---
title: "Turbine blade profile and terminology"
subject: "Turbo Machines"
chapter: 2
key: "semester_7/turbo_machines:turbine-blade-profile-terminology"
source: https://www.bctengineer.com/q/semester_7/turbo_machines/turbine-blade-profile-terminology
site: BCT Engineering
---

# Turbine blade profile and terminology

Turbo Machines · Chapter 2: Velocity Vector Diagram

- Asked in: year not given, 6 marks
- Page: https://www.bctengineer.com/q/semester_7/turbo_machines/turbine-blade-profile-terminology
- Whole chapter: https://www.bctengineer.com/md/semester_7/turbo_machines/chapter-2

## Question

With a neat sketch, explain the typical turbine blade profile. Define chord, camber line, pitch, stagger angle, blade angles and solidity, and state how the profile differs for impulse and reaction blading.

## Short answer

Turbine blade terminology: chord (LE to TE straight line), camber line (mean), pitch, stagger angle (chord to axial), blade angles (camler tangents at LE/TE), solidity (c/s). Impulse blades: constant passage area, similar blade angles, pressure drop only in nozzle. Reaction blades: converging passage, pressure drop in stator and rotor.

## Answer

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.

```text
     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
| Feature | Impulse blade | Reaction blade |
|---|---|---|
| Passage shape | Constant area, symmetrical, deeply curved | Converging passage, aerofoil-like |
| Inlet and outlet angles | Nearly equal ($\beta_1 \approx \beta_2$) | Outlet angle smaller than inlet angle |
| Turning | Large (up to about 160 degrees) | Moderate |
| Pressure across rotor | Constant | Falls |
| Trailing edge | Thin | Thin, more rounded at the leading edge |

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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which pair of definitions is correct?
   A. Chord: mean line of the blade; camber line: straight line joining the edges
   B. Chord: distance between adjacent blades; camber line: blade height
   C. Chord: straight line joining leading and trailing edges; camber line: mean line between pressure and suction surfaces
   D. Chord: angle with the axis; camber line: pitch
   Answer: C. The chord joins the edges; the camber line is the mean line between the concave and convex surfaces, and more camber gives more turning. (Key point: Chord and camber)

2. Solidity is defined as:
   A. Pitch divided by chord
   B. Blade height divided by chord
   C. Chord divided by blade height
   D. Chord divided by pitch
   Answer: D. sigma = c/s; it indicates how well the blades guide the flow. (Key point: Solidity)

3. The stagger angle is the angle between:
   A. The chord and the axial direction
   B. The camber line and the chord
   C. The relative velocity and the absolute velocity
   D. Two adjacent blades
   Answer: A. Stagger angle is measured between the chord line and the axial direction. (Key point: Stagger angle)

4. How does impulse blade profile differ from reaction blade profile?
   A. Impulse has converging passages and falling pressure
   B. Impulse has a constant-area, symmetrical, deeply curved passage with nearly equal inlet and outlet angles
   C. Reaction blades have large turning up to 160 degrees
   D. Both have identical passages
   Answer: B. Impulse blades are symmetrical with constant passage area and large turning; reaction passages converge and pressure falls. (Key point: Impulse vs reaction blading)

## Related in Turbo Machines

- [Impulse stage: blade angles, work and thrust](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Asingle-stage-impulse-steam-numerical) — Turbo Machines, chapter 2
- [Parsons reaction turbine blade efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Areaction-turbine-parsons-efficiency) — Turbo Machines, chapter 2
- [Impulse turbine blade work and diagram efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Aimpulse-turbine-diagram-efficiency) — Turbo Machines, chapter 2
- [Axial gas turbine stage design calculation](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Aaxial-turbine-stage-gas-numerical) — Turbo Machines, chapter 2

## Related in other subjects

- [Single-stage impulse turbine blade work and efficiency](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Ade-laval-turbine-velocity-diagram-numerical) — Fluid Machines, chapter 5
- [Kaplan turbine discharge and blade angles](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Akaplan-turbine-numerical) — Fluid Machines, chapter 3
- [Impulse and reaction turbine performance characteristics](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aimpulse-reaction-performance) — Hydro Power, chapter 7
- [Reaction and impulse turbine types and performance](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aturbine-types-reaction-impulse-suitability) — Hydropower Engineering, chapter 7