KLU - Project Report[Red.]

A STUDY ON

EVOLUTION, MANUFACTURING AND WORKING OF TURBINES

A mini project submitted for the partial fullfillment of the requirement for the award of degree of

BACHELOR OF TECHNOLOGY

IN

MECHANICAL ENGINEERING

Submitted by

DEPARTMENT OF MECHANICAL ENGINEERING

K L UNIVERSITY

GREEN FIELDS, VADDESWARAM.

GUNTUR Dt.

2011-2012

Under the guidance of M.NAGESWARA RAO, Asst. Professor

S.NAGA BABU (09101448)

T.NAREN (09101450) R.PATTABHIRAM (09101456) V.NAVEEN PAUL (09101451) G.PRUDHVI (09101461)B.PARAMESWARA RAO (09101455) M.RAGHAVENDRA (09101465)

K L UNIVERSITY DEPARTMENT OF MECHANICAL ENGINEERING

CERTIFICATE

This is to certify that the report entitled, “ EVOLUTION, MANUFACTURING AND

WORKING OF TURBINES ” has been carried out by these students :

as part of mini project for the academic year 2011-2012 of K L University , is the

record of the bonafide work carried out by them.

Sri M.NAGESWARA RAO

Asst. Professor Dept. of Mechanical Engg. K L UNIVERSITY

S.NAGA BABU (09101448) R.PATTABHIRAM (09101456) T.NAREN (09101450) G.PRUDHVI (09101461)

V.NAVEEN PAUL (09101451) M.RAGHAVENDRA (09101465)

B.PARAMESWARA RAO (09101455)

Dr. Y.V.HANUMANTHA RAO

Head of the Department,Dept. of Mechanical Engg.K L UNIVERSITY

ACKNOWLEDGEMENT

We would like to take this opportunity to express our heartfelt thanks to all those who helped in the course of our mini project.

We express our sincere thanks to our college K L UNIVERSITY and Dr. Y.V.HANUMANTHA RAO (HOD), Dept. of Mechanical Engineering for giving us a chance to come up with a mini project and get exposed to the real engineering.

We specially thank and warmly acknowledge the continuous encouragement, invaluablesupervision, timely suggestions and inspired guidance offered by our guide M.NAGESWARA RAO,Asst. Professor,Dept. of Mechanical Engineering, K L University, in bringing this report to a successful completion. Last but not the least, we thank our parents and the Almighty whose blessings are always therewith us.

CONTENTS

Abstract……………………………………………………………………….. i

CHAPTER 1: EVOLUTION AND CLASSIFICATION 1

1.1 Evolution of turbines 2

1.2 Major types of turbines used today 4

1.3 classification of Steam turbines 5

1.4 classification of Wind turbines 6

1.5 classification of Gas turbines 7

CHAPTER 2: THE STEAM TURBINE 8

2.1 Introduction 9

2.2 Parts of a steam turbine 9

2.3 Steam Turbine Blades 12

2.4 Classification of blade profiles 12

2.5 Dynamics in blades 15

2.6 Blading Materials 17

2.7 Manufacturing of a Steam turbine blade 18

2.8 Working of a steam turbine 24

2.9 The Rankine cycle 27

CHAPTER 3: THE GAS TURBINE 30

3.1 Manufacturing of a gas turbine 31

3.2 Process chart for manufacturing 35

3.3 Working of a Gas turbine – An overview 36

3.4 Theory of operation 37

3.5 The Brayton Cycle 39

3.6 The Actual Gas Turbine cycle 41

CHAPTER 4: THE WIND TURBINE 42

4.1 Introduction 43

4.2 Parts of a wind turbine 43

4.3 Production of the nacelle 44

4.4 Production of the blades 45

4.5 Manufacturing of the tower 47

4.6 Description of a wind turbine blade 49

4.7 The Aerodynamic profile 49

4.8 Power produced by a wind turbine 53

4.9 Efficiency of a Wind turbine 53

CHAPTER 5: RESULTS AND DISCUSSIONS 55

5.1 Results 56

5.2 Problem on steam turbine efficiency 56

5.3 Problem on Gas turbine efficiency 57

Conclusion …………………………………………………………………… 58

References ……………………………………………………………………. 59

ABSTRACT

A TURBINE is a prime mover/rotary engine that extracts energy from a fluid flow and converts it into useful work. The working fluid contains potential energy which is in turn converted into kinetic energy. There are several procedures/methods which are employed in turbines to collect this energy. From the invention of the turbine in 1913 to the highly advanced present day turbines, there have been many developments in the design, operating procedures, manufacturing and working of turbines.

Our study is mainly aimed at manufacturing and working of the turbines, considering the historical developments also. The entire process study is divided as follows:

Phase I: History, classification and overview of turbines.

In this phase, the historical developments and different types of turbines are studied.

Phase II: Manufacturing of Turbines.

A turbine is a combination of many static and rotating components/members. At high operating levels, these components are subjected to high stresses and other deformations. For the stability of a turbine, various considerations in design, manufacturing are employed. Therefore, these components are to be carefully manufactured. In this phase, we study the various manufacturing processes involved in the manufacturing a turbine & its components.

Phase III: Working Procedures of different types of turbines.

In this phase, the operating and working procedure for different turbines stated in phase I & II are studied in detail.

CHAPTER 1

EVOLUTION AND CLASSIFICATION

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1.1 EVOLUTION OF TURBINES

A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work. The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and impart rotational energy to the rotor. Early turbine examples are windmills and water wheels. Turbine has evolved in due process of time with significant changes in design and uses.

The Romans were grinding corn with a waterwheel as early as 70 B.C. On these old waterwheels, only the very lowest part of the wheel was submerged beneath a moving body of water (as shown in fig. below), and the entire wheel was turned as the river flowed past it, pushing against its paddles. This was a prototype for what came to be called an impulse

turbine, which is one that is driven by the force of a fluid directly striking it.

Fig. – undershot water wheel

The undershot waterwheel was followed by overshot wheel during medieval times (as shown in fig. below). This first made its appearance in Germany around the middle of the twelfth century and became the prototype for the modern reaction turbine. Contrasted to the impulse turbine whose energy source is kinetic energy, the energy source for an overshot wheel (or reaction turbine) is potential energy. This is because it is the weight of the water

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acting under gravity that is used to turn the wheel. Renaissance engineers studied the waterwheel and realized that the action of water on a wheel with blades would be much more effective if the entire wheel were somehow enclosed in a kind of chamber.

Fig. – overshot water wheel

French mining engineer Claude Burdin published his results in 1828. It was in this publication that Burdin coined the word "turbine" which he took from the Latin "turbo" meaning a whirling or spinning top. It was Burdin's student, Benoit Fourneyron, who improved and developed his master's work and who is considered to be the inventor of the modern hydraulic turbine. Fourneyron built a six-horsepower turbine and later went on to build larger machines that worked under higher pressures and delivered more horsepower. His main contribution was his addition of a distributor which guided the water flow so that it acted with the greatest efficiency on the blades of the wheel. His was a reaction type turbine, since water entering through the vanes of the distributor (that was fitted inside the blades) then acted on the blades of the wheel. Following Fourneyron's first turbine, which happened to be a hydraulic or water turbine, other turbines were developed that used the energy of a different material like gas or steam.

In the Impulse turbine, a nozzle transforms water under a high head into a powerful jet. The momentum of this jet is destroyed by striking the runner, which absorbs the resulting

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force. If the velocity of the water leaving the runner is nearly zero, all of the kinetic energy of the jet has been transformed into mechanical energy, so the efficiency is high.

In the Reaction turbine, the pressure change occurs in the runner itself at the same time that the force is exerted. The force still comes from rate of change of momentum, but not as obviously as in the impulse turbine. Many turbines combine impulse and reaction, so that it is difficult to separate them conceptually.

1.2 MAJOR TYPES OF TURBINES USED TODAY Although there are many types and variants of turbines, three major types of turbines are

used. They are: Steam Turbines Gas Turbines Wind Turbines

These three types of turbines are widely used because of the economic aspects, natural resources available and other factors. So we study the manufacturing processes, working procedures involved with these turbines.

Side view of typical Gas turbine

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1.3 Steam Turbine- Classification and Types

The first steam turbine, at its time indeed did spark off the industrial revolution throughout the west. However, the turbine at that time was still an inefficient piece of heavy weighing high maintenance machine. The power to weight ratio of the first reciprocating steam turbine was extremely low, and this led to a great focus improving the design, efficiency and usability of the basic steam turbine, the result of which are the power horses that currently produce more than 80% of today’s electricity at power plants!

