What is Mixed Flow Turbine? | What are types of Mixed Flow Turbines?

A mixed-flow turbine is a combination of the axial flow turbine and radial flow turbine. These types of turbines are most commonly used in hydroelectric power plants. This article mainly explains the different aspects of the mixed flow turbine.

What is a Mixed Flow Turbine?

A combination of axial flow and radial flow turbines is known as a mixed flow turbine.

In the case of a mixed flow turbine, the inlet side water flows at an angle between the radial and axial designs. This reduces the curvature of the flow path and effectively decreases the secondary flow formation compared to radial turbines.

The Francis turbine is a most common example of mixed flow where the water enters radially and exits axially.

With radial turbines, the liquid enters the turbine tangentially (a straight line to the right or left of the top of the circle) and vortexes like water when you empty a toilet. Just consider that a spiral of liquid rotates a fan blade in a similar direction as the liquid.

In contrast, in the axial turbines, the water flows perpendicularly to the rotor blades instead of flowing in a similar direction to the rotor blades. Wind turbines are the most common example of axial flow turbine.

Windmills use rotor blades to capture the wind flowing by the blades of the rotor. John B. McCormick mixed two turbines into one to increase unit efficiency. Energy efficiency is a quickly evolving issue all over the world, and designs that improve the energy efficiency of systems are valuable.

The Francis turbines are probably the most common as they are suitable for a wider range of heads (typically from 3 meters to 600 meters). These turbines are used for high head range applications to increase the flow rate and power. Otherwise, the runner or impeller will become very small for suitable manufacturing. In the case of low-head, the propeller turbine is best unless the output power is also low.

Advantage of Mixed Flow Turbines

The axial flow turbine has an efficiency between 70% to 75%. This efficiency is low because this turbine has a space between the blades of the rotor through which the fluid flows. One of the best methods to catch all the air blowing off the wind turbine is to convert the wind turbine into a complete circle. If you convert the wind turbine into a perfect full circle, the turbine won’t enclose the air in a meaningful way. Therefore, there is no way to contain a 100% efficiency of the axial flow turbine.

The efficiency of radial turbines is up to 80%. This means that this turbine has more efficiency than the axial turbine. The axial flow turbine has less than 100 percent efficiency because of the “secondary flow” phenomenon. This phenomenon starts when the blade’s liquid discharges from the blades, and this liquid can’t provide any more help to turn the rotor.

Just remember the toilet example again. When flushing the wastewater, the water exterior the bowl doesn’t fully circulate with the remaining water in the bowl. This exterior water is called the secondary flow. The more secondary water in the stream, the less water will help to spin the rotor. That’s why the radial turbine doesn’t have 100% efficiency.  

The combination of these two turbines (axial and radial turbines) increases the efficiency from approx. 85% to 90%. This is because in a mixed-flow turbine, the fluid introduces into the turbine in a radial direction which minimizes the secondary flow, and the efficiency of the axial turbine section maximizes due to the forced flow of the fluid. In this way, the efficiency of the mixed flow turbine becomes very high. That’s why these turbines are recommended.  

Types Mixed Flow Turbines

1) Mixed flow Turbochargers 

Part load performance is important in the design and manufacture of automotive turbochargers because of the increasing need to improve fuel economy in passenger vehicles. In the driving cycle of an automobile, a turbine that has the capacity to achieve high power at a lower speed and a higher-pressure ratio is desirable.

In general, the radial turbine has the highest performance at a 0.7-speed ratio. Still, at low speed and high-pressure ratio, the speed ratio of the blades is low due to that the rotor experiences a high value of positive incidence at the inlet of the turbine. According to the main concerns, a mixed flow turbine offers excellent benefits for this purpose.

2) Mixed Flow Hydraulic Turbine 

In this turbine, the fluid in the runner first flows in the radial direction to the turbine axis and then in the axial direction. Water flows from the turbine housing to the stator guiding mechanism. The draft tube of this turbine has a curved design.

The mixed flow turbine has more efficiency than all other types of turbines. But the mixed flow turbine doesn’t have a flat performance curve as those of adjustable-blade turbines.

Therefore, when the load fluctuation of a hydroelectric power plant is large, and the number of devices is low, the power generation effect of a mixed flow turbine is not as good as that of an adjustable-blade turbine. In contrast, the mixed flow turbines have better cavitation properties than the cavitation properties of adjustable-blade turbines.

3) Francis Turbine

These types of mixed-flow turbines are widely used all over the world due to their wide range of heads (i.e., 3m to 600m). The high head applications require a high flow rate and output. If not, the runners will become too small to be reasonably manufactured.

These types of turbines are best suitable for medium heads (i.e., 120m to 300m) applications. They are available in various designs and sizes. Francis turbines may have a vertical axis or horizontal axis.

Vertical axis turbines use for machines that have up to 2m diameter or more. These turbines generally take up low area than horizontal turbines. This permits the turbine runner to sink deeper with the lowest drilling effort and makes servicing the top-mounted generator easier.

The most famous Francis turbine designs have a cast-steel or cast volute casing. The volute casing assists in distributing the fluid evenly to all entrance gates with up to 24 guide blades or pivoted gates. This gate fully opens or fully closes according to the power requirements. 

In maximum turbines, a standard regulating speed ring uses to drive the entrance gates. These gates are coupled by pins in such a way that no damage occurs if any of the gate channels become blocked by debris. One or two hydraulic servomotors rotate the regulating speed ring. A governor uses to control the hydraulic servomotors.

4) Deriaz Turbine

These types of mixed-flow turbines use variable-pitch swiveled vanes to increase part load functioning in medium-sized units. This design is best suitable for high head applications and some pumped storage applications. Compared to Kaplan turbines, it has the advantage of a slower runaway speed (faster load drop), which can save a lot of generator expenses.

Deriaz turbines are not common as other mixed flow turbines.

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