Understanding Hydraulic Turbine Efficiency and Regulation

The question asks to identify a hydraulic turbine type known for its high design efficiency and the ability to maintain this efficiency across a wide range of regulations, which essentially means operating efficiently even at part load conditions.

Let's examine the characteristics of the given hydraulic turbine types concerning their efficiency under varying loads:

• Pelton Turbine: This is an impulse turbine used for high heads. Its efficiency is high at the design point. While its efficiency drop at part loads is less severe compared to some reaction turbines, it doesn't possess the specific mechanism for maintaining high efficiency over a very wide range as described. Regulation is done by controlling the jet size with a needle valve.
• Francis Turbine: This is a reaction turbine suitable for medium heads. It has high efficiency at the design point. However, its efficiency tends to drop significantly at part loads due to the fixed runner blade geometry which is optimized only for the design flow conditions. Regulation is done by adjusting guide vanes.
• Propeller Turbine: This is a reaction turbine used for low heads and high flow rates. It has fixed runner blades, similar to a Francis turbine in this respect. Its efficiency curve is relatively steep, meaning efficiency drops considerably at part loads compared to the design point. Regulation is done by adjusting guide vanes.
• Kaplan Turbine: This is a type of propeller turbine, also used for low heads. The distinguishing feature of a Kaplan turbine is that it has adjustable runner blades in addition to adjustable guide vanes. This provides a double regulation mechanism.

Kaplan Turbine's High Efficiency Over Wide Regulation Range

The key to the Kaplan turbine's performance characteristic described in the question lies in its adjustable runner blades. When the load on the turbine changes, requiring a different flow rate, the guide vanes are adjusted, and simultaneously, the angle of the runner blades is also adjusted. This combined adjustment ensures that the angle at which water enters the runner blades is optimized for the current flow rate. By changing the blade angle, the turbine can maintain efficient energy transfer from the water to the runner over a much broader range of flow rates and operating conditions (part loads).

Because the Kaplan turbine can adapt its blade geometry to the changing flow conditions, it can sustain high efficiency from near-full load down to relatively low part loads, unlike turbines with fixed runner blades (like Propeller or Francis turbines) or those with single regulation (like Pelton).

Conclusion

Therefore, the hydraulic turbine type that is designed for high efficiency and, more importantly, maintains this high efficiency over a wide range of regulations (loads) is the Kaplan turbine, thanks to its unique double regulation system involving adjustable guide vanes and runner blades.