Classification of Steam Turbines

Steam Turbines can be classified on the basis of a number of factors. Some of the important methods of steam turbine classification are enunciated below: On the basis of Stage Design: Steam turbines use different stages to achieve their ultimate power conversion goal. Depending on the stages used by a particular turbine, it is classified as Impulse Turbine, or Reaction type. On the Basis of the Arrangement of its Main Shaft: Depending on the shaft arrangement of the steam turbine, they may be classified as Single housing (casing), tandem compound (two or more housings, with shafts that are coupled in line with each other) and Cross compound turbines (the shafts here are not in line). On the Basis of Supply of Steam and Steam Exhaust Condition: They may be classified as Condensing, Non Condensing, Controlled or Automatic extraction type, Reheat (the steam is bypassed at an intermediate level, reheated and sent again) and Mixed pressure steam turbines (they have more than one source of steam at different pressures). On the basis of Direction of Steam Flow: They may be axial, radial or tangential flow steam turbines. On the Basis of Steam Supply: Superheated steam turbine or saturated steam turbine.

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1.4 CLASSIFICATION OF WIND TURBINES

Wind turbines can rotate about either a horizontal or a vertical axis, the former being both older and more common.

Horizontal axis Wind Turbine: Horizontal-axis wind turbines (HAWTs) have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind. Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor. Most have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive an electrical generator.

Vertical axis Wind Turbine: Vertical-axis wind turbines (VAWTs) have the main rotor shaft arranged vertically. Key advantages of this arrangement are that the turbine does not need to be pointed into the wind to be effective. This is an advantage on sites where the wind direction is highly variable, for example when integrated into buildings. The key disadvantages include the low rotational speed with the consequential higher torque and hence higher cost of the drive train, the inherently lower power coefficient, the 360 degree rotation of the aero foil within the wind flow during each cycle and hence the highly dynamic loading on the blade, the pulsating torque generated by some rotor designs on the drive train, and the difficulty of modeling the wind flow accurately and hence the challenges of analyzing and designing the rotor prior to fabricating a prototype.

Subtypes

Darrieus wind turbine:

Darrieus turbines, were named after the French inventor, Georges Darrieus. They have good efficiency, but produce large torque ripple and cyclical stress on the tower, which contributes to poor reliability. They also generally require some external power source, or an additional Savonius rotor to start turning, because the starting torque is very low. The torque ripple is reduced by using three or more blades which results in greater solidity of the rotor. Solidity is measured by blade area divided by the rotor area.

Savonius wind turbine:

These are drag-type devices with two (or more) scoops that are used in anemometers, flettner vents (commonly seen on bus and van roofs), and in some high-reliability low-efficiency power turbines. They are always self-starting if there are at least three scoops.

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1.5 CLASSIFICATION OF GAS TURBINES

A Gas Turbine is a heat engine that converts the energy of fuel into work by using compressed, hot gas as the working medium and that usually delivers its mechanical output power either as torque through a rotating shaft or as jet power in the form of velocity through an exhaust nozzle. Also known as combustion turbine.

Jet engines: Air breathing jet engines are gas turbines optimized to produce thrust from the exhaust gases, or from ducted fans connected to the gas turbines. Jet engines that produce thrust primarily from the direct impulse of exhaust gases are often called turbojets, whereas those that generate most of their thrust from the action of a ducted fan are often called turbofans.

Turboprop engines: A turboprop engine is a type of turbine engine which drives an external aircraft propeller using a reduction gear. Turboprop engines are generally used on small subsonic aircraft, but sometimes used on large military and civil aircraft,

Aero derivative gas turbines: Aero derivatives are also used in electrical power generation due to their ability to be shut down, and handle load changes more quickly than industrial machines. They are also used in the marine industry to reduce weight.

Auxiliary power units: APUs are small gas turbines designed for auxiliary power of larger machines, such as those inside an aircraft. They supply compressed air for aircraft ventilation (with an appropriate compressor design), start-up power for larger jet engines, and electrical and hydraulic power.

Industrial gas turbines: Used for power generation

Turbo shaft engines: Turbo shaft engines are often used to drive compression trains (for example in gas pumping stations or natural gas liquefaction plants) and are used to power almost all modern helicopters. The first shaft bears the compressor and the high speed turbine, while the second shaft bears the low speed turbine. This arrangement is used to increase speed and power output flexibility.

Micro turbines: Micro turbines are touted to become widespread in distributed power and combined heat and power applications. They are one of the most promising technologies for powering hybrid electric vehicles. They range from hand held units producing less than a kilowatt, to commercial sized systems that produce tens or hundreds of kilowatts. Basic principles of micro turbine are based on micro combustion

.

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CHAPTER 2

THE STEAM TURBINE

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2.1 INTRODUCTION The first steam turbine, at its time indeed did spark off the industrial revolution

throughout the west. However, the turbine at that time was still an inefficient piece of heavy weighing high maintenance machine. The power to weight ratio of the first reciprocating steam turbine was extremely low, and this led to a great focus improving the design, efficiency and usability of the basic steam turbine, the result of which are the power horses that currently produce more than 80% of today’s electricity at power plants.

Because a steam turbine runs on of steam produced by a boiler, it can support many

different types of fuels. Natural gas, coal, nuclear, wood, municipal solid waste and more can all be used to run a steam turbine. As a result, facilities that have an excess of waste products such as oil or wood tend to implement steam turbines. The steam turbine can be fitted to match a facilities pressure and temperature requirements. Furthermore, steam turbines can be retrofitted into an existing steam system.

2.2 PARTS OF STEAM TURBINE

a) Blades

For starters, a simple turbine works just like a windmill. Only, in the steam turbines of today, rather than striking the blades directly, the blades are designed in such a way as to produce maximum rotational energy by directing the flow of the steam along its surface. So the primary component that goes into a steam turbine is its blades. The blades of a steam turbine are designed to behave like nozzles, thus effectively tapping both the impulse and reaction force of the steam for higher efficiency. Nozzle design itself is a complex process, and the nozzle shaped blade of the turbine is probably one of the most important parts in its construction. We study more on blades in section 2.2.

Fig. –steam turbine blades

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b) Shafts

The shaft is a power transmitting device and is used to transmit the rotational movement of the blades connected to it at one end via the rotor to the coupling, speed reducer or gear at the other end.

Fig. –shaft of a steam turbine

c) Outer Casing

The steam turbine is surrounded by housing or an outer casing which contains the turbine and protects the device components from external influence and damage. It may also support the bearings on which the shafts rest to provide rigidity to the shaft. Usually split at the center horizontally, the casing parts are often bolted together for easy opening, checking and steam turbine maintenance, and are extremely sturdy and strong.

Fig. – Outer casing of a steam turbine

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d) Governor

The governor is a device used to regulate and control or govern the output of the steam turbine. This is done by means of control valves which control the steam flow into the turbine in the first place.

Fig. – Governors used in a steam turbine

e) Oil System

A steam turbine has thousands of moving parts and all these parts not only have to move in high velocities, but also need to be protected from wear and tear over the years. This is done by effective lubrication by the oil system, which governs the pressure, flow and temperature of the turbine oil, the bearing oil and lubrication of other moving parts.

f) Pipes

The pipe is an all-important steam turbine component that brings the steam from the boiler to the turbine. This has to be done without an appreciable loss in pressure, and at the same time, must be able to withstand all these pressures safely. The pipes should be easy to clean and are prone to deposits on their inner surfaces. Deposits on the inner surface of the steam pipe reduce the net steam flow area, throwing forth a negative effect on the efficiency.

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2.3 STEAM TURBINE BLADES A turbine blade is the individual component which makes up the turbine section of

the steam turbine. The blades are responsible for extracting energy from high temperature & high pressure steam produced by the combustor. The turbine blades are often the limiting component of the steam turbines. To survive in this difficult environment , turbine blades often use super alloys, exotic materials and many different methods of cooling , such as internal air channels , boundary layer cooling etc.Therfore design and manufacturing of turbine blades are of high interest and need to be done very carefully.

Blades are the heart of a steam turbine, as they are the principal elements that convert the thermal energy into kinetic energy. The efficiency and reliability of a turbine depend on the proper design of the blades. It is therefore necessary for all engineers involved in the steam turbines engineering to have an overview of the importance and the basic design aspects of the steam turbine blades. Blade design is a multi-disciplinary task. It involves the thermodynamic, aerodynamic, mechanical and material science disciplines. A total development of a new blade is therefore possible only when experts of all these fields come together as a team. The development process of a new profile took years of development and testing in the earlier years. But with the advent of CFD and FEM packages, there is a significant reduction in design and testing times. The feasibility of 3-D designs also has improved because of the advances in these software packages.

