Hydraulic characteristics of a siphon-shaped overflow tower in a long water conveyance system: CFD simulation and analysis

The siphon-shaped overflow tower is a new type of pressure-suppressing structure used in long water conveyance systems,and it plays a crucial role in guaranteeing the system’s stability and safety during hydraulic transient processes.The flow in the tower is characteristic of weir flow in a closed duct,and is thus a complex air-water two-phase flow.Intensive studies of the flow patterns,the pressure pulsations,and the discharge capacity are necessary for better understanding of the flow processes and for the purpose of design.In this paper,we simulate the flow in a siphon-shaped overflow tower under both steady and unsteady flow conditions.Through a steady-flow field simulation,the relationship between the overflow discharge and the pressure in the connected pipeline is analyzed and an empirical formula for evaluating the discharge capacity is provided.Through a transient-flow field simulation,the negative-pressure distributions on the weir crest,the pressure pulsations on the crest and in the falling pond,and the transformation of the air-water two-phase flow in the downstream outlet pipe are analyzed.Moreover,the major influencing factors of the flow patterns,especially,the sectional area of the air vents,are clarified.It is indicated that the siphon-shaped overflow tower can regulate the pressure surge during hydraulic transients and guarantee the safety and stability of the pipeline system,if the shape and the vents are properly designed.     

雷达信号处理机并行自动测试系统设计

为了测试和评价某型雷达信号处理机的功能与性能,提供了一种以PCI总线为基础,结合虚拟仪器、数据库和直接数字频率合成等技术的多机并行自动测试系统设计,实现了为雷达信号处理机提供雷达回波模拟、高稳定度ADC采集时钟和多通道多类型信号输入,重点完成对雷达信号处理机多类型通讯总线测试和关键性能实时检测与分析,结合典型产品定量给出测试结果并分析,验证了测试系统的有效性。     

水泵水轮机四象限工作区流动特性数值分析

抽水蓄能电站工况转换频繁,过渡过程中水泵水轮机可历经全特性4个象限,不稳定的复杂流动演变使过渡过程中事故频发。本文针对某低比转速模型水泵水轮机进行四象限全流道数值计算,得到同一开度下水轮机内部流态的演变规律。结果显示,在水泵区部分负荷工况,导叶区产生旋转失速现象,失速区的个数随流量大小变化;失速涡阻塞流道,使局部压力升高,产生周向传播的低频压力波动。在水泵制动区,当转速较高时,离心力作用使得水流不能均匀进入转轮,在转轮内产生旋转失速现象。在水轮机反"S"区,转轮进口产生的回流漩涡结构随流量在某些工况点的突变,导致沿转轮进口展向的流速分布呈现随流量而突变的现象。入流分布在飞逸点附近的突变使水流对转轮的做功特性发生突然变化,可能是导致空载不稳定的原因。     

有源电力滤波器谐波检测算法的研究

通过对三相四线制系统中三相不平衡情况的研究,提出了一种基于改进的瞬时无功功率的谐波和无功电流的检测方法。详细阐述了这种算法的理论推导,表明应用改进检测算法的检测环节,不用先分离出零序电流,就可准确检测出电网中的谐波及无功电流。最后采用MATLAB软件进行仿真实验,证明该检测算法的正确性。