Figure P 11.93 shows a cutaway of a cross-flow or "Banki"...

Figure $\mathrm{P} 11.93$ shows a cutaway of a cross-flow or "Banki" turbine $[55],$ which resembles a squirrel cage with slotted curved blades. The flow enters at about 2 o'clock and passes through the center and then again through the blades, leaving at about 8 o'clock. Report to the class on the operation and advantages of this design, including idealized velocity vector diagrams.

Key Concepts

Advantages of the Banki Turbine Design

The Banki, or cross-flow, turbine design offers several benefits including simplicity of construction, ease of maintenance, and low fabrication costs. Additionally, the ability to capture energy over two-pass interactions within the turbine increases efficiency, particularly in low-head or variable-flow environments. The design is also robust and capable of self-cleaning, which is valuable in turbulent water conditions.

Velocity Vector Diagram Analysis

Velocity vector diagrams are an idealized way to represent the changes in fluid velocity as it interacts with moving blades in the turbine. By visually breaking down the components of the velocity (such as relative velocity, tangential velocity, and absolute velocity), these diagrams help in analyzing the performance of the turbine and in identifying how the fluid dynamics contribute to effective energy extraction.

Cross-Flow Turbine Operation

This concept refers to the way fluid flows through the turbine so that it meets the blades twice — once on the inlet pass as it travels through the turbine's internal channel, and a second time as it continues to exit. The design enables improved energy extraction as the fluid's kinetic energy is harnessed on two consecutive passages, enhancing overall turbine efficiency compared to single-pass designs.

Blade Design and Geometry

The specific design of curved, slotted blades is crucial for guiding the fluid smoothly through the turbine. The curvature and slots help optimize the fluid's pressure and velocity distribution along the blade, reducing energy losses due to turbulence and separation. This design maximizes the torque produced by the turbine and assists in efficient energy conversion.

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