Constructional features of a blade The blade can be divided into 3 parts:

The profile, which converts the thermal energy of steam into kinetic energy, with a certain efficiency depending upon the profile shape. The root, which fixes the blade to the turbine rotor, giving a proper anchor to the blade, and transmitting the kinetic energy of the blade to the rotor. The damping element, which reduces the vibrations which necessarily occur in the blades due to the steam flowing through the blades. These damping elements may be integral with blades, or they may be separate elements mounted between the blades.

Each of these elements will be separately dealt with in the following sections.

2.4 CLASSIFICATION OF BLADE PROFILES There are two basic types of profiles - Impulse and Reaction. In the impulse type of profiles, the entire heat drop of the stage occurs only in the stationary blades. In the reaction type of blades, the heat drop of the stage is distributed almost equally between the guide and moving blades. Though the theoretical impulse blades have zero pressure drop in the moving blades, practically, for the flow to take place across the moving blades, there must be

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a small pressure drop across the moving blades also. Therefore, the impulse stages in practice have a small degree of reaction. These stages are therefore more accurately, though less widely, described as low-reaction stages. The typical impulse and reaction stages are plotted in the following figure.

Fig. - Impulse & Reaction stages

The presently used reaction profiles are more efficient than the impulse profiles at part loads. This is because of the more rounded inlet edge for reaction profiles. Due to this, even if the inlet angle of the steam is not tangential to the pressure-side profile of the blade, the losses are low. However, the impulse profiles have one advantage. The impulse profiles can take a large heat drop across a single stage, and the same heat drop would require a greater number of stages if reaction profiles are used, thereby increasing the turbine length.

The Steam turbines use the impulse profiles for the control stage (1st stage), and the reaction profiles for subsequent stages. There are three reasons for using impulse profile for the first stage.

a) In most of the turbines, if the first stage is a reaction stage, the lower half of the moving blades does not have any inlet steam, and would ventilate. Therefore, most of the stage heat drop should occur in the guide blades.

b) The heat drop across the first stage should be high, so that the wheel chamber of the outer casing is not exposed to the high inlet parameters.

c) Nozzle control gives better efficiency at part loads than throttle control. d) The number of stages in the turbine should not be too high, as this will increase

the length of the turbine.

a) H.P. BLADE PROFILES

If circles are drawn tangential to the suction side and pressure side profiles of a blade and their centers are joined by a curve, this curve is called the camber line. This camber line intersects the profile at two points A and B. The line joining these points is called chord, and the length of this line is called the chord length. A line which is tangential to the inlet and

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outlet edges is called the bitangent line. The angle which this line makes with the circumferential direction is called the setting angle. Pitch of a blade is the circumferential distance between any point on the profile and an identical point on the next blade.

Fig.- High Pressure Blade Profile

H.P. BLADE ROOTS

The root is a part of the blade that fixes the blade to the rotor or stator. Its design depends upon the centrifugal and steam bending forces of the blade. It should be designed such that the material in the blade root as well as the rotor / stator claw and any fixing element are in the safe limits to avoid failure.

The roots are T-root and Fork-root. The fork root has a higher load-carrying capacity than the T-root. It was found that machining this T-root with side grip is more of a problem. It has to be machined by broaching, and the broaching machine available could not handle the sizes of the root. The typical roots used for the HP moving blades for various steam turbine applications are shown in the following figure:

Fig-root profiles for a HP turbine blade

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b) L.P. BLADE PROFILES

The LP blade profiles of moving blades are twisted and tapered. These blades are used when blade height-to-mean stage diameter ratio (h/Dm) exceeds 0.2.

LP BLADE ROOTS

The roots of LP blades are T- roots and Fork roots.

2.5 DYNAMICS IN BLADES The excitation of any blade comes from different sources. They are:

a) Nozzle-passing excitation

As the blades pass the nozzles of the stage, they encounter flow disturbances due to the pressure variations across the guide blade passage. They also encounter disturbances due to the wakes and eddy in the flow path. These are sufficient to cause excitation in the moving blades. The excitation gets repeated at every pitch of the blade. This is called nozzle-passing frequency excitation. The order of this frequency = no. of guide blades x speed of the machine. Multiples of this frequency is considered for checking for resonance.

b) Excitation due to non-uniformities in guide-blades around the periphery

These can occur due to manufacturing inaccuracies, like pitch errors, setting angle variations, inlet and outlet edge variations, etc. For HP blades, due to the thick and cylindrical cross-sections and short blade heights,

the natural frequencies are very high. Nozzle-passing frequencies are therefore necessarily considered, since resonance with the lower natural frequencies occurs only with these orders of excitation.

In LP blades, since the blades are thin and long, the natural frequencies are low. The excitation frequencies to be considered are therefore the first few multiples of speed, since the nozzle-passing frequencies only give resonance with very high modes, where the vibration stresses are low.

The HP moving blades experience relatively low vibration amplitudes due to their thicker sections and shorter heights. They also have integral shrouds. These shrouds of adjacent blades butt against each other forming a continuous ring. This ring serves two purposes – it acts as a steam seal, and it acts as a damper for the vibrations. When vibrations occur, the vibration energy is dissipated as friction between shrouds of adjacent blades.

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For HP guide blades of Wesel design, the shroud is not integral, but a shroud band is

riveted to a number of guide blades together. The function of this shroud band is mainly to seat the steam. In some designs HP guide blades may have integral shrouds like moving blades. The primary function remains steam sealing.

In industrial turbines, in LP blades, the resonant vibrations have high amplitudes due to the thin sections of the blades, and the large lengths. It may also not always be possible to avoid resonance at all operating conditions. This is because of two reasons. Firstly, the LP blades are standardized for certain ranges of speeds, and turbines may be selected to operate anywhere in the speed range. The entire design range of operating speed of the LP blades cannot be outside the resonance range. It is, of course, possible to design a new LP blade for each application, but this involves a lot of design efforts and manufacturing cycle time. However, with the present-day computer packages and manufacturing methods, it has become feasible to do so.

Secondly, the driven machine may be a variable speed machine like a compressor or a boiler-feed-pump. In this case also, it is not possible to avoid resonance.

In such cases, where it is not possible to avoid resonance, a damping element is to be used in the LP blades to reduce the dynamic stresses, so that the blades can operate continuously under resonance also.

There may be blades which are not adequately damped due to manufacturing inaccuracies. The need for a damping element is therefore eliminated. In case the frequencies of the blades tend towards resonance due to manufacturing inaccuracies, tuning is to be done on the blades to correct the frequency. This tuning is done by grinding off material at the tip to increase the frequency, and by grinding off material at the base of the profile to reduce the natural frequency. The damping in any blade can be of any of the following types:

a) Material damping: This type of damping is because of the inherent damping properties of the material which makes up the component.

b) Aerodynamic damping: This is due to the damping of the fluid which surrounds the component in operation.

c) Friction damping: This is due to the rubbing friction between the components under consideration with any other object.

Out of these damping mechanisms, the material and aerodynamic types of damping are very small in magnitude. Friction damping is enormous as compared to the other two types of damping. Because of this reason, the damping elements in blades generally incorporate a feature by which the vibrational energy is dissipated as frictional heat.

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The frictional damping has a particular characteristic. When the frictional force between the rubbing surfaces is very small as compared to the excitation force, the surfaces slip, resulting in friction damping. However, when the excitation force is small when compared to the frictional force, the surfaces do not slip, resulting in locking of the surfaces. This condition gives zero friction damping, and only the material and aerodynamic damping exists. In a periodically varying excitation force, it may frequently happen that the force is less than the friction force. During this phase, the damping is very less. At the same time, due to the locking of the rubbing surfaces, the overall stiffness increases and the natural frequency shifts drastically away from the individual value. The response therefore also changes in the locked condition. The resonant response of a system therefore depends upon the amount of damping in the system and the natural frequency of the system.

2.6 BLADING MATERIALS

Among the different materials typically used for blading are 403 stainless steel, 422

stainless steel, A-286, and Haynes Stellite Alloy Number 31 and titanium alloy. The 403 stainless steel is essentially the industry’s standard blade material and, on impulse steam turbines, it is probably found on over 90 per cent of all the stages. It is used because of its high yield strength, endurance limit, ductility, toughness, erosion and corrosion resistance, and damping. It is used within a Brinell hardness range of 207 to 248 to maximize its damping and corrosion resistance. The 422 stainless steel materials are applied only on high temperature stages (between 700 and 900°F or 371 and 482°C), where its higher yield, endurance, creep and rupture strengths are needed.

The A-286 material is a nickel-based super alloy that is generally used in hot gas expanders with stage temperatures between 900 and 1150°F (482 and 621°C). The Haynes Stellite Alloy Number 31 is a cobalt-based super alloy and is used on jet expanders when precision cast blades are needed. The Haynes Stellite Number 31 is used at stage temperatures between 900 and 1200°F (482 and 649°C). Another blade material is titanium. Its high strength, low density, and good erosion resistance make it a good candidate for high-speed or long-last stage blading, but it is very expensive.

