Reaction Turbines

Lecture slides by

Sachin Kansal

NATIONAL INSTITUTE OF TECHNOLOGY

KURUKSHETRA

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Objectives

• Have an intuitive understanding Reaction Turbines, its

type and components

• Calculate the work done and efficiencies of various

Reaction Turbines

• Design the turbines according to various combination

of parameters

• Understand the function, working and types of draft

tube

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Giant Hydro Power Plants of India

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Reaction Turbine

In a reaction turbine, water at the inlet of the turbine

possesses kinetic energy as well as pressure energy

As water flows through the runner, a part of pressure

energy goes on changing into kinetic energy.

Thus the water through the runner is under pressure and

the runner is completely enclosed in an airtight casing.

Casing and the runner are always full of water.

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Different types of reaction turbine

Different types of reaction turbine are

A) Inward radial flow reaction turbine: Water flows from

outward to inward

B) Outward radial flow reaction turbine: Water flows from

inward to outward

C) Mixed flow : Water enters radially but leaves axially

e.g. Francis Turbine

D) Axial flow turbine: Water enters and leaves axially

a. Kaplan turbine:- Runner blades are adjustable

b. Propeller turbine:- Runner blades are fixed

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Francis Turbine

A Francis Turbine is:

(i) Mixed Flow Turbine : Water enters radially and leaves

axially to the direction of rotation of the shaft

(ii) Reaction Turbine : At the inlet of the turbine, both

kinetic, as well as pressure energy, are available

(iii) Medium head and medium flow rate

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Components

A) Penstock

B)Spiral Casing

C)Guide Blades

D)Governing

Mechanism

E) Runner

F) Draft Tube

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Components

(A) Penstock

A penstock is a large diameter conduit, which carries

water from a dam or a reservoir to the turbine house

Since Francis turbine requires a large volume of water

than the Pelton wheel, the size of the penstock is bigger

in the case of Francis turbine

Material: Generally steel is used

(B)Spiral Casing

Water from the penstock enters into the spiral casing

which completely surrounds the runner.

This casing is also known as scroll casing or volute

casing

The cross-section area of this casing decreases

uniformly along the circumference to keep the fluid velocity

constant in magnitude along its path towards the guide

vane.

This is so because the rate of flow

along the fluid path in the volute

decreases due to continuous entry

of the fluid to the runner through

the openings of the guide vanes.

Material: For low head: Concrete

casing with steel plate lining

For medium head: Welded rolled

steel plate casing

For high head: Cast steel

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(C) Guide Vane Mechanism

A series of airfoil-shaped vanes called the guide vanesor wicket gates, are mounted on the casing.

Guide vanes are fixed between the two rings in the formof a wheel however, they can swing about their ownaxis.

The basic purpose of the guide vanes is to convert apart of pressure energy at its entrance into the kineticenergy and to direct the water or fluid on to the runnerblades at an angle appropriate to the design.

Guide blades can move on its pivot centers and hencecan change the area of flow.

Depending on load fluctuations, the governingmechanism changes the position of guide blades andhence the area of flow so that the turbine rotates withconstant speed

Material: Cast steel

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(D) Runner

It is the most important component of the Francis

turbine.

The runner of a Francis turbine consists of a series of

curved vanes evenly arranged around the

circumference in the annular space between two plates.

The runner vanes are so shaped that water enters the

runner radially at the outer periphery and leaves it

axially at the inner periphery.

Most of the portion of pressure energy is converted into

kinetic energy as water flows through the runner.

The driving force on the runner is both due to impulse

(deviation in the direction of flow) and reaction (change

in kinetic and pressure energy) effects.

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(D) Draft Tube

It is a pipe or passage of gradually increasing cross-

sectional area towards its outlet end

It connects the runner exit to the tailrace

As the pressure of the reaction turbine decreases

continuously as water passes through the guide vanes

and the runner, it does below atmospheric pressure at

the outlet of the runner

The draft tube is used to discharge the water to the

tailrace by increasing the pressure above atmospheric

The draft tube must be submerged below the level of

water in the tailrace

Material: Steel plate

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Working of Francis Turbine

Water through the penstock under pressure enters the

spiral casing which completely surrounds the runner.

From casing, water passes through a series of guide

vanes, which directs the water to the runner at a proper

angle

The pressure energy of water reduces continuously as it

passes over the guide vanes and moving vanes.

