is adjusted, both the unstable form and the core factor affecting the overall machine stability change. A specific indicator is proposed to measure the instability of each component in a compressor, which can be used to determine the best region for stability extension technologies, such as a holed casing treatment, in different compressor applications.     

Impact of the thermal effect on the load-carrying capacity of a slipper pair for an aviation axial-piston pump

A thermal hydraulic model based on the lumped parameter method is presented to analyze the load-carrying capacity of a slipper pair in an aviation axial-piston pump under specified operating conditions. Both theoretical and experimental results are presented to demonstrate the validity of the thermal hydraulic model. The results illustrate that the squeezing force and thermal wedge bearing force are the main factors that affect the film thickness and load-carrying capacity. At high oil temperature and high load pressure, the film thickness decreases with increasing clamping force due to a combined action of the squeezing bearing force and the thermal wedge bearing force, but the load-carrying capacity will increase. An increase of the film thickness is proven to be beneficial under high shaft rotational speed but especially dangerous as it strongly increases the ripple amplitude of the film thickness, which leads to decreasing the load-carrying capacity. The structural parameters of the slipper can be optimized to achieve desired performance, such as the slipper radius ratio and orifice length diameter ratio. To satisfy the requirement of the load-carrying capacity, the slipper radius ratio should be selected from 1.4 to 1.8, and the orifice length diameter ratio should be selected from 4 to 5.     

A novel environmental control system based on membrane dehumidification

This paper conducts a simulation study of a novel aircraft environmental control system based on membrane dehumidification(MD-ECS),and compares the system with the up-to-date four-wheel high pressure de-water system(4WHPDW-ECS).Mathematical models for the two systems are established,and a system simulation using a numerical technique is performed to analyze and compare the cooling performance of the two systems.Simulation results show that the cooling capacity of MD-ECS is much higher than that of 4WHPDW-ECS under the same working conditions,indicating that the novel system is theoretically feasible and promising.The effects of the sweep ratio of the membrane dehumidifier on the dehumidification and cooling performance of the system is also investigated.     

模糊评价法在分散实习成绩评定中的应用

分散实习中,当某些学生工地偏远,指导老师监管困难时,其成绩评定只能依赖实习成果成绩与答辩成绩,这是一个多因素综合评价问题,采用模糊评价法能避免评价过程中主观性因素对结果的影响,研究提出了分散实习模糊评价体系,给出了综合评判义性失效时的解决办法,讨论了模糊评价方法的可行性。     

基于熵权法开展读者满意度测评的实证研究——长沙商贸旅游职业技术学院数字图书馆为例

基于熵权法测评高职院校数字图书馆的服务水平,用数字来量化图书馆读者满意度,找出高职院校数字图书馆服务的薄弱环节,明确服务改善的具体内容。通过构建高职院校数字图书馆读者满意度评价指标体系和基于熵值法权重确定的模糊综合测评模型,对长沙商贸旅游职业技术学院数字图书馆读者满意度进行测评,并提出相应的对策。     

浅谈激发学生对英语学习的兴趣

“兴趣是最好的老师。”培养学生的学习兴趣、提高教学质量一直是英语教学改革的重点。本文从因材施教，逐步探索适合我院学生的英语教学模式，强调能力的培养，创新英语课堂教学、做学生的良师益友，充分调动学生学习英语的积极性等三方面对这一问题加以论述。     

基于Matlab的无人机全过程飞行仿真

为了对小型无人机全过程飞行仿真进行研究,根据无人机飞行控制回路和制导回路,搭建了模块化的系统仿真结构,将Matlab/Simulink和Stateflow有机结合起来,建立了易于维护和扩展的仿真环境。利用仿真环境对无人机的控制规律和制导规律进行了仿真,结果表明了控制规律和制导规律的合理性,以及全过程仿真环境的实用性。     

固定翼飞机水上迫降漂浮特性计算方法研究

漂浮特性是固定翼飞机水上迫降性能的重要组成部分,直接关系到飞机水上迫降后乘员能否成功撤离。为了更好地研究这一问题,提高固定翼飞机水上迫降漂浮特性计算和分析的效率及精度,在阿基米德静力学原理理论基础上,通过对CATIA软件中优化模块进行二次开发,建立了一种飞机水上迫降漂浮特性计算方法;并以波音737-700飞机为原型机,分析飞机水上迫降后破口面积及重心位置对飞机漂浮特性的影响。     

基于振动原理的无接触流量测量实验及模拟研究

利用清华大学SMC中心试验设备,通过对气体流过管道时由于湍流而诱发振动的实验研究,确定了完全发展管道流的情况下,由湍流诱发的管壁振动加速度脉动值的标准方差与平均流量的量化关系,同时进行初步数值模拟.为无接触式气体流量测量研究打下良好基础.     

一种外环水平结构双轴光纤惯导系统旋转方案设计方法

目前国内外长航时高精度自主惯导系统多采用双轴旋转调制自动补偿技术,而旋转方案设计对系统导航精度影响至关重要.双轴惯导系统按结构可分为外环水平结构和外环航向结构两类.分析了外环水平结构双轴惯导系统在旋转方案设计中的局限性,考虑到光纤陀螺输出特性,指出外环水平结构双轴光纤惯导系统不宜采用传统旋转方案,并提出了一种该类双轴光纤惯导系统的旋转方案设计方法,最后对所设计旋转方案与十六位置旋转方案进行试验对比,试验表明此设计方案导航精度提升了71％.