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2.7 MANUFACTURING OF A STEAM TURBINE BLADE

The following steps are involved in the machining of a Steam Turbine Blade:

a) Size Milling b) Size Grinding c) Facing d) Root Bottom Width Milling e) Neck Milling f) Total Length Milling g) Convex Profile h) Concave Profile i) Pitch Milling j) Pitch Grinding k) Finishing

Size Milling of the raw material

Fig. - Size Milling of the raw material work piece

The fixturing elements at the head and root of the blade structure are ultimately removed to leave the final shaped item, but during the machining process itself their accuracy and form have a crucial impact on the success of the overall operation.

For Root Rectangle, It is done by standard end mill- only roughing necessary. Now for Root Trapezoid or dovetail, special roughing and finishing necessary. The Head Counter sinking is carried out by counter-bore tool.

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The Head rectangle Standard is done by standard end-mill and only roughing is required. Now the Head Cylindrical shape is done by the standard end-mill followed by roughing and finishing.

Whichever processing methods are employed, the first step is to machine the reference surfaces by which the work piece will be clamped during the subsequent machining. Several tools are suitable for this operation, and the long edge cutter is particularly recommended.

Fig.- Bending of work piece

It is possible that the blade work piece may deform or bend during subsequent stages of the machining process, the result of machining away 80% of the original rolled or annealed raw material and the residual stresses thus created. This is particularly possible for large blades, 400–600 mm long, which may bend by as much as 2 mm. Reworking the fixturing elements during the machining process, so that the position of the work piece in the machining centers is modified to account for the deformation, can counteract this phenomenon.

Machining the root of the blade

The machining process to shape the root of the blade will depend on several factors, notably the dimensions of the finished item. Small blades are often machined directly from round bar stock, which is then is milled to a square shape. 160 mm larger blades are often made from rectangular bar stock or forging. Normally these blades are first machined with cutting tools, and then broached or ground. Turbine blades can be divided into two classes, stator and rotor blades, and in normal practice these two designs have different mounting systems and different styles of root, to accommodate the different loadings they receive in use. Stator blades normally have one small slot in one side of the root, which is relatively easy to machine with solid carbide or index able insert end mills. Rotor blades may have different mounting systems, such as a “Christmas tree” profile, or deep slots machined in a trapezoidal cross-section. These variations in the profile and geometry of the blade’s root will require different machining strategies.

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Machining a ‘Christmas tree’ profile

For machining the Christmas tree profile on a blade, it can be helpful to change the fixturing arrangement, and make the tool axis parallel to the blade length. It may also then be possible to use a special adaptor on the Christmas tree profile to hold the blade during subsequent roughing operations, and so avoid the need for machining (and later removing) separate fixturing elements onto the work piece. A milling strategy using long edge milling cutters, applying down milling for each side of the profile, will allow maximized metal removal rates and tool life.

Using a set of different diameter cutters mounted in this manner results in large differences in effective cutting speed between the largest and the smallest cutter. An alternative is to employ solid tools, particularly if there are difficulties with accessibility or the complexity of the shapes being produced.

Fig.- Christmas tree profile machined

Machining a deep slot in the blade root by end milling

The type of work piece material will have a large influence over the machining parameters when machining slots into the blade roots. In many cases it will be stainless steel, and thus problems of chip adhesion to the cutting tool will occur. However, carefully selected tooling and the correct machining methods will counteract these difficulties. The blade’s size and material, and the slot’s position and form, will determine the machining strategy. In most cases it will be better to leave the machining of the slots, along with their roughing and finishing, until after the other machining operations are complete. That way the machining of the blade profile itself can be carried out without any slots in the blade root which might conceivably affect the clamping and stability of the work piece. In addition any bending or deformation in the work piece that occurs during profiling, due to the release of internal stresses, can be compensated for when the item is remounted prior to the finishing operations, an approach which should also help to maximize the quality of the final blade.

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In general, machining deep slots in the blade root can be divided into:

Slot milling (L-style with end mill). Plunge milling (with end mill). Trochoidal milling (with end mill).

Machining the blade body

Machining the blade rhombus is a critical step in blade manufacture, and a wide variety of potential machining solutions are available depending on the design of the blade and the types of cutting machinery available. A comprehensive description of all these different methods is beyond the scope of this book, but the basic principles can be outlined, emphasizing the machining principles which underlie them: optimizing the cutting tool engagement, reducing vibrations, using the tooling as effectively as possible and maximizing productivity.

a) Roughing the rhombus – parallel to the blade axis, using one tool

This is a very common machining approach, using two separate cutting steps to reach the full depth of cut. In most cases this method allows the cutting force to be reduced more effectively than by reducing the feed per tooth, as it allows the chip thickness to be modified towards the recommended target values.

Fig.- cutting forces acting along the blade material

Vibrations and heavy axial pressure on the inserts will occur if the feed forces cause any movement or deflection of the work piece. If this occurs the feed direction should be modified so the forces act in directions where the blade fixturing arrangement supports the work piece most effectively.

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Vibrations can also be reduced by adopting cutting paths which machine the metal in small triangular steps, in both the longitudinal and lateral directions. This approach requires modifications to the cutting speed and feed, along with no more than 60% of the usual maximum depth of cut, and the modified cutting forces will also produce changes in the wear patterns seen on the cutting inserts.

Fig.- End milling the profile in two directions

b) Roughing the rhombus – parallel to the blade axis, using two tools of different

diameter

The use of two different tools to machine the rhombus is an effective strategy in many situations. A first cut, producing a slot perpendicular to the blade axis, can be made with an end mill or a slot milling cutter. This slot then provides clearance for a subsequent cutting tool of different diameter, which should experience a less severe cutting environment and generate lower vibrations while it machines along the blade’s longitudinal axis.

Fig.- Roughing the rhombus – parallel to the blade axis (using two tools of different

diameter)

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Roughing the rhombus – machining the roof slopes

This penultimate operation in roughing the blade’s contour uses a roughing tool whose size will depend on the design of the blade, and on the radius between the roof slope and the blade’s root.

The pressure side – peripheral milling

Roughing the pressure side of the blade – the concave side – is usually the last stage of the roughing process, and also one of the most complex. Modern designs of turbine blades maximize their efficiency through complicated surface geometries, and machining these surfaces requires a careful machining strategy to account for both the profile of the blade, and changes in the effective stiffness of the work piece as the machining operation proceeds. Peripheral milling is an effective way to carry out this operation, with a depth of cut between 1–5 mm.

Fig.- Roughing the pressure side – peripheral milling

Semi Finishing

The semi finishing operation requires a 5-axis milling operation, and will directly influence the surface quality of the final finished blade. Therefore the aim should always be to achieve a very regular, uniform level of residual material – if necessary, through two separate semi finishing operations. Normally this operation is done by turn milling.

Fig.- Semi-Finishing the Blade by turn milling

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Convex and Concave Profiling

The convex and concave profiling is done by a fly tool bit and finished on a milling machine.

Finishing the Blade

Fig.- Finishing the Blade

Finishing the blade is probably the most difficult 5-axis machining operation, but its success will greatly depend on the quality of the other machining steps carried out previously. The most suitable tool depends on the type and size of the blade, and also on the spindle speed and the feed available in the machining center. The principal problems when finishing are vibrations, and the quality of the pre-finished surfaces. During the cutting process the tool follows a helical path around the blade, a path controlled by a specialized CAD-CAM system.

Other components of the turbine are manufactured by simple machining processes

Hence, this is about the manufacturing of steam turbine.

2.8 WORKING OF A STEAM TURBINE

A steam turbine is a mechanical device that converts thermal energy in pressurized steam into useful mechanical work. The original steam engine which largely powered the industrial revolution in the UK was based on reciprocating pistons. This has now been almost totally replaced by the steam turbine because the steam turbine has a higher thermodynamic efficiency and a lower power-to-weight ratio and the steam turbine is ideal for the very large power configurations used in power stations. The steam turbine derives much of its better thermodynamic efficiency because of the use of multiple stages in the expansion of the steam.This results in a closer approach to the ideal reversible process.

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Steam turbines are made in a variety of sizes ranging from small 0.75 kW units used as mechanical drives for pumps, compressors and other shaft driven equipment, to 1,500,000kW turbines used to generate electricity. Steam turbines are widely used for marine applications for vessel propulsion systems. In recent times gas turbines , as developed for aerospace applications, are being used more and more in the field of power generation once dominated by steam turbines.

