Information AboutTurbine |
| CATEGORIES ABOUT TURBINE | |
| mechanical engineering | |
| turbines | |
| jet engines | |
| power engineering | |
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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 Windmill s and Water Wheel s. 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. THEORY OF OPERATION A working fluid contains Potential Energy (pressure Head ) and Kinetic Energy (velocity head). The fluid may be Compressible or Incompressible . Several physical principles are employed by turbines to collect this energy: ; describes the transfer of energy for impulse turbines. ; describes the transfer of energy for reaction turbines. Turbine designs will use both these concepts to varying degrees whenever possible. Wind Turbine s 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. Crossflow Turbine s are designed as an impulse machine, with a nozzle, but in low head applications maintain some efficiency through reaction, like a traditional water wheel. Turbines with multiple stages may utilize either reaction or impulse blading at high pressure. Steam Turbines are usually more impulse while Gas Turbines 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 with the base of the blade solely impulse. 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. Classical turbine design methods were developed in the mid 19th century. Vector analysis related the fluid flow with turbine shape and rotation. Graphical calculation methods were used at first. Formulas for the basic dimensions of turbine parts are well documented and a highly efficient machine can be reliably designed for any fluid flow condition. Some of the calculations are empirical or 'rule of thumb' formulae, and others are based on Classical Mechanics . As with most engineering calculations, simplifying assumptions were made. Velocity triangles can be used to calculate the basic performance of a turbine stage. Gas exits the stationary turbine nozzle guide vanes at absolute velocity Va1. The rotor rotates at velocity U. Relative to the rotor, the velocity of the gas as it impinges on the rotor entrance is Vr1. The gas is turned by the rotor and exits, relative to the rotor, at velocity Vr2. However, in absolute terms the rotor exit velocity is Va2. The velocity triangles are constructed using these various velocity vectors. Velocity triangles can be constructed at any section through the blading (for example: hub , tip, midsection and so on) but are usually shown at the mean stage radius. Mean performance for the stage can be calculated from the velocity triangles, at this radius, using the Euler equation: : Whence: : where: : acceleration of gravity : enthalpy drop across stage : turbine entry total (or stagnation) temperature : turbine rotor peripheral velocity : delta whirl velocity The turbine pressure ratio is a function of and the turbine efficiency. Modern turbine design carries the calculations further. Computational Fluid Dynamics dispenses with many of the simplifying assumptions used to derive classical formulas and computer software facilitates optimization. These tools have led to steady improvements in turbine design over the last forty years. The primary numerical classification of a turbine is its ''specific speed''. This number describes the speed of the turbine at its maximum efficiency with respect to the power and flow rate. The specific speed is derived to be independent of turbine size. Given the fluid flow conditions and the desired shaft output speed, the specific speed can be calculated and an appropriate turbine design selected. The specific speed, along with some fundamental formulas can be used to reliably scale an existing design of known performance to a new size with corresponding performance. Off-design performance is normally displayed as a Turbine Map or characteristic. TYPES OF TURBINES
:Water and wind turbines have a thermodynamic cycle that is part of Weather .
USES OF TURBINES Almost all electrical power on Earth is produced with a turbine of some type. Very high efficiency turbines harness about 40% of the thermal energy, with the rest exhausted as waste heat. Most Jet Engine s rely on turbines to supply mechanical work from their working fluid and fuel as do all nuclear ships and power plants. Turbines are often part of a larger machine. A Gas Turbine , for example, may refer to an internal combustion machine that contains a turbine, ducts, compressor, combustor, heat-exchanger, fan and (in the case of one designed to produce electricity) an alternator. However, it must be noted that the collective machine referred to as the turbine in these cases is designed to transfer energy from a fuel to the fluid passing through such an internal combustion device as a means of propulsion, and not to transfer energy from the fluid passing through the turbine to the turbine as is the case in turbines used for electricity provision etc. Reciprocating piston engines such as Aircraft Engine s can use a turbine powered by their exhaust to drive an intake-air compressor, a configuration known as a Turbocharger (turbine Supercharger ) or, colloquially, a "turbo". Turbines can have incredible power density (with respect to volume and weight). This is because of their ability to operate at very high speeds. The Space Shuttle 's main engines use Turbopumps (machines consisting of a pump driven by a turbine engine) to feed the propellants (liquid oxygen and liquid hydrogen) into the engine's combustion chamber. The liquid hydrogen turbopump is slightly larger than an automobile engine (weighing approximately 700 lb) and produces nearly 70,000 Hp (52.2 MW ). Turboexpander s are widely used as sources of refrigeration in industrial processes. EXTERNAL LINKS |
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