The steam turbine working principle lies in the change of heat energy contained in steam which is converted into mechanical energy transmitted to the turbine rotor. This happens in several different steam turbine stages. Each turbine stage always consists of a stationary circular blade and a rotating blade.
Heat energy in steam is shown by the amount of enthalpy (h).
h = u + p.V
u = internal energy, p.V = work flow
First, heat energy must be converted to kinetic energy, this process occurs in the nozzle (see picture above). In steam turbines, nozzles are mounted on the sides of the turbine stator and also at the rotor blades, hereinafter known as reaction stage. In this nozzle, water steam increases the speed (kinetic energy increase), and this acceleration causes differential pressure between the upstream sides nozzle and downstream nozzle.
Second, the kinetic energy is transformed into a rotary energy of the turbine rotor that occurs only on the rotating blade (rotor side).
Stages on the turbine has a speed difference, as shown in the picture above. At each level, a velocity triangle is drawn, one on the rotating inlet side of the blade, and the second at the outlet side. The absolute speed (c) in the inlet and outlet have different magnitude, since the kinetic energy of water vapor is converted to mechanical energy in the rotor.