Steam Turbine Principle

The steam energy is converted mechanical work by expansion through the turbine. The expansion takes place through a series of fixed blades (nozzles) and moving blades each row of fixed blades and moving blades is called a stage. The moving blades rotate on the central turbine rotor and the fixed blades are concentrically arranged within the circular turbine casing which is substantially designed to withstand the steam pressure. On large output turbines the duty too large for one turbine and a number of turbine casing/rotor units are combined to achieve the duty. These are generally arranged on a common centre line (tandem mounted) but parallel systems can be used called cross compound systems. There are two principles used for design of turbine blades: The Impulse Blading and The Reaction Blading.

Impulse Blading

The impulse blading principle is that the steam is directed at the blades and the impact of the steam on the blades drives them round. The day to day example of this principle is the pelton wheel. In this type of turbine the whole of the stage pressure drop takes place in the fixed blade (nozzle) and the steam jet acts on the moving blade by impinging on the blades.

Fig.- Impulse & Reaction blading

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Fig.-Velocity diagram impulse turbine stage

z represents the blade speed , V r represents the relative velocity, V wa & V wb- represents the tangential component of the absolute steam in and steam out velocities The Power developed per stage = Tangential force on blade x blade speed.

Power /stage= (V w a - V wb).z/1000 kW per kg/s of steam

Reaction Blading

The reaction blading principle depends on the blade diverting the steam flow and gaining kinetic energy by the reaction. The Catherine wheel (firework) is an example of this principle. For this turbine principle the steam pressure drop is divide between the fixed and moving blades.

Fig.- Velocity diagram reaction turbine stage

z represents the blade speed , V r represents the relative velocity, V wa & V wb- represents the tangential component of the absolute steam in and steam out velocities

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The Power developed per stage = Tangential force on blade x blade speed.

Power /stage= (V w a - V wb).z/1000 kW per kg/s of steam

The blade speed z is limited by the mechanical design and material constraints of the blades.

2.9 THE RANKINE CYCLE

Fig.- The Rankine cycle

The figure above is known as a T-s diagram (Temperature versus Entropy).

Step 1 to 2 represents the work done by the boiler.

Step 2 to 3 represents the work done by the turbine.

Step 3 to 4 represents the work done by the condenser.

Step 4 to 1 represents the work done by the pump.

There are three types of steam turbines: condensing, non-condensing, and extraction. Condensing turbines are not used for combined heat and power applications and therefore will not be addressed here. Non-condensing steam turbines are also referred to as “back pressure” steam turbines. Here, steam is expanded over a turbine and the exhaust steam is used for to meet a facilities steam needs. The steam is expanded until it reaches a pressure that the facility can use. The figure below schematically shows the process of a back pressure steam turbine.

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The various energy streams flowing in a simple steam turbine system are as indicated in the diagram below. It is clear that the working fluid is in a closed circuit apart from the free surface of the hot well. Every time the working fluid flows at a uniform rate around the circuit it experiences a series of processes making up a thermodynamic cycle. The complete plant is enclosed in an outer boundary and the working fluid crosses inner boundaries (control surfaces). The inner boundary defines a flow process. The various identifiers represent the various energy flows per unit mass flowing along the steady-flow streams and crossing the boundaries. This allows energy equations to be developed for the individual units and the whole plant. When the turbine system is operating under steady state conditions the law of conservation of energy dictates that the energy per unit mass of working agent entering any system boundary must be equal to the rate of energy leaving the system boundary.

Fig.-Steady Flow Energy Equations

Boiler

The energy streams entering and leaving the boiler unit are as follows:

F + A + h d = h 1 + G + hl b hence F + A = G + h 1 - h d + hl b

Turbine

The energy streams entering and leaving the turbine are as follows:

h 1 = T + h 2 + hl t hence 0 = T - h 1 + h 2 + hl t

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Condenser Unit

The energy streams entering and leaving the condenser unit are as follows:

W i + h 2 = W o + h w + hl c hence W i = W o + h w - h 2 + hl c

Feed Water System

The energy streams entering and leaving the Feed Water System are as follows:

h w + d e + d f= h d + hl f hence d e + d f = - h w + h d + hl

The four equations on the right can be arranged to give the energy equation for the whole turbine system enclosed by the outer boundary.

That is the energy of the fuel (F) per unit mass of the working agent (water) is equal to the sum of

- The mechanical energy available from the turbine minus that used to drive the pumps

[T - (d e+ d f) ].

- The energy leaving the exhaust using the air temperature as the datum.

- The energy gained by the water circulating through the condenser [W o - W i]

- The energy gained by the atmosphere surrounding the plant [Σ hl]

The overall thermal efficiency of a steam turbine plant can be represented by the ratio of the net mechanical energy available to the energy within the fuel supplied.

Hence, this is about the working of a steam turbine.

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CHAPTER 3

THE GAS TURBINE

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3.1 MANUFACTURING OF A GAS TURBINE (JET ENGINE)

Building and assembling the components of a jet engine takes about two years, after a design and testing period that can take up to five years for each model. The research and development phase is so protracted because the engines are so complex: a standard Boeing 747 engine, for example, contains almost 25,000 parts.

Fig.- Materials used in a typical gas turbine jet engine

BUILDING COMPONENTS

(a) Fan blade

Turbine blades rotate due to combustion of fuel in combustion chamber. They undergo various forces, stresses and other conditions such as:

• Mechanical forces • Creep • Fatigue • Thermo-Mechanical fatigue • High temperature environment

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• Oxidation, Hot corrosion

Ni-based super alloys are used for manufacturing Turbine Blades. The Various stages involved in manufacturing of these blades are:

• HPTR (casting) • LPTR (forging) • 1st nozzle vane (casting) • 2nd nozzle vane (casting) • Stator blades (forging)

(b) Compressor disc

The disc, the solid core to which the blades of the compressor are attached, resembles a big, notched wheel. It must be extremely strong and free of even minute imperfections, as these could easily develop into fractures under the tremendous stress of engine operation. For a long time, the most popular way to manufacture the disc entailed machine-cutting a metal blank into a rough approximation of the desired shape, then heating and stamping it to precise specifications (in addition to rendering the metal malleable, heat also helps to fuse hairline cracks). Today, however, a more sophisticated method of producing discs is being used by more and more manufacturers.Called powder metallurgy, it consists of pouring molten metal onto a rapidly rotating turntable that breaks the metal into millions of microscopic droplets that are flung back up almost immediately.

Turbine blades are made by forming wax copies of the blades and then immersing the copies in a ceramic slurry bath. After each copy is heated to harden the ceramic and melt the wax, molten metal is poured into the hollow left by the melted wax. Due to the table's spinning. As they leave the table, the droplets' temperature suddenly plummets (by roughly 2,120 degrees Fahrenheit—1,000 degrees Celsius—in half a second), causing them to solidify and form a fine-grained metal powder. The resulting powder is very pure because it solidifies too quickly to pick up contaminants.

In the next step, the powder is packed into a forming case and put into a vacuum. Vibrated, the powder sifts down until it is tightly packed at the bottom of the case; the vacuum guarantees that no air pockets develop. The case is then sealed and heated under high pressure (about 25,000 pounds per square inch). This combination of heat and pressure fuses the metal particles into a disc. The disc is then shaped on a large cutting machine and bolted to the fan blades.

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(c) Compressor blades

Casting, an extremely old method, is still used to form the compressor blades. In this process, the alloy from which the blades will be formed is poured into a ceramic mold, heated in a furnace, and cooled. When the mold is broken off, the blades are machined to their final shape.

(d) Combustion chamber

Combustion chambers must blend air and fuel in a small space and work for prolonged periods in extreme heat. To accomplish this, titanium is alloyed to increase its ductility—its ability to formed into shapes. It is then heated before being poured into several discrete and very complex, segment molds.

(e) Turbine disc and blades

The turbine disc is formed by the same powder metallurgy process used to create the compressor disc. Turbine blades, however, are made by a somewhat different method than that used to form compressor blades, because they are subjected to even greater stress due to the intense heat of the combustor that lies just in front of them. First, copies of the blades are formed by pouring wax into metal molds. Once each wax shape has set, it is removed from the mold and immersed in a ceramic slurry bath, forming a ceramic coating about .25-inch (.63-centimeter) thick. Each cluster is then heated to harden the ceramic and melt the wax. Molten metal is now poured into the hollow left by the melted wax. The internal air cooling passages within each blade are also formed during this stage of production.

The metal grains in the blade are now aligned parallel to the blade by a process called directional solidifying. The grain direction is important because the turbine blades are subjected to so much stress; if the grains are aligned correctly, the blade is much less likely to fracture. The solidifying process takes place in computer-controlled ovens in which the blades are carefully heated according to precise specifications. The metal grains assume the correct configuration as they cool following their removal from the ovens.

