Ethiopian researchers from Mekelle University have conducted an assessment of the impact of blade shapes on the operation of a cross-flow turbine used in small hydroelectric power plants (HPPs). They found that the correct choice of blade profile can significantly increase the energy output of the turbines, which is especially relevant in remote and rural areas of the country, where centralized power supply faces difficulties. The results of their work have been published in the journal Energy.
A cross-flow turbine, also known as the Bánki-Michell turbine, is a simple and inexpensive type of hydro turbine designed for a pressure of 2 to 200 meters. It has a design feature: water passes through the blades twice, first into the wheel and then back out, making it possible to use of the flow energy more fully. These turbines can operate even with muddy or sandy water. Despite these advantages, cross-flow turbines are less efficient than turbines that are more expensive and complex to produce.
To increase their efficiency without complicating the design, the researchers analyzed the impacts of blade geometry on internal hydrodynamic processes. They considered four profile types during modeling: flat, round, sharp and aerodynamic. They performed calculations at various rotor speeds, from 270 to 940 rpm, with a constant water pressure of 10 meters. Analysis was conducted via an advanced SST k–ω turbulence model, which is widely used in turbomachinery thanks to its high accuracy in modeling flow separations, recirculation and transition zones. The scientists also assessed pressure distribution, speed, torque and water content of the flow in the working chamber.
The results showed that aerodynamic blades demonstrated the greatest efficiency. It reached 83.5% thanks to stable flow, minimal turbulence and reduced losses at the outlet. They performed especially well in the speed range of 670–800 rpm. Sharp blades also showed good results, especially in the medium speed range, but were less robust against deviations from the optimal mode. Flat and round blades, while being easier to manufacture, demonstrated worse performance, losing up to 20–25% of potential energy due to pronounced turbulence and flow separation. Another important conclusion of the study was that up to 70% of all useful energy is extracted at the first stage of water passing through the turbine, during initial contact of the jet with the blades. This is why the exact direction of the flow and the shape of the blade at the inlet are critical to overall efficiency.
In the future, the Ethiopian researchers plan to verify the obtained data on physical prototypes. They also propose the possibility of creating hybrid blades, such as blades with a rounded inlet part for increased mechanical strength and an aerodynamically optimized outlet to minimize losses.
