Flowing water is directed on to the blades of a turbine runner, creating a force on the blades. Since the runner is spinning, the force acts through a distance (force acting through a distance is the definition of work). In this way, energy is transferred from the water flow to the turbine.Water turbines are divided into two groups. Reaction turbines and impulse turbines.The precise shape of water turbine blades is a function of the supply pressure of water, and the type of impeller selected.



Figure 3:The insides of a water turbine.


In a reaction turbine, the blades sit in a much larger volume of fluid and turn around as the fluid flows past them. A reaction turbine doesn't change the direction of the fluid flow as drastically as an impulse turbine: it simply spins as the fluid pushes through and past its blades. Wind turbines are perhaps the most familiar examples of reaction turbines. Most water turbines in use are reaction turbines. They are used in low and medium head applications. In reaction turbines pressure drop occurs in both fixed and moving blades.


Figure 4:The basic motion of the reaction turbine.


Figure 5: A typical reaction turbine from a geothermal power plant


Impulse turbines change the velocity of a water jet. The jet impinges on the turbine's curved blades which change the direction of the flow. The resulting change in momentum (impulse) causes a force on the turbine blades. Since the turbine is spinning, the force acts through a distance (work) and the diverted water flow is left with diminished energy. Prior to hitting the turbine blades, the water's pressure (potential energy) is converted to kinetic energy by a nozzle and focused on the turbine. No pressure change occurs at the turbine blades, and the turbine doesn't require a housing for operation.


Figure 6: The basic motion of the impulse turbine.


Figure 7:An impulse turbine like this works when the incoming fluid hits the buckets and bounces back again.


Aspects Impulse Turbine Reaction turbine
Conversion of fluid energy. All the available energy of fluid is converted into kinetic energy by nozzle. Only a portion of the fluid energy is converted into KE before fluid enters the runner.
Changes in pressure and velocity. The pressure is constant (atmosphere)throughout the action of water on runner. Water enters the runner with an excess of pressure and then both velocity and pressure change as water pass through runner.
Action of water on blades. Blades are only in action when they are in front of nozzle. Blades are in action all the time.
Admittance of water over the wheel. Water may be allowed to enter a part or whole of the wheel circumference. Water is admitted over the circumference of the wheel.
Water tight casing. Water tight casing required. Not necessary.
Extent to which water fills the wheel/turbine. Runner and blades are not completely filled or covered by water. Water completely fills all the passages between the blades throughout the operation of the turbine.
Installation unit. Always installed above the tail race. No draft tube is used. Unit may be installed above or below the tail race use of draft tube is made.
Relative velocity of water. When water glides over the moving blades, its relative velocity either remains constant or reduces slightly due to friction. Since there is continuous drop in pressure during flow through the blade passages, the relative velocity increases.
Flow regulation. Flow regulated by needle valve fitted into the nozzle. Flow regulated by guide vanes.

Figure8:Schematic of impulse and reaction turbines, where the rotor is the rotating part, and the stator is the stationary part of the machine.
taken by:http://en.wikipedia.org/wiki/Turbine