The next and final stages in preparing turbine blades are machine-shaping and either laser drilling or spark erosion. First, the blade is honed to the final, desired shape through a machining process. Next, parallel lines of tiny holes are formed in each blade as a supplement to the interior cooling passageways. The holes are formed by either a small

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laser beam or by spark erosion, in which carefully controlled sparks are permitted to eat holes in the blade.

(f) Exhaust system

The inner duct and the afterburners of the exhaust system are molded from titanium, while the outer duct and the nacelle (the engine casing) are formed from Kevlar. After these three components have been welded into a subassembly, the entire engine is ready to be put together.

(g) Final assembly

Engines are constructed by manually combining the various subassemblies and accessories. An engine is typically built in a vertical position from the aft end forward, on a fixture that will allow the operator to manipulate the engine easily during build up. Assembly begins with bolting the high pressure turbine (that closest to the combustor) to the low-pressure turbine (that furthest from the combustor). Next, the combustion chamber is fastened to the turbines. One process that is used to build a balanced turbine assembly utilizes a CNC (Computer Numerically Controlled) robot capable of selecting, analyzing, and joining a turbine blade to its hub. This robot can determine the weight of a blade and place it appropriately for a balanced assembly.

Once the turbines and combustion chamber have been assembled, the high and low pressure compressors are attached. The fan and its frame comprise the forward most subassembly, and they are connected next. The main drive shaft connecting the low pressure turbine to the low pressure compressor and fan is then installed, thus completing the engine core.

After the final subassembly, the exhaust system, has been attached, the engine is ready to be shipped to the aircraft manufacturer, where the plumbing, wiring, accessories, and aerodynamic shell of the plane will be integrated.

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3.2 PROCESS CHART FOR MANUFACTURING

• BILLET CHART • VISUAL INSPECTION • PLUGGING OF INT.CAVITY • GRINDING • TRANSFERING OF MELT NO. • GRINDING • TRANSFERING OF MARKS • VISUAL INSPECTION • GRINDING • MILLING • BENCH WORK • INSPECTION • POLISHING • ZYGLO INSPECTION • BUFFING • BLOWING • INT.INSPECTION • CUTTING OF SPECIMEN • SET MAKING • PLUGGING OF INT.CAVVITY • INSPECTION • ALITIZING • INSPECTION • ULTRASONIC WASHING • INSPECTION • DIFFUSION ANNEALING • INSPECTION • REMOVAL OF PLUG STRIPS • PLUGGING OF CAVITY • GRINDING • BENCH WORK • ROUGH MILLING • FINAL MILLING • BENCH WORK

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In a gas turbine engine manufacture, the various parts are made individually as part of subassemblies; the subassemblies then come together to form the whole engine. One such part is the fan blade, situated at the front of the engine. Each fan blade consists of two blade skins produced by shaping molten titanium in a hot press. When removed, each blade skin is welded to a mate, with a hollow cavity in the center. To increase the strength of the final product, this cavity is filled with a titanium honeycomb.

3.3 WORKING OF A GAS TURBINE

Fig.- gas turbine

A gas turbine, also called a combustion turbine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between. Gas turbine engines have a great power-to-weight ratio compared to reciprocating engines. That is, the amount of power you get out of the engine compared to the weight of the engine itself is very good. Gas turbine engines are smaller than their reciprocating counterparts of the same power. The main disadvantage of gas turbines is that, compared to a reciprocating

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engine of the same size, they are expensive. Because they spin at such high speeds and because of the high operating temperatures, designing and manufacturing gas turbines is a tough problem from both the engineering and materials standpoint. Gas turbines also tend to use more fuel when they are idling, and they prefer a constant rather than a fluctuating load. That makes gas turbines great for things like jet aircraft and power plants, but explains why you don't have one under the hood of your car.

3.4 THEORY OF OPERATION

Energy is added to the gas stream in the combustor, where fuel is mixed with air and ignited. In the high pressure environment of the combustor, combustion of the fuel increases the temperature. The products of the combustion are forced into the turbine section. There, the high velocity and volume of the gas flow is directed through a nozzle over the turbine's blades, spinning the turbine which powers the compressor and, for some turbines, drives their mechanical output. The energy given up to the turbine comes from the reduction in the temperature and pressure of the exhaust gas. This Energy can be extracted in the form of shaft power, compressed air or thrust or any combination of these and used to power aircraft, trains, ships, generators, or even tanks.

Gasses passing through an ideal gas turbine undergo three thermodynamic processes. These are isentropic compression, isobaric (constant pressure) combustion and isentropic expansion. Together these make up the Brayton cycle.

In a practical gas turbine, gases are first accelerated in either a centrifugal or radial compressor. These gases are then slowed using a diverging nozzle known as a diffuser; these processes increase the pressure and temperature of the flow. In an ideal system this is isentropic. However, in practice energy is lost to heat, due to friction and turbulence. Gases then pass from the diffuser to a combustion chamber, or similar device, where heat is added. In an ideal system this occurs at constant pressure (isobaric heat addition). As there is no change in pressure the specific volume of the gases increases. In practical situations this process is usually accompanied by a slight loss in pressure, due to friction. Finally, this larger volume of gases is expanded and accelerated by nozzle guide vanes before energy is extracted by a turbine. In an ideal system these are gases expanded isentropically and leave the turbine at their original pressure. In practice this process is not isentropic as energy is once again lost to friction and turbulence.

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If the device has been designed to power a shaft as with an industrial generator or a turboprop, the exit pressure will be as close to the entry pressure as possible. In practice it is necessary that some pressure remains at the outlet in order to fully expel the exhaust gasses. In the case of a jet engine only enough pressure and energy is extracted from the flow to drive the compressor and other components. The remaining high pressure gasses are accelerated to provide a jet that can, for example, be used to propel an aircraft.

Fig.- process diagram for the working of a gas turbine

As with all cyclic heat engines, higher combustion temperatures can allow for greater efficiencies. However, temperatures are limited by ability of the steel, nickel, ceramic, or other materials that make up the engine to withstand high temperatures and stresses. To combat this many turbines feature complex blade cooling systems.

As a general rule, the smaller the engine the higher the rotation rate of the shaft must be to maintain tip speed. Blade tip speed determines the maximum pressure ratios that can be obtained by the turbine and the compressor. This in turn limits the maximum power and efficiency that can be obtained by the engine. In order for tip speed to remain constant, if the diameter of a rotor is reduced by half, the rotational speed must double. For example large Jet engines operate around 10,000 rpm, while micro turbines spin as fast as 500,000 rpm.More sophisticated turbines (such as those found in modern jet engines) may have multiple shafts (spools), hundreds of turbine blades, movable stator blades, and a vast system of complex piping, combustors and heat exchangers.

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Thrust bearings and journal bearings are a critical part of design. Traditionally, they have been hydrodynamic oil bearings, or oil-cooled ball bearings. These bearings are being surpassed by foil bearings, which have been successfully used in micro turbines and auxiliary power units.

Air breathing jet engines are gas turbines optimized to produce thrust from the exhaust gases, or from ducted fans connected to the gas turbines. Jet engines that produce thrust primarily from the direct impulse of exhaust gases are often called turbojets, whereas those that generate most of their thrust from the action of a ducted fan are often called turbofans or (rarely) fan-jets. More sophisticated turbojets are also built, where their thrust and light weight are sufficient to power large aircraft.

3.5 THE BRAYTON CYCLE

Fig.- P-V & T-S diagram of a Brayton cycle

Since fresh air enters the compressor at the beginning and exhaust are thrown out at the end, this cycle is an open cycle.

By utilizing the air-standard assumptions, replacing the combustion process by a constant pressure heat addition process, and replacing the exhaust discharging process by a constant pressure heat rejection process, the open cycle described above can be modeled as a closed cycle, called ideal Brayton cycle. The ideal Brayton cycle is made up of four internally reversible processes as shown in the figure above .They are :

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1-2 Isentropic compression (in a compressor) 2-3 Constant pressure heat addition 3-4 Isentropic expansion (in a turbine) 4-1 Constant pressure heat rejection

In an ideal Brayton cycle, heat is added to the cycle at a constant pressure process (process 2-3).

qin = h3 - h2 = cP(T3 - T2)

Heat is rejected at a constant pressure process (process 4 -1).

qout = h4 - h1 = cP(T4 - T1)

Then the thermal efficiency of the ideal Brayton cycle under the cold air-standard assumption is

given as:

Process 1-2 and process 3-4 are isentropic processes, thus,

Since P2 = P3 and P4 = P1,

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Considering all the relations above, the thermal efficiency becomes,

Where, rP = P2/P1 is the pressure ratio and k is the specific heat ratio.

In most designs, the pressure ratio of gas turbines range from about 11 to 16.

