quality:correction, structural:scholar_edition
Impulse turbines

An impulse turbine has fixed nozzles that orient the steam flow into high speed jets. These jets contain significant kinetic energy, which is converted into shaft rotation by the bucket-like shaped rotor blades, as the steam jet changes direction. A pressure drop occurs across only the stationary blades, with a net increase in steam velocity across the stage.As the steam flows through the nozzle its pressure falls from inlet pressure to the exit pressure (atmospheric pressure or, more usually, the condenser vacuum). Due to this high ratio of expansion of steam, the steam leaves the nozzle with a very high velocity. The steam leaving the moving blades has a large portion of the maximum velocity of the steam when leaving the nozzle. The loss of energy due to this higher exit velocity is commonly called the carry over velocity or leaving loss.

The law of moment of momentum states that the sum of the moments of external forces acting on a fluid which is temporarily occupying the control volume is equal to the net time change of angular momentum flux through the control volume.

The swirling fluid enters the control volume at radius r 1 {\displaystyle r_{1}} with tangential velocity V w 1 {\displaystyle V_{w1}} and leaves at radius r 2 {\displaystyle r_{2}} with tangential velocity V w 2 {\displaystyle V_{w2}}.

A velocity triangle paves the way for a better understanding of the relationship between the various velocities. In the adjacent figure we have:

Then by the law of moment of momentum, the torque on the fluid is given by:

For an impulse steam turbine: r 2 = r 1 = r {\displaystyle r_{2}=r_{1}=r}. Therefore, the tangential force on the blades is F u = m ˙ (V w 1 − V w 2) {\displaystyle F_{u}={\dot {m}}\left(V_{w1}-V_{w2}\right)}. The work done per unit time or power developed: W = T ω {\displaystyle W=T\omega }.

When ω is the angular velocity of the turbine, then the blade speed is U = ω r {\displaystyle U=\omega r}. The power developed is then W = m ˙ U (Δ V w) {\displaystyle W={\dot {m}}U(\Delta V_{w})}.