In an impulse turbine all the fluid’s pressure energy is converted into kinetic energy in a fixed nozzle before the water reaches the runner, so the runner spins purely from the momentum of high-velocity jets at atmospheric pressure (degree of reaction = 0). In a reaction turbine the runner flows full and pressure continues to drop across the moving blades, so the wheel is driven by a combination of kinetic energy and a pressure (reaction) force while fully submerged (degree of reaction > 0). That single distinction — where the pressure drop happens — is the heart of the impulse vs reaction turbine comparison.
What is an impulse turbine and how does it work?
An impulse turbine extracts energy from the kinetic energy of a free jet. The total available head is first converted to velocity in one or more nozzles. The high-speed jet then strikes curved buckets or vanes on the runner periphery, and the change in momentum of the jet produces the driving force. Because the conversion is complete in the nozzle, the pressure on both sides of the runner is the same (atmospheric), and only a part of the runner is in contact with water at any instant.
- Energy conversion: pressure energy → kinetic energy in the nozzle, before the runner.
- Runner condition: open to atmosphere; buckets run in air, struck by jets.
- Pressure across runner: constant (no pressure drop).
- Best suited to: high head, low flow rate.
- Classic example: Pelton wheel (also Turgo, cross-flow).
The jet velocity leaving the nozzle follows from energy conversion of the net head H:
V = Cv √(2gH) where V is jet velocity (m/s), g = 9.81 m/s², H = net head (m), and Cv is the nozzle velocity coefficient (≈ 0.97–0.99).
What is a reaction turbine and how does it work?
A reaction turbine develops power from both a pressure drop and a change in velocity as water passes through the runner. The runner is enclosed in a casing and runs completely full of water; guide vanes give the flow a swirl, and the blades are shaped so that the passage acts like a moving nozzle. As water flows through, its pressure falls below the inlet pressure, and this reaction force, together with the kinetic component, turns the runner. Because the flow is continuous and full, reaction machines handle far larger volumes.
- Energy conversion: partly in fixed guide vanes, partly across the moving runner.
- Runner condition: fully submerged and flowing full inside a casing.
- Pressure across runner: drops from inlet to outlet (pressure energy still present at inlet).
- Best suited to: medium-to-low head, large flow rate.
- Classic examples: Francis (mixed flow, medium head) and Kaplan / propeller (axial flow, low head).
Degree of reaction: the defining parameter
The two families are distinguished quantitatively by the degree of reaction, R — the fraction of the stage’s total energy conversion that occurs as a pressure drop in the runner:
R = (pressure energy change in runner) ÷ (total energy change in stage)
- Impulse turbine:
R = 0— no pressure change in the runner; all conversion is in the nozzle. - Reaction turbine:
0 < R < 1— Francis runners typically operate around R ≈ 0.4–0.6, and high-specific-speed Kaplan units higher still.
For both types the theoretical power developed is P = ρ g Q H η, where ρ is water density (kg/m³), Q is discharge (m³/s), H is net head (m), η is overall efficiency, and P is in watts. The runner force itself comes from Euler’s equation, P = ρQ(u₁Vw₁ − u₂Vw₂), where u is blade speed and Vw the whirl (tangential) velocity component.
Impulse vs reaction turbine: comparison table
| Parameter | Impulse turbine | Reaction turbine |
|---|---|---|
| Energy at runner inlet | Kinetic only (jet) | Kinetic + pressure |
| Where pressure drops | Entirely in nozzle, before runner | Across the runner (and guide vanes) |
| Degree of reaction (R) | R = 0 | 0 < R < 1 |
| Runner condition | At atmospheric pressure, partly in air | Fully submerged, flowing full |
| Flow control | Nozzle / spear & deflector | Adjustable guide vanes (and Kaplan blades) |
| Head & discharge | High head, low flow | Medium-low head, high flow |
| Casing | Not pressurised (open housing) | Spiral / volute casing, pressurised |
| Draft tube | Not used | Required (recovers exit kinetic energy) |
| Examples | Pelton, Turgo, cross-flow | Francis, Kaplan, propeller |
Which turbine is used where?
Selection is governed mainly by the available head and flow, expressed through specific speed:
- High head (above ~250 m), small flow: Pelton (impulse) — e.g. mountainous hydro schemes.
- Medium head (~30–250 m), moderate flow: Francis (reaction) — the most widely installed type worldwide.
- Low head (below ~30 m), very large flow: Kaplan / propeller (reaction) — run-of-river and barrage plants.
How is it demonstrated and measured in a teaching lab?
In a fluid mechanics laboratory the contrast is shown on bench-top test rigs fed by a hydraulic supply. On a Pelton wheel test rig, students see a discrete jet striking the buckets in open air and vary the nozzle/spear setting; on a Francis or Kaplan turbine test rig, the runner is enclosed and flowing full, with adjustable guide vanes. Typical measurements and computed quantities include:
- Discharge (Q) via a venturi/orifice meter or collecting tank (m³/s).
- Net head (H) from inlet pressure gauge and tailrace level (m).
- Speed (N) with a tachometer (rpm) and shaft torque (T) via a rope brake or spring-balance dynamometer (N·m).
- Brake power
P = 2πNT/60and overall efficiencyη = P ÷ (ρgQH).
Plotting efficiency, power and torque against speed for each rig lets students plot characteristic curves and confirm that the impulse machine peaks near a blade-speed-to-jet-speed ratio of about 0.46, while the reaction machine’s behaviour shifts with guide-vane opening — turning the degree-of-reaction concept into measured data.
Scientico India manufactures and exports ISO 9001:2015 and CE certified Pelton, Francis and Kaplan turbine test rigs and the full range of Fluid Mechanics Lab Equipment for engineering colleges and universities in India and across 60+ countries.
Frequently Asked Questions
What is the main difference between an impulse and a reaction turbine?
In an impulse turbine all pressure energy is converted to kinetic energy in the nozzle before the runner, so the runner spins at atmospheric pressure from water jets (degree of reaction = 0). In a reaction turbine the runner flows full and pressure drops across the moving blades, so it is driven by both kinetic and pressure (reaction) forces while fully submerged (degree of reaction > 0).
Is a Pelton wheel an impulse or reaction turbine?
A Pelton wheel is an impulse turbine. Its nozzles convert all the head into a high-velocity jet that strikes the buckets in open air, and there is no pressure drop across the runner. Francis and Kaplan turbines, by contrast, are reaction turbines.
What is degree of reaction in a turbine?
Degree of reaction R is the fraction of the stage’s total energy conversion that occurs as a pressure drop in the runner. R = 0 for an impulse turbine (no runner pressure change) and 0 < R < 1 for a reaction turbine, with Francis runners typically around R = 0.4 to 0.6.
How is turbine efficiency measured in a lab?
Measure discharge Q with a venturi/orifice meter or collecting tank, net head H from pressure gauge and tailrace, speed N with a tachometer, and shaft torque T with a rope brake. Brake power is P = 2 pi N T / 60 and overall efficiency is eta = P divided by (rho g Q H).
Lab Equipment Featured in This Guide
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