外文翻译-水轮机和水力发电.doc

1、毕 业 设 计（论 文）外 文 参 考 资 料 及 译 文 译文题目：hydraulic-turbines and hydro-electric power 水轮机和水力发电学生姓名： 学 号： 专 业： 所在学院： 指导教师： 职 称： 20xx年 2月 27日hydraulic-turbines and hydro-electricThere has been practically no increase in the efficiency of hydraulic turbines since about 1925, when maximum efficiencies reached

2、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 the physical size and horsepower capacity of individual units.In addition, there has been

3、 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 of this progress with larger units, higher specific speed, and simplification and improveme

4、nts 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. 1、types of hydraulic turbinesHydraulic turbines may be grouped in two general classes: the impulse type which utilizes the kinetic energy of a high-velocit

5、y 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 runner and water passages. The reaction group is divided into two general types: the Franci

6、s, 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 representative.1.1 impulse wheelsWith the impulse wheel the potential energy of the water in the pen

7、stock 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 the runner. At each revolution the bucket enters, passes through, and passes out of the jet, d

8、uring 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 bucket is subjected to the centrifugal force tending to separate the bucket from its disk. On account of the stresses so produced and also the scouring effects of th

9、e 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. Bronze and annealed cast steel are normally used.1.2 Francis runnersWith the Francis type the water enters f

10、rom 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 tube sealed below the tail-water level. All the runner passages are completely filled with water, which acts

11、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 difference in pressure acting on the front and back of the runner buckets. The draft tube allows maximum util

12、ization 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 the runner, thus utilizing a part of the kinetic energy of the water leaving the runner blades.1.3 propelle

13、r runnersnherently 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 turbine results in a lower-cost generator and somewhat smaller powerhouse substructure and superstructure. Pr

14、opeller-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 propellers may have individual blades fastened to the hub.1.4 adjustable-blade runnersThe adjustable-blade pr

15、opeller 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 angle 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 ope