Pelton Turbine:
What most likely comes to mind when you think about hydro-turbines is a pelton style design. For instance, standard water-wheel would be a crude version of one. These turbines are called impulse ones because they generate power from the water hitting it. The cup shaped design allows it to capture and contain the "hits";a slit through the middle allows water to exit once all the energy is transferred. Because it realizes on intense "hits" it is best suited for high velocity water, thus it demands high heads (distance between intake and the turbine).
Kaplan:
This is the opposite of a Pelton style blade. It is similar to a propeller on a ship because they have similar principles. A Kaplan likes low velocity water but at bulk quantities like thousands of gallons per second but at low velocities. The huge amounts of water are forced to fall on the blades at a 90 degree angle due to the box things (pitch vanes) around the circumference. These vanes can move to restrict or allow water entry to the turbine. They can also change their angles for optimum efficiency.
The key difference here is that water pushes through the blades causing them to rotate due to their unique hydro-foil shape hence the name reaction turbine. In a Pelton, the water hits the blade. These blades can change their angle depending on the velocity and flow rate. I would assume that if the blades are less steep, their would be a higher flow rate and more steep if the flow rate was low but at a higher velocity. If there is a less steep blade, the contact area is larger so more low velocity water can push against it. In a high velocity, low volume application, the blades should be steeper so that the most of the velocity could be used to rotate the blade rather than whack it.
Francis:
A hybrid of the 2 designs. The cup shaped blade resembles a pelton and the wide, long, blades resemble a Kaplan. See the Pelton and Francis designs to learn about this type. This application would be a medium ground of the 2 extremes.
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