Turbine blade profile and terminology

A turbine blade is an aerofoil-shaped body that turns and accelerates (or only turns) the flowing fluid. Cascades of such blades form the nozzle (stator) ring and the rotor ring of a stage.

     leading edge        camber line
         ______---------___
        /  ___-----------__ \__
       (  (_________________)  > trailing edge
        \_____________________/
        |<------- chord c ------>|

  cascade:   |<- pitch s ->|
          \     \     \          blade
           \     \     \         row
        ---> flow, angle measured from axial

Terms

• Chord ($c$): straight line joining the leading and trailing edges.
• Camber line: mean line midway between the pressure (concave) and suction (convex) surfaces. More camber means more turning.
• Pitch ($s$): distance between corresponding points of adjacent blades in the row.
• Stagger angle ($\xi$): angle between the chord and the axial direction.
• Blade inlet and outlet angles: angles of the camber line (tangent) at the leading and trailing edges. Flow angles $\alpha$, $\beta$ are the actual fluid directions; the difference is the incidence or deviation.
• Solidity ($\sigma = c/s$): ratio of chord to pitch; it fixes how well the blades guide the flow. Turbines use $\sigma \approx 1$ to 2.
• Aspect ratio: blade height to chord.
• Turning angle ($\theta$): $\beta_1 + \beta_2$ for the rotor, the total deflection of the flow.

Impulse and reaction profiles

Reaction blades give a smooth acceleration of the flow in the passage, so profile loss is lower, but leakage over the tips is higher because of the pressure difference across the rotor.