- Asked 2/22
- 2075 Chaitra · 5 marks
- 2074 Chaitra · 5 marks
Impulse versus reaction turbines
Short answer
In an impulse turbine, the entire stage pressure drop occurs in nozzles, so moving blades experience only impulse force while pressure stays constant. In a reaction turbine, pressure drops in both fixed and moving blades, generating impulse plus reaction forces. Moving blade passages are converging; leakage losses are larger but efficiency is higher.
Differentiate between the operation of impulse and reaction turbines?
In an impulse turbine the whole pressure drop of a stage occurs in fixed nozzles, and the high-velocity jet only changes direction on the moving blades. In a reaction turbine the pressure drops in both the fixed and moving blades, so the moving blades are also pushed by the reaction of the steam accelerating through them.
| Point | Impulse turbine | Reaction turbine |
|---|---|---|
| Pressure drop | Whole drop occurs in the nozzles only | Drop occurs in both fixed and moving blades |
| Pressure on moving blades | Constant over the moving blades | Falls continuously across moving blades |
| Force on blades | Impulse of high-velocity jet only | Impulse + reaction of steam accelerating in the blade passage |
| Blade profile | Symmetrical, constant passage area | Aerofoil shape, converging passage |
| Fixed element | Nozzles | Fixed (guide) blades acting as nozzles |
| Steam admission | Partial or full (nozzle arcs) | Full admission all round |
| Blade velocity / speed | Very high speed per stage (needs compounding) | Lower speed, many stages |
| Number of stages | Fewer | More (for same pressure drop) |
| Efficiency | Lower | Higher |
| Size / cost | Smaller, cheaper for given power | Larger, costlier (more stages) |
| Tip leakage | Small (no pressure difference across blades) | Larger, needs close clearances and dummy piston |
| Example | De Laval, Curtis, Rateau | Parsons turbine |
Degree of reaction is 0 for a pure impulse stage and usually 0.5 for a Parsons (50% reaction) stage. Large modern turbines use impulse stages at the HP end and reaction stages in the IP/LP parts.
✓ Answer checked by Claude Opus 5.5 (high effort)
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