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Text 1. Types of Turbines
A turbine is a rotary engine that extracts energy from a fluid flow. Claude Burdin coined the term from the Latin turbinis or vortex during an 1828 engineering competition. The simplest turbines have one moving part, a rotor assembly, which is a shaft with blades attached. Moving fluid acts on the blades or the blades react to the flow, so that they rotate and impart energy to the rotor. Early turbine examples are windmills and water wheels. Gas, steam and water turbines usually have a casing around the blades that focuses and controls the fluid. The casing and blades may have variable geometry that allows efficient operation for a range of fluid-flow conditions. A device similar to a turbine but operating in reverse is a compressor or pump. The axial compressor in many gas turbine engines is a common example.
Impulse turbines change the direction of flow of a high velocity fluid jet. The resulting impulse spins the turbine and leaves the fluid flow with diminished kinetic energy. There is no pressure change of the fluid in the turbine rotor blades. Before reaching the turbine, the fluid’s pressure head is changed to velocity head by accelerating the fluid with a nozzle. Impulse turbines do not require a pressure casement around the runner since the fluid jet is prepared by a nozzle prior to reaching turbine. Newton’s second law describes the transfer of energy for impulse turbines.
Reaction turbines develop torque by reacting to the fluid’s pressure or weight. The pressure of the fluid changes as it passes through the turbine rotor blades. A pressure casement is needed to contain the working fluid as it acts on the turbine stage(s) or the turbine must be fully immersed in the fluid flow (wind turbines). The casing contains and directs the working fluid and, for water turbines, maintains the suction imparted by the draft tube. Francis turbines and most steam turbines use this concept. For compressible working fluids multiple turbine stages may be used to harness the expanding gas efficiency. Newton’s third law describes the transfer of energy for reaction turbines.
Wind turbines use an airfoil to generate lift from the moving fluid and impart it to the rotor (this is a form of reaction). Wind turbines also gain some energy from the impulse of the wind by deflecting it at an angle. Turbines with multiple stages may utilize either reaction or impulse blading at high pressure. Steam turbines are usually more impulse while gas turbines are more reaction type designs. At low pressure the operating fluid medium expands in volume for small changes in pressure. Under these conditions (termed Low Pressure Turbines) blading becomes strictly a reaction type design. The reason is due to the effect of the rotation speed for each blade. As the volume increases the blade height increases and the base of the blade spins at a slower speed relative to the tip. This change in speed forces a designer to change from impulse at the base to a high reaction style tip.
Exercise 3. Answer the questions on text 1.
1. What is the purpose of a turbine?
2. How do the simplest turbines operate?
3. What do gas, steam and water turbines have to control the fluid?
4. What device is similar to a turbine?
5. What type of turbines (impulse or reaction) requires a pressure casement?
6. Are wind turbines referred to reaction or impulse turbines?
7. Are steam turbines or gas turbines more reaction type design?
Exercise 4. Say whether the given statements are true or false. If a statement is wrong, correct. If true, enrich it with details.
1. Gas, steam and water turbines usually have a casing around the blades.
2. A device similar to a turbine but operating in reverse is a compressor or pump.
3. Impulse turbines require a pressure casement.
4. Reaction turbines do not require a pressure casement.
5. Wind turbines may utilize either reaction or impulse blading at high pressure.
6. Steam turbines are usually more reaction while gas turbines are more reaction type designs.
Exercise 5. Memorize the following words and word combinations to text 2.
Guide vane – направляюча лопатка
ratio – коефіцієнт
impinge – наштовхуватися, ударятися
alloy – сплав
melt – плавитись
brittle – крихкий
counterpart – двійник, аналог
shroud – бандаж, кожух
boundary layer – пограничний шар, суміжний шар
damping – амортизація, демпфування, гасіння (коливань)
flutter – вібрація; нестійке коливання
nuclear – ядерний
internal combustion machine – двигун внутрішнього згорання
duct – канал, прохід, трубопровід
heat-exchanger – радіатор, теплообмінник
alternator – генератор
propulsion – рух
exhaust – випускання (газів), вихлоп; випускати, вивільняти, видаляти
with respect to – що стосується, відносно
propellant – паливо (ракетне)
oxygen – кисень
hydrogen – водень
Exercise 6. Read and translate text 2.