Francis Turbine

Build the simpler kind so you can feel the difference.

1. Fix flat vanes around a disc.
2. Direct a jet of water at one vane from a nozzle.
3. Watch it spin, and note the water leaves at atmospheric pressure.

The runner blades must turn the flow, not just be pushed by it.

1. Cut identical curved blades and fix them between two discs.
2. Curve them so water entering at the rim is turned toward the centre and downward.
3. Keep the spacing even — an unbalanced runner will not spin freely.

Water must arrive at the runner already spinning, from every direction at once.

1. Build a ring of fixed vanes around the runner.
2. Angle them so water enters the runner tangentially rather than radially.
3. Feed the ring from a spiral casing so the flow is even all the way round.

The outlet is part of the machine, not just a drain.

1. Fit a tube below the runner that widens gradually as it descends.
2. Keep its lower end submerged in the tailwater.

Turn the demonstration into a measurement.

1. Measure flow rate: catch the outflow for a timed period.
2. Measure head: the vertical drop from supply surface to tailwater.
3. Water power in watts ≈ 1000 × 9.81 × Q(m³/s) × H(m).
4. Measure shaft output by lifting a known mass over a measured time.
5. Efficiency = output / water power

James Bicheno Francis was an English-born engineer who ran the water power system at Lowell, Massachusetts — a city built around textile mills driven by canals. His job was to sell water power fairly and reliably, which meant he needed to know exactly how much power a wheel delivered.

Samuel Howd had patented an inward-flow turbine in 1838. Francis took that arrangement, redesigned the runner and guide vanes properly, and — crucially — tested everything. Lowell Hydraulic Experiments (1855) is a book of careful measurements, and it did more for turbine engineering than the machine alone would have. He did not patent the design.

Choosing between turbine types is a matter of head and flow. A Pelton wheel suits very high head and small flow — a jet hitting buckets in open air. A Francis suits medium head and medium flow. A Kaplan, essentially a propeller with adjustable blades, suits low head and very large flow. Francis turbines cover the widest middle ground, which is why they run the majority of the world's installed hydroelectric capacity.

The failure mode to know about is cavitation. Where local pressure falls below the vapour pressure of water, bubbles form and then collapse violently against the blade, hammering pits into the metal. It is why draft tube design and submergence depth are taken so seriously, and why runners are inspected for pitting.