The difference in pressure at stationary guide vanes and

moving runner is responsible for the motion of the runner

vanes.

Finally, water is discharged to the tailrace through a draft

tube

https://www.youtube.com/watch?v=aVWH1DhV5-k

Velocity Triangles, Work done and

Efficiency of Francis TurbineThe velocity triangles at the inlet and outlet of the Francis

turbine are drawn as shown in Fig.

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The general expression for work done by the runner can

be derived in the same manner as in the case of a series

of radial curved vanes

𝑊𝐷 ⁄𝑠𝑒𝑐 = 𝑚 ̇(𝑉𝑤1 𝑢1 ± 𝑉𝑤2 𝑢2 )

𝑊𝐷 ⁄𝑠𝑒𝑐 = 𝜌𝑎𝑉1 (𝑉𝑤1 𝑢1 ± 𝑉𝑤2 𝑢2 )

𝑖𝑓 𝛽 < 90° → +𝑣𝑒 𝑠𝑖𝑔𝑛 𝑡𝑎𝑘𝑒𝑛

𝑖𝑓 𝛽 > 90° → −𝑣𝑒 𝑠𝑖𝑔𝑛 𝑡𝑎𝑘𝑒𝑛

For maximum output, the runner of

the Francis turbine is so designed

that there occurs a radial discharge

at the outlet tip of the blades

For radial discharge at the outlet,

𝛽 = 90° and 𝑉𝑤2 = 0, as shown in Fig

∴ 𝑾𝑫⁄𝒔𝒆𝒄 = �̇� (𝑽𝒘𝟏 𝒖𝟏 )𝑵𝒎/𝒔𝒆𝒄 15

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Hydraulic efficiency =

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Working Proportions for Francis Turbine

1. Speed Ratio (𝑲𝒖): The ratio of the peripheral velocityat the inlet (𝑢1 ) to theoretical velocity (√2𝑔𝐻) is calledspeed ratio. Its value lies between 0.6 to 0.9 i.e

2. Flow Ratio (𝑲𝒇): The ratio of flow velocity at the inlet(𝑉𝑓1 ) to theoretical velocity (√2𝑔𝐻) is called flow ratio.Its value lies between 0.15 to 0.30 i.e.

Breadth Ratio (𝒏) : The ratio of the width of the runner(𝐵) to the outside diameter of the runner (𝐷) is calledthe breadth ratio. Its value ranges from 0.1 to 0.4 i.e.

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Design Aspects of Francis Turbine

Sr. no Parameter Equation Approximate

value

1 Flow ratio (Kf) 0.15 to 0.30

2 Speed ratio (Ku) 0.6 to 0.9

3 Breadth ratio (n) 0.1 to 0.4

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Total Discharge through Francis Turbine

∴ 𝑄 = 𝜋𝐷1 𝐵1 × 𝑉𝑓1 = 𝜋𝐷2 𝐵2 × 𝑉𝑓2 If the thickness of the vanes are taken into

consideration, then the area through which flow takes

place is given by, (𝜋𝐷1 − 𝑛𝑡)𝐵

Hence,𝑄 = (𝜋𝐷1 − 𝑛𝑡)𝐵1 × 𝑉𝑓1 = (𝜋𝐷2 − 𝑛𝑡)𝐵2 × 𝑉𝑓2

Key Point for Reaction Turbine

Energy per unit weight is known as Head

Head balance : Head utilized = Head available at the inlet -

Head at the outlet

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Axial Flow Reaction Turbine

In an axial flow reaction turbine, the water flows parallel

to the axis of the rotation of the shaft.

It is used under the low head and high discharge

conditions.

For the axial flow reaction turbine, the shaft of the turbine

is vertical.

The lower end of the shaft is made larger which is known

as “Hub” or “Boss”

The vanes are fixed on the hub and hence hub acts as a

runner for axial flow reaction turbine

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Types of Axial Flow Reaction Turbine

Different types of axial flow reaction turbine are

1. Kaplan Turbine and

2. Propeller Turbine

When the vanes are fixed to the hub and they are not

adjustable, the turbine is known as the Propeller turbine

If the vanes on the hub are adjustable the turbine is

known as a Kaplan turbine

The runner blades are adjusted automatically by

servo-mechanism so that at all loads the flow enters

them without shock.