3.6 ACTUAL GAS TURBINE CYCLE

The actual gas-turbine cycle is different from the ideal Brayton cycle since there are irreversibilities. Hence, in an actual gas-turbine cycle, the compressor consumes more work and the turbine produces less work than that of the ideal Brayton cycle. The irreversibilities in an actual compressor and an actual turbine can be considered by using the adiabatic efficiencies of the compressor and turbine. They are:

Another difference between the actual Brayton cycle and the ideal cycle is that there are pressure drops in the heat addition and heat rejection processes.

Hence, this is about the working of a gas turbine.

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CHAPTER 4

THE WIND TURBINE

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4.1 INTRODUCTION

A wind turbine installation consists of the necessary systems needed to capture the wind's energy, point the turbine into the wind, convert mechanical rotation into electrical power and other systems to start, stop, and control the turbine.

4.2 PARTS OF A WIND TURBINE A typical wind turbine consists of three major systems.

a) Nacelle

The Nacelle contains the most important parts of a wind turbine, such as the main shaft, bearings, gear-box, generator, brake, nacelle frame, hydraulic systems for brakes and lubrication, cooling systems, Transmission system, Generator, Control & safety systems. These are very important and they play a crucial role in the operating a wind turbine. b) Blades & Rotor System

The rotor system captures wind energy and converts into rotational kinetic energy. This is accomplished through blades that connect to a rotor hub that is connected to the main shaft.

c) Tower

Tower is the “body” of a wind turbine. The blades and the nacelle are mounted on the tower, which provides a stronger base and support. A tower typically supports the nacelle with bearings in which a horizontal rotation axis with blades can rotate. The tower is supported on the ground with a base.

Other sub-components are as follows:

Generator and Gear Box Lightning protection rod Anemometer Controller Rotor High speed and Low speed shafts Yaw drive and Yaw rotor Braking system Power cable

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The manufacturing of a wind turbine is a typical assembly process involving the assembly of the above mentioned components. These components are especially manufactured/ordered/procured from outside sources & finally gathered into one place. The main manufacturing and assembly process is as follows:

4.3 PRODUCTION OF THE NACELLE

Fig.- The Nacelle

The nacelle, like the tower, is manufactured off site in a factory. Unlike the tower, however, it is also put together in the factory. It houses the frame, gear box, generator, drive shafts and controls. It is usually made of fiber reinforced plastic or fiber glass etc. The whole process involves several steps as follows: a) Frame Assembly

Firstly the yaw system is assembled with yaw motors, hydraulic components, columns. This assembly then undergoes a rotational test and is connected to the rear frame. Then the service crane is installed along with the rail beams. Cables are run to the control cabinet and this part of assembly is sent to the next stage for gear box assembly.

b) Gear Box Assembly

The frame assembly is placed within the lower housing. Then the main gear box, power shaft, power transformer are attached as shown. Gearbox is a standard component in non-direct drive turbines. The purpose is to increase the rotational speed. There are different types of gears in a gearbox: Planetary gears and spur/helical gears.

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c) Generator Assembly Here the generator is assembled with all other electrical connections in the cabinet.

Then the nacelle is verified and checked. If it passes the verification test, then it is sent to the final phase of assembly. d) Upper Housing Assembly

After all the verification tests are passed, the upper housing is assembled and the nacelle is ready to be sent for installation.

4.4 PRODUCTION OF THE BLADES

The Manufacturing of Wind turbine blade consists of five stages. They are a) Manufacturing the BEAM of the blade b) Manufacturing of the SHELL ‘S of the blade c) Assembly of the BEAM & SHELLS d) Curing of the blade e) Trimming & Polishing of the blade

Fig.- % weight of components in a blade

Wood, Foam, Fiber Glass, Epoxy Resin are the important materials used in the

manufacturing of a wind turbine blade. There are two methods used for the manufacturing process. They are:

a) Epoxy Prepregmolding b) Vacuum assisted Resin Transfer Molding(VARTM)

In Epoxy Prepregmolding process, fiber glass soaked with epoxy resin is laid into

layers and placed in a mould. These layers are pressed and cured at higher temperatures in a kiln.

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In Vacuum Assisted Resin Transfer Molding process, fiberglass is laid in a pre-form and placed in a closed mold. In this mold, epoxy resin is sucked in using vacuum and then cured to form a blade. VARTM has resulted in a simpler process, although it is a time intensive process.

A typical manufacturing process of a blade involves the following steps :

a) Manufacture of the BEAM

The fiber-glass and carbon fiber materials are impregnated with epoxy resin base.Several cloth lengths are cut and placed in the mould. Then the setup is subjected to a curing process.

Fig: Manufacturing of a Beam

(b) Manufacture of the SHELL

Paint is applied as a protective coating on the blade. Then fiber-glass is used to manufacture the shell by following the same process as that of the beam.

Fig: Manufacturing of a shell

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(c) Assembly and Curing process

After the two shells are manufactured, the beam is placed in the middle of the two shells and they are glued together. Then the entire assembly unit is placed in a shell mould and passed through a kiln for one last time. This ensures that the blade is solid and compact.

(d) Trimming and polishing of the blade

After the curing process, the blade is removed from the mould and sent to the polishing & finishing section. Here the leading edge and trailing edge are finished and polished for any irregularities. Then the blade is sent to inspection and later to assembly.

4.5 MANUFACTURING THE TOWER OF THE WIND TURBINE A typical wind turbine tower would be in the height range of 14 - 29 meters.

Depending on the height the tower is made up of steel rings .The manufacturing of the TOWER involves five stages

a) Reception and Quality control of the steel plates b) Shaping of the steel plates c) Welding d) Shot peening, painting, drying e) Assembly of the auxiliary equipment

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a) Reception and quality control of steel plates

The cylinders/rings forming the wind turbine tower are made from steel plates. These steel plates are manufactured by flame cutting & priming. In flame cutting, an oxy acetylene gas is directed through a torch, thereby producing a controlled flame. Priming is an anti-corrosion method in which the metal surface is treated and cleansed with chemicals

b) Shaping of steel plates The basic shaping process by which the slab is formed into the final product is rolling.

The sheets are inserted in a machine with three large rollers that shape the rings.

(c) Welding of the rings

These rings are submerged arc welded forming sections of different lengths. Submerged arc welding (SAW) is a common arc welding process which requires a continuously fed consumable solid or tubular electrode. The molten weld and the arc zone

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are protected from atmospheric contamination by being “submerged” under a blanket of granular fusible flux consisting of lime, silica, manganese oxide, calcium fluoride, and other compounds.

Fig: Welding of the rings

(d) Shot peening, painting and drying

Once the tower plating is finished, it is then given a surface treatment, consisting of a double-steel shot peening and three coats of paint. This provides a C-5 level protection. Shot peening a surface spreads it plastically, causing changes in the mechanical properties of the surface. Shot peening is often used to relieve tensile stresses built up in the grinding process and replace them with beneficial compressive stresses. Depending on the part geometry, part material, shot material, shot quality, shot intensity, shot coverage, shot peening can increase fatigue life up to 1000%.It is a cold working process. After this, the structure is placed inside the painting and drying tunnel. Note: C-5 level protection indicates protection from very high risk environments, such as Industrial areas with high humidity and aggressive atmospheres, Coastal and offshore areas with high salinity.

(e) Assembly of the auxiliary equipment

Once the tower is dry, all the service elements & equipment (such as platforms,mountings and ladders) are mounted on it.

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4.6 DESCRIPTION OF A WIND TURBINE BLADE

The front and rear sides of a wind turbine rotor blade have a shape roughly similar to that of a long rectangle, with the edges bounded by the leading edge, the trailing edge, the blade tip and the blade root. The blade root is bolted to the hub. The radius of the blade is the distance from the rotor shaft to the outer edge of the blade tip. Some wind turbine blades have moveable blade tips as air brakes, and one can often see the distinct line separating the blade tip component from the blade itself. If a blade were sawn in half, one would see that the cross section has streamlined asymmetrical shape, with the flattest side facing the oncoming air flow or wind. This shape is called the blade aerodynamic profile

Fig.- description of a Wind turbine blade

4.7 THE AERODYNAMIC PROFILE

Aerodynamics is the science and study of the physical laws of the behavior of objects in an air flow and the forces that are produced by air flows.

The shape of the aerodynamic profile is decisive for blade performance. Even minor alterations in the shape of the profile can greatly alter the power curve and noise level. Therefore a blade designer does not merely sit down and outline the shape when designing a new blade. The shape must be chosen with great care on the basis of past experience. For this reason, blade profiles were previously chosen from a widely used catalogue of airfoil profiles developed in wind tunnel research by NACA (The United States National Advisory Committee for Aeronautics) around the time of the Second World War.

