What is a Reaction Turbine : Working & Its Applications

We know that there are different types of turbines available which are used to generate electricity but its classification can be done mainly on the nozzle position & runner blades like reaction turbine and impulse turbine. But almost 60 to 70% of the turbines used around the world is reaction turbine to generate electricity very efficiently. Unlike an impulse turbine, a reaction turbine is dipped within the water to use the pressure energy of water to produce power. Once the turbine is dipped in the water then the turbine uses the weight of water for rotating the blades of the turbine efficiently instead of striking the base of the wheel. So, this article discusses an overview of a reaction turbine, working with applications.

What is Reaction Turbine?

The turbine which develops torque by responding to the pressure of liquid or the mass or the gas is known as a reaction turbine. The reaction turbine operation can be done by using Newton’s third motion law which states that actions & reactions are equivalent but reverse within the direction. So the turbine generates force because of the movement of water over the fixed blades. The Reaction turbine diagram is shown below.

In this type of turbine, when the water enters the turbine’s wheel with some pressure & supplies above the vanes, then turbine’s wheel moves completely & may be submerged under the tailrace or may discharge into the atmosphere.

Reaction Turbine Construction and Working

The construction of a reaction turbine can be done by using different components like guide vanes, spiral casing, draft tube & runner blades.

Guide Vanes

In a reaction turbine, a guide vane is connected directly to the spiral casing. The main function of this component is to verify whether the direction of water hitting the impeller blade is in the turbine axis or not. Otherwise, the water will form a strong vortex when it flows throughout the volute casing. So this is the main reason that impeller blades will be not efficient to turn.

In the latest turbines, the vane’s angles are flexible. So based on the load of the turbine, the water supply can adjust by changing the guide vanes angle.

Spiral Casing or Volute Housing

A spiral casing is also known as volute housing that has an identical reduction within the cross-sectional region along the boundary. So this region ensures that we have a stable velocity supply that is striking the impeller blades. Here, the cross-sectional area is small, so there is an opening to supply the water from the entrance of the cross-sectional area into the impeller blade.

When the water flows within the area then the pressure of water will decrease. Thus, the cross-sectional area within the circumferential direction reduces to generate a consistent force, so that uniform water velocity will hit the impeller blade.

Draft Tube

The main function of a draft tube is to make a link between the tail run and the exit of the impeller. This tube has a cross-sectional area so it increases based on its length. Once the water leaves the impeller blades at less pressure, then the cross-sectional area of the draft tube will increase, so it helps in recovering the pressure of water because it moves in the direction of the tailrace.

Impeller Blade or Runner

The impeller blade or runner’s main function is to drive the reaction turbine through the energy of water pressure. So turbine efficiency can be determined through its design because these turbines include adaptable runner blades. The present turbines permit these impeller blades to tilt in the region of the axis of the turbine, which allows these blades to adjust the pressure on them based on the existing pressure & load of the turbine.

Reaction Turbine Working

The reaction turbine working is not complex, so it can be easily understood. In a reaction turbine, a rotor includes moving nozzles that release water with high force. When the water leaves the nozzles, then they will experience a reaction force that revolves around the rotor at a maximum speed.

In addition, a reaction force can be produced through the moving water above the runner blades. So the reaction force generated on the runner blades can cause the runner to revolve. The water comes into the draft tube after moving on the runner blades & after that to the trail race.

Reaction Turbine Efficiency

The reaction turbine’s overall efficiency can be defined as the ratio of the generated power by the turbine to the supplied energy by the turbine. So, the maximum efficiency of this turbine can be given as

η = 2cos2α/1+ cos2α

So α = 90o for the maximum efficiency where ‘α’ is the absolute velocity vector’s angle at the opening.

Reaction Turbine Types

There are different types of reaction turbines which include the following.

• Francis Turbine
• Propeller Turbine
• Gravity Turbine
• Kinetic Turbine

Francis Turbine

Francis turbine is an improved version of the propeller turbine where the direction of water supplies axially & radially into the runner. The main components of this turbine mainly include the gate, draft tube & spiral.

In the center part of this Francis turbine, usually, the flow channels are arranged in spiral housing through internally changeable influence blades. The rotor of this turbine typically includes a minimum of nine or above-fixed blades. Once water directly supplies above & around the runner, then the turbine starts rotating.

Propeller Turbine

The propeller turbine includes a propeller-shaped runner, which can be observed in submarines & ships. This turbine is equipped with 3 to 6 impeller blades where the flow of water is in contact with these impeller blades. In this turbine, the flow of water can be changed through adjustable wicket gates or guide vanes. The guide vanes help in pushing the water into the runner to transmit its energy toward the blades.

There are different types of propeller turbines like a bulb, starflo, tube, kalpan, VLH, DIVE, and Tyson. The applications of propeller turbines mainly include hydraulic sites through high flow rates. These turbines are simply arranged at the position where the height & the load are stable. In partial load, the energy efficiency curve of this turbine is very poor.

Gravity Turbine

The main function of a gravity turbine is to change the force from gravity to rotational. And also converts the kinetic energy (K.E) of the gravity force into electricity.

