to Doble because it had brand name recognition. Turgo and cross-flow turbines were later impulse designs. Theory of operation 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. Reaction turbines Reaction turbines are acted on by water, which changes pressure as it moves through the turbine and gives up its energy. They must be encased to contain the water pressure (or suction), or they must be fully submerged in the water flow. Newton's third law describes the transfer of energy for reaction turbines. Most water turbines in use are reaction turbines and are used in low (<) and medium () head applications. In reaction turbine pressure drop occurs in both fixed and moving blades. It is largely used in dam and large power plants Impulse turbines Impulse turbines change the velocity of a water jet. The jet pushes on the turbine's curved blades which changes 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. An impulse turbine is one in which the pressure of the fluid flowing over the rotor blades is constant and all the work output is due to the change in kinetic energy of the fluid. 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. Newton's second law describes the transfer of energy for impulse turbines. Impulse turbines are often used in very high (>300m/1000 ft) head applications. Power The power available in a stream is; where: power (J/s or watts) turbine efficiency density of fluid (kg/m3) acceleration of gravity (9.81 m/s2) head (m). For still water, this is the difference in height between the inlet and