Impulses-reaction Turbine (Reaction Turbine)

If steam expands both in nozzle as well as in blades of turbine, i.e., pressure at inlet of the turbine is more than that of outlet, it is known as impulse-reaction turbine. In this case, expansion of steam in nozzle creates impulse on blades and reaction due to minor expansion of steam during passing through moving blades. The small drop in pressure of steam in the moving blades gives back pressure to the moving blades in the direction opposite to the velocity. In this turbine there are stages of fixed blades and moving blades; fixed blades act as nozzles that create an impact on the moving blades by reducing the pressure and increasing the velocity.

Since, the moving blade channels are also of nozzle shape. There is increase in kinetic energy due to the expansion of steam while flowing through the blade, that produces reaction in opposite direction (by Newton’s third law of motion). Blades rotate due to both the impulse effect and the reaction force of steam jets. Such turbines are called impulse-reaction turbine or simply a reaction turbine.

The degree of reaction (R) of these turbines is defined as

where mb = Moving blades

          fb = Fixed blades

Note:

• If enthalpy drops in moving blade are zero, degree of reaction will be zero. This is a case of pure impulse turbine.
• If enthalpy drops in fixed blade are zero, degree of reaction will be one. This is a case of pure reaction turbine.
• If enthalpy drops in moving blades are equal to enthalpy drop in fixed blade, the degree of reaction will be ½. This is a case of Parson’s reaction turbine