Kinetic Turbine

These turbines are also known as free-flow turbines and the main function of these turbines is to generate electricity from the kinetic energy (K.E) present in the water flow instead of the potential energy (P.E) from the head. These systems may function in man-made channels, rivers, ocean currents, or tidal waters.

These turbines utilize the steam of water in a natural way. They do not need the water diversion through man-made channels, riverbeds, or pipes. Kinetic systems do not need large civil works; but, they can utilize existing structures like tailraces, channels & bridges.

Reaction Turbine Solved Problems

Example1: The inlet & outlet diameters of a Francis turbine are 1.2m & 2.0m. The blade’s width is stable at 0.2m. The runner turns at 250rpm speed with an 8m^3 discharge for each sec. At the inlet, the vanes are radial and at the outlet, the discharge is outwards radially. Calculate the guide vane angle & blade angle at the inlet and outlet.

From the above information,

Inlet diameter (D1) = 2.0 m

Outlet diameter (D2) = 1.2 m

Diameter of Breath B1=B2=0.2m

Speed (N) = 250rpm

Discharge (Q) = 8m^3/s

At inlet, flow velocity is (Vfi) = Q/KfπD1B1

Assume Kf = 0.95

Substitute the values given in the above equation, then we can get

Vf1 = 6.7m/s

Vf1 = Kf√2gH => 6.7 = 0.25X√2×9.81xH

H = 36.62m

At inlet, tangential velocity u1 = π*D1*B1/60 => π*2*250/60 = 26.18 m/s

We know that, Hydraulic efficiency is

ηk =Vw1u1/gH

Assume ηk = 95% => 0.95

0.95 = Vw1x26.18/9.81×36.61

Vw1 = 13.3 m/s

Guide vane value at inlet (α) = Vf1/Vw1 = 6.7/13.03 => 0.514

α = tan-1(0.514) = 27.2 degrees

We know that Vf1/Vf2 = Kf2 π D2 B2/ Kf1 π D1 B1

Since B1=B2 & Kf1=Kf2

Vf1/Vf2 = D2/D1 => 6.7/Vf2 = 1.2/2.0

Vf2 = 11.16 m/sec

At outlet, tanget velocity

μ2 = π*D2*N/60 = π*1.2*250/60 => 15.7m/s

At outlet (ɸ) => tan ɸ = Vf2/ μ2 => 11.167/15.71 => 0.711

ɸ = tan-1 (0.711) = 35.4o

Example2: The speed of a reaction turbine is 450 rpm in a 115m head. The inlet diameter is 1.2m whereas the flow area is 0.4m^2. Absolute & relative velocities at the inlet will make 20 degrees & 60 degrees angles respectively including the velocity of tangential. Determine the developed power & Hydraulic efficiency. Assume the spin velocity at the outlet is zero.

From the above data, speed (N) = 450 rpm

Head (H) = 115m

At inlet, Diameter (D1) is 1.2m

Area of flow (A) = 0.4m^2

Absolute velocity angle (α) = 20 degrees

Relative velocity angle (θ) = 60 degrees

The reaction turbine’s Tangential velocity μ1 = π*D1*N/60 => 3.4×1.2×450/60 = 28.27 m/sec

From velocity triangle at inlet is

Tan (α) = Vf1/Vw1 => tan20o = Vf1/Vw1

From the above equation, we can get Vf1 = Vw1xtan20o => 0.364 Vw1

Tan θ = Vf1/Vw1- μ1 => tan(60) = 0.364 Vw1/Vw1 – 28.27

1.732 = 0.364Vw1/Vw1-28.27

1.732 (Vw1 -28.27) = 0.364Vw1

1.368 VW1 = 48.96

Vw1 = 35.78 m/s

Thus, Vf1 = 0.364×25.78 -= 13m/sec

Flow rate of volume (Q) = AxVf1-0.4×13 => 5.12 m3/sec

The developed power (P) = w*Q*H => 9.81×5.2×115 => 5865.37 kw

Hydraulic efficiency (ηh) = Vw1* μ1/gH => 35.78×28.2/9.8X115 = 0.897 =>89.7%

Advantages

The advantages of a reaction turbine include the following.

• The speed of rotation is high.
• High efficiency.
• Blade efficiency is high.
• High pressure and temperature used.
• The exhaust system is oil-free.
• Both kinetic energy & pressure at the inlet.
• Stage spacing is low.
• Elevated capacity & weight ratio.
• These are portable.
• Less complex design.
• Simple to design.

Disadvantages

The disadvantages of a reaction turbine include the following.

• Blade tip wear problem.
• The blade is not symmetric.
• Generates thrust force.
• Airtight casing.
• Cavitation problem.
• It cannot be arranged reversible.
• Small steam turbine effectiveness is poor.
• High maintenance cost.
• It needs high maintenance.

Applications

The applications of a reaction turbine include the following.

• These turbines are used for generating electricity in wind power mills and hydropower plants.
• This type of turbine gets maximum output power from a low accessible water head & high velocity.
• This turbine develops torque by responding to the weight or pressure of a liquid.

Thus, this is all about an overview of a reaction turbine and its working with applications. These are used to produce electricity in hydroelectric power plants. Here is a question for you, what is a wind turbine?