23 April, 2007

Water Turbines and its Classification

Water turbine is a device that convert the energy in a stream of fluid into mechanical energy by passing the stream through a system of fixed and moving fan like blades and causing the latter to rotate. A turbine looks like a large wheel with many small radiating blades around its rim.

Classification of Water turbines
According to the type of flow of water : The water turbines used as prime movers in hydro electric power stations are of four types.They are
  • axial flow : having flow along shaft axis
  • inward radial flow : having flow along the radius
  • tangential or peripheral : having flow along tangential direction
  • mixed flow : having radial inlet axial outlet
If the runner blades of axial flow turbines are fixed,those are called propeller turbines.
According to the action of water on moving blades water turbines are of 2 types namely impulse ad reaction type turbines.
Impulse Turbines :These 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. Pelton wheels and de Laval turbines use this process exclusively. 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 : These 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 efficiently. Newton's third law describes the transfer of energy for reaction turbines.
According to the Head and quantity of water available the water turbines are of 2 types.Those are high head - low flow and low to medium head and high to medium discharge turbines.
According to the name of the originator water turbines are of 3 types namely Pelton Wheel,Francis tubine and Kaplan turbine.

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