What type of blade design is used in a turbine that has blades shaped progressively from impulse at the base to reaction at the tip?

The design being referred to in the question is a specific configuration that blends both impulse and reaction principles in the turbine blades. In this type of design, the blades are shaped so that they operate on the impulse principle near the base of the turbine and transition to a reaction principle towards the tip. This configuration allows for enhanced efficiency and performance across different operating conditions, utilizing the strengths of both types of blade designs.

In the case of impulse blades, the energy from the fluid impacting the blades causes them to turn, while reaction blades rely on a pressure difference created by the flow area changes along the blade. By incorporating both designs into a single turbine blade system, the turbine can benefit from the fast response of impulse at lower speeds and the smooth, continuous power generation associated with reaction at higher speeds. This results in better overall efficiency and versatility for the turbine's operational range.

The other designs, such as impulse-only or reaction-only, do not allow for this transitional effectiveness, limiting their adaptability and efficiency in varying conditions. Variable geometry refers to adjustable blade configurations, which is a different concept focusing on altering blade angles rather than integrating both impulse and reaction principles in a single blade design.