1、 外文资料翻译 学院 (直属系 ): 能源与环境学院 年级、 专业 : 2009 级 水利水电工程 学 生 姓 名 : 李 巧 龙 学 号 : 312009080801230 指 导 教 师 : 杨 耀 完 成 时 间 : 2013 年 5 月 27 日 西华大学外文资料翻译 1 hydraulic turbines 1 introduction Power may be developed from water by three fundamental processes : by action of its weight, of its pressure, or of its veloci
2、ty, or by a combination of any or all three. In modern practice the Pelton or impulse wheel is the only type which obtains power by a single process the action of one or more high-velocity jets. This type of wheel is usually found in high-head developments. There has been practically no increase in
3、the efficiency of hydraulic turbines since about 1925, when maximum efficiencies reached 93% or more. As far as maximum efficiency is concerned, the hydraulic turbine has about reached the practicable limit of development. Nevertheless, in recent years, there has been a rapid and marked increase in
4、the physical size and horsepower capacity of individual units. In addition, there has been considerable research into the cause and prevention of cavitation, which allows the advantages of higher specific speeds to be obtained at higher heads than formerly were considered advisable. The net effect o
5、f this progress with larger units, higher specific speed, and simplification and improvements in design has been to retain for the hydraulic turbine the important place which it has long held at one of the most important prime movers. 2 types of hydraulic turbines Hydraulic turbines may be grouped i
6、n two general classes: the impulse type which utilizes the kinetic energy of a high-velocity jet which acts upon only a small part of the circumference at any instant, and the reaction type which develops power from the combined action of pressure and velocity of the water that completely fills the
7、runner and water passages. The reaction group is divided into two general types: the Francis, sometimes called the reaction type, and the propeller type. The propeller class is also further subdivided into the fixed-blade propeller type, and the adjustable-blade type of which the Kaplan is represent
8、ative. 2.1 impulse wheels With the impulse wheel the potential energy of the water in the penstock is transformed into kinetic energy in a jet issuing from the orifice of a nozzle. This jet discharge freely into the atmosphere inside the wheel housing and strikes against the bowl-shaped buckets of t
9、he runner. At each revolution the bucket enters, passes through, and passes out of the jet, during which time it receives the full impact force of the jet. This produces a rapid hammer blow upon the bucket. At the same time the 西华大学外文资料翻译 2 bucket is subjected to the centrifugal force tending to sep
10、arate the bucket from its disk. On account of the stresses so produced and also the scouring effects of the water flowing over the working surface of the bowl, material of high quality of resistance against hydraulic wear and fatigue is required. Only for very low heads can cast iron be employed. Br
11、onze and annealed cast steel are normally used. 2.2 Francis runners With the Francis type the water enters from a casing or flume with a relatively low velocity, passes through guide vanes or gates located around the circumstance, and flows through the runner, from which it discharges into a draft t
12、ube sealed below the tail-water level. All the runner passages are completely filled with water, which acts upon the whole circumference of the runner. Only a portion of the power is derived from the dynamic action due to the velocity of the water, a large part of the power being obtained from the d
13、ifference in pressure acting on the front and back of the runner buckets. The draft tube allows maximum utilization of the available head, both because of the suction created below the runner by the vertical column of water and because the outlet of he draft tube is larger than the throat just below
14、 the runner, thus utilizing a part of the kinetic energy of the water leaving the runner blades. 2.3 propeller runners nherently suitable for low-head developments, the propeller-type unit has effected marked economics within the range of head to which it is adapted. The higher speed of this type of
15、 turbine results in a lower-cost generator and somewhat smaller powerhouse substructure and superstructure. Propeller-type runners for low heads and small outputs are sometimes constructed of cast iron. For heads above 20 ft, they are made of cast steel, a much more reliable material. Large-diameter
16、 propellers may have individual blades fastened to the hub. 2.4 adjustable-blade runners The adjustable-blade propeller type is a development from the fixed-blade propeller wheel. One of the best-known units of this type is the Kaplan unit, in which the blades may be rotated to the most efficient an
17、gle by a hydraulic servomotor. A cam on the governor is used to cause the blade angle to change with the gate position so that high efficiency is always obtained at almost any percentage of full load. By reason of its high efficiency at all gate openings, the adjustable-blade propeller-type unit is particularly applicable to low-head developments where conditions are such that the units must be operated at varying load and varying head. Capital cost and maintenance for such units are necessarily higher than for fixed-blade propeller-type units operated at the point of maximum efficiency.
