Expert: Paul Soderman - 10/23/2011

Question
Sir,
I read in Gas turbine engines by Farookhi that "Relative inlet angle to rotor B2 and Relative exit angle B3 are equal in mag for an impulse turbine?". Also " The Relative velocity vector at inlet to rotor is equal to the Relative velocity vector at the exit to rotor". I know that these two statements are connected, but I am not able to figure out a reason for them.

Answer
Without seeing the book I can't be sure.  But we know that an impulse turbine such as a Pelton wheel has a fluid jet that impacts the inner surface of a curved rotor blade or bucket and gives up kinetic energy to the wheel.  Because the flow has to follow the bucket surface, relative to the bucket the flow is tangent to the surface at the leading edge (inlet) and trailing edge (exit) of the rotor. Beyond that I am not sure what Farookhi is getting at.