Francis Turbine Test Rig Experiment: Procedure, Calculations, and Performance Curves

Francis Turbine Test Rig Experiment: Procedure, Calculations, and Performance Curves

The Francis turbine experiment demonstrates the operation of a mixed-flow reaction turbine — the most widely installed turbine type in the world for medium-head hydroelectric power. Students measure turbine efficiency, output power, and speed under varying load conditions, plotting characteristic curves. This guide covers theory, apparatus, procedure, observation table, and calculations for engineering laboratory use.

Theory: Francis Turbine Operating Principle

The Francis turbine is a reaction turbine — water enters the runner under both pressure and velocity. Unlike the Pelton wheel (impulse), pressure drops across the runner as energy is extracted. Water enters radially through guide vanes (wicket gates) and exits axially through the draft tube.

Specific speed: Ns = N√P / H^(5/4) — Francis turbines operate at Ns = 60–300

Output power: P = 2πNT/60 (W)

Input water power: P_in = ρgQH (W)

Overall efficiency: η = P/P_in × 100%

Apparatus Description

• Francis turbine runner (mixed-flow, radial-axial) with adjustable guide vanes
• Spiral casing (volute) for uniform water distribution to guide vanes
• Draft tube for pressure recovery at runner exit
• Rope brake dynamometer for mechanical load measurement
• Pressure gauges at inlet and outlet for net head measurement
• Flow measurement by venturi meter or electromagnetic flowmeter
• Digital tachometer for speed measurement
• Centrifugal pump + sump for water recirculation

Experimental Procedure

Setup and No-Load Reading

1. Start the pump and fully open the guide vanes — measure inlet pressure P₁ and outlet P₂
2. Calculate net head: H = (P₁ – P₂)/ρg + (V₁² – V₂²)/2g
3. Measure flow rate Q using the venturi meter or rotameter
4. Record no-load speed N₀ (rpm) at the given head

Load Test (Constant Head)

1. Apply rope brake load in steps (add weights W in increments)
2. At each load: record N (rpm), W (N), spring balance S (N)
3. Calculate torque: T = (W – S) × r_drum
4. Calculate output power: P_out = 2πNT/60
5. Calculate efficiency: η = P_out / (ρgQH) × 100%
6. Continue until speed drops to ~50% of no-load speed

Observation Table

Run	N (rpm)	W (N)	S (N)	T (N·m)	P_out (W)	Q (m³/s)	H (m)	η (%)
No load		0	0	0	0			0
1
2
3
4

Graphs to Plot

• Speed N vs. Torque T
• Speed N vs. Output Power P_out (shows maximum power speed)
• Speed N vs. Efficiency η (shows maximum efficiency speed)
• Unit power P₁₁ vs. unit speed N₁₁ (dimensionless characteristic curve)

Expected Results

• Maximum efficiency: 75–88% (lab rig scale losses are higher than full-scale turbines)
• Maximum efficiency occurs at approximately 70–80% of no-load speed
• Torque is maximum at zero/low speed (stall); power peaks at an intermediate speed

Viva Questions

• What is the difference between an impulse and a reaction turbine?
• Why does the Francis turbine use a draft tube?
• Define specific speed. At what specific speed does the Francis turbine operate?
• How do the guide vane openings affect turbine speed and efficiency?
• Compare Francis, Pelton, and Kaplan turbines for head and flow conditions

Get a Quote

Scientico India exports CE-certified Francis Turbine Test Rigs to engineering colleges in UAE, Saudi Arabia, Kenya, Bangladesh, Philippines, and 60+ countries. Contact [email protected] for a proforma invoice within 24 hours.

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