This gives better part-load efficiency for Kaplan

turbine

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Kaplan Turbine

A Kaplan Turbine is:

(i) Axial Flow Turbine : Water enters axially and leaves

axially to the direction of rotation of the shaft

(ii) Reaction Turbine : At the inlet of the turbine, both

kinetic, as well as pressure energy, are available

(iii) Low head and high flow rate

https://www.youtube.com/watch?v=0p03UTgpnDU

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Main parts of the Kaplan & Propeller

turbine

Components of the

Kaplan turbine and

Propeller turbine are

similar to that of the

Francis turbine, only the

runner is different.

Main parts of the Kaplan

& Propeller turbine are:

(A)Scroll casing

(B)Guide vane

mechanism

(C)Hub with vanes or

runner

(D) Draft tube

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Hub with vanes or runner

The lower end of the shaft is made larger which is

known as “Hub” or “Boss”.

The vanes are fixed on the hub and hence hub acts as a

runner for axial flow reaction turbine

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Working of Kaplan Turbine The water from the penstock enters the casing

and then moves to the guide vanes

From the guide vanes, the water turns through

90°and flows axially through the runner as shown

in Fig

Work done, Efficiency and PowerDeveloped

Expressions for work done, efficiency and power

developed by Kaplan & Propeller turbine are similar to thatof the Francis turbine

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Discharge through Runner of Kaplan &

Propeller TurbineThe discharge through the runner is obtained by,

where,𝐷𝑜 = Outer diameter of the runner

𝐷𝑏 = Diameter of the hub

𝑉𝑓1 = Velocity of flow at the inlet

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Working Proportions for Kaplan Turbine

1. The peripheral velocity at inlet and outlet are equal,

The velocity of flow at inlet and outlet are equal,

Area of flow at inlet and outlet are equal,

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Draft Tube Theory

The draft tube is an integral part of the reaction turbine

It is an airtight diverging conduit with a cross-sectional

area increasing along its length.

One end of this diverging tube is connected to runner exit

and the other is located below the level of the tailrace.

The functions of the draft tube are:

• When water flows through the turbine it’s kinetic and

pressure energy is utilized to generate shaft power.

Even though when the water leaves the turbine it

possesses high kinetic energy and negative pressure

head. If water is discharged through a draft tube having

gradually increasing cross-sectional area, the velocity is

largely reduced at the outlet of the draft tube, and thus

resulting in a gain in kinetic head and also increases

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Draft Tube Theory

the negative pressure head at the turbine exit so that net

working head on the turbine increases. So the output of the

turbine and efficiency also increases.

• By providing a draft tube, a turbine can be installed

above the tailrace without the loss of any head. This

helps to make inspection and maintenance of a turbine

easy.

Consider a capital draft-tube as shown in Fig

Let,𝐻𝑠 = Vertical height of draft tube above the tailrace

𝑦 = distance of the bottom of draft tube from the tailrace

Applying Bernoulli’s equation to the inlet(section 2-2) and

outlet (section 3-3) of the tube as shown in Fig.

Assuming section 3-3 as a datum line, we get,

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Efficiency of Draft Tube

It is defined as the ratio of actual conversion of the kinetic

head into pressure head in the draft tube to the kinetic

head at the inlet of the draft tube.

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Types of Draft Tube

Different types of draft tubes used in reaction turbine are

The conical draft-tubes and Moody spreading draft-tubes

are most efficient while simple elbow tubes and elbow

draft-tubes with circular inlet and rectangular outlet require

less space as compared to other draft-tubes.

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Difference between Inward flow and

Outward flow Reaction Turbine

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Reference Links

• Reaction Turbine: https://youtu.be/JB_VwxhAeGU

• Francis Turbine: https://youtu.be/1eYcjdGrXYA

https://youtu.be/BFoJOZbZ9FI

• Draft Tube: https://youtu.be/HWX06fwSJWM

• Axial Flow Turbine: https://youtu.be/TyygDiQPzaA

https://youtu.be/JB_VwxhAeGUhttps://youtu.be/1eYcjdGrXYAhttps://youtu.be/BFoJOZbZ9FIhttps://youtu.be/BFoJOZbZ9FIhttps://youtu.be/HWX06fwSJWMhttps://youtu.be/TyygDiQPzaA