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Fig.- NACA blade profiles

The NACA 44 series profiles were used on older wind turbines (up to and including the 95 kW models). This profile was developed during the 1930’s, and has good all-round properties, giving a good power curve and a good stall. The blade is tolerant of minor surface imperfections, such as dirt on the blade profile surface. The NACA 63 profiles developed during the 1940’s. These have slightly different properties than the NACA 44 series. The power curve is better in the low and medium wind speed ranges, but drops under operation at higher wind speeds. Likewise this profile is more sensitive with regard to surface dirt. In certain climate zones with little rain, accumulated dirt, grime and insect deposits may impair and reduce performance for longer periods. The LM 19 blades, specifically developed for wind turbines, used on the 500 kW, have completely new aerodynamic profiles and are therefore not found in the NACA catalogue. These blades were developed in a joint research project some years ago and further developed and wind tunnel tested by FFA (The Aerodynamic Research Institute of The Swedish Ministry of Defense).

THE CHANGE OF FORCES ALONG THE BLADE

Fig.-Airflow around a blade profile, near the blade tip The drawings shown previously mainly illustrate the air flow situation near the blade tip. In principle these same conditions apply all over the blade, however the size of the forces and their direction change according to their distance to the tip. We will use as an example

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the cross section near the blade tip of a 450 kW wind turbine operating in a wind speed of 10 m/s In the stationary situation (A) in the left hand drawing, wind pressure is 80 N/m^2 with a velocity of 10m/s. The force becomes slightly larger than the force at the tip, as the blade is wider at the root. The pressure is once again roughly at a right angle to the flat side of the blade profile, and as the blade is more twisted at the root; more of the force will be directed in the direction of rotation, than was the case at the tip. On the other hand the force at the root has not so great a torque-arm effect in relation to the rotor axis and therefore it will contribute about the same force to the starting torque as the force at the tip. During the operational situation as shown in the center drawing (B), the wind approaching the profile is once again the sum of the free wind ‘v’ of 10 m/s and the head wind from the blade rotational movement through the air. The head wind near the blade root of the 450 kW wind turbine is about 15 m/s and this produces a resulting wind over the profile of 19 m/s. This resulting wind will act on the blade section with a force of about 500 N/m^2.

In the drawing on the right (C) force is broken down into wind pressure against the tower, and the blade driving force in the direction of rotation. In comparison with the blade tip the root section produces less aero dynamic forces during operation, however more of these forces are aligned in the correct direction, that is, in the direction of rotation. The change of the size and direction of these forces from the tip in towards the root, determine the form and shape of the blade. Head wind is not so strong at the blade root, so therefore the pressure is likewise not so high and the blade must be made wider in order that the forces should be large enough. The resulting wind has a greater angle in relation to the plane of rotation at the root, so the blade must likewise have a greater angle of twist at the root. It is important that the sections of the blade near the hub are able to resist forces and stresses from the rest of the blade. Therefore the root profile is both thick and wide, partly because the thick broad profile gives a strong and rigid blade and partly because greater width, as previously mentioned, is necessary on account of the resulting lower wind speed across the blade. On the other hand, the aerodynamic behavior of a thick profile is not so effective. Further out along the blade, the profile must be made thinner in order to produce acceptable aerodynamic properties, and therefore the shape of the profile at any given place on the blade is a compromise between the desire for strength (the thick wide profile) and the desire for good aerodynamic properties (the thin profile) with the need to avoid high aerodynamic stresses (the narrow profile).

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WHEN THE WIND SPEED CHANGES

The description so far was made with reference to an example where wind speed was at a constant 10 m/s. We will now study what happens during alterations in the wind speed. In order to understand blade behavior at different wind speeds, it is necessary to understand a little about how lift and drag change with a different angle of attack. This is the angle between the resulting wind ‘w’ and the profile chord. In the drawing below, the angle of attack is called ‘a’ and the setting angle is called ‘b’. The setting angle has a fixed value at any one given place on the blade, but the angle of attack will grow as the wind speed increases.

Fig.- relationship between coefficients of lift & drag, angle of attack

During the change of wind speed from 5 to 15 m/s there is a significant increase in lift, and this increase is directed in the direction of rotation. Therefore power output of the wind turbine is greatly increased from 15 kW to 475 kW. During the change of wind speed from 15 to 25 m/s, there is a drop in lift accompanied by an increase in drag. This lift is even more directed in the direction of rotation, but it is opposed by drag and therefore output will fall slightly to 425 kW.

PROBLEM DESCRIPTION

The first problem is common to all control and safety systems: A wind turbine is without constant supervision, apart from the supervision of the control system itself. The periods between normal qualified maintenance schedules is about every 6 months, and in the intervening 4,000 hours or so the control system must function trouble-free, whether the

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wind turbine is in an operational condition or not. In almost every other branch of industry there is a much higher degree of supervision by trained and qualified staff. On factory production lines, operatives are normally always present during production.

POWER GENERATION USING A WIND TURBINE

Wind turbines convert the kinetic energy of the wind into mechanical energy. On the top of the large turbines there's a generator that converts the mechanical energy into electrical energy. Gears connect the generator to the turbine shaft and the electronic systems of the turbine regulate the motion in order to produce electric current of a specific frequency (60 Hz or 50 Hz depending on which country) and voltage.

4.8 POWER PRODUCED BY A WIND TURBINE

The formula used to calculate the wind power produced by a wind turbine

P = c x d x (D^2) x (v^3)

P: Power produced in Watts [W]

c: power coefficient

d: density of wind in [kg/m^3]

D: turbine blade diameter [m]

v: velocity of wind [m/s^2]

The constant ‘c’ depends on the system of units used and the specific variables in the equation. The formula implies that once the wind speed doubles, the wind power increases by a factor of eight! Furthermore, if the diameter of the blades is doubled, the power increases by a factor of four! The faster the wind and the bigger the blades, the more power input we can get.

4.9 EFFICIENCY OF A WIND TURBINE

Efficiency is generally determined by the ratio of energy output to energy input. According to the Second Law of Thermodynamics, the efficiency of a device can never reach 100%; there is always some loss in energy and this applies to wind turbines too. For a wind 53

turbine the energy input is the kinetic energy of the wind and the energy output is mechanical rotating energy. According to Betz's law (1919), the wind turbine efficiency of converting the kinetic energy of the wind into mechanical rotating energy can never exceed 59%. This is the best possible conversion rate, as was proven by a physicist.

Efficiency is also determined by the design of the windmill. A given design has peak efficiency at a certain wind speed. Below or above this specific speed value, the efficiency may stay the same or even drop down. For example, if a wind turbine has a top efficiency of 50% at a given wind speed of 10 meters per second, it will probably drop to 45% or lower if the wind velocity increases or decreases. Another thing to keep in mind is that the generator converting mechanical to electrical energy, has conversion efficiency as well, which is well below 100%. This fact lowers the overall efficiency of power conversion to a smaller value of 20-30%. The design of the blades and the turbine play an important role in a windmill’s performance. The design should satisfy the load requirements and generate energy at the lowest cost. This can be done by optimizing the performance characteristics of the windmill. Power output versus wind speed or power output verses rotor angular velocity are two important characteristics that are taken into consideration during optimization. Enhancing this improves the overall quality of the power produced.

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CHAPTER 5

RESULTS AND DISCUSSIONS

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5.1 RESULTS

In the previous chapters, we discussed about the working processes of steam, gas and wind turbines with their manufacturing procedure too. Based on these, we calculated the efficiencies of Gas and steam turbines which are as follows:

5.2 PROBLEM ON STEAM TURBINE EFFICIENCY

A cycle steam power plant is to be designed for a steam temperature at turbine inlet of 3600C and an exhaust pressure of 0.08 bar. After isentropic expansion of steam in the turbine, the moisture content at the turbine exhaust is not to exceed 15%. Determine the greatest allowable steam pressure at the turbine inlet, and calculate the Rankine cycle efficiency for these steam conditions. Estimate also the mean temperature of heat addition.

Sol: As state 2s (Fig shown below), the quality and pressure are known.

S2s = Sf + x2s sfg = 0.5926+0.85(8.2287-0.5926)

= 7.0833 kJ/kg K

Since S1 = s2s

S1 = 7.0833 kJ/kg K

At state 1, the temperature and entropy are thus known. At 3600C, Sg = 5.0526 kJ/kg K, which is less than S1. So from the table of superheated steam, at t1 = 3600C and S1 = 7.0833 kJ/kg K, the pressure is found to be 16.832 bar (by interpolation).

The greatest allowable steam pressure is

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p1 = 16.832 bar

h1 = 3165.54 kJ/kg

h2s = 173.88 + 0.85 x 2403.1 = 2216.52 kJ/kg

h3 = 173.88 kJ/kg

h4s - h3 = 0.001 x (16.83 – 0.08) x 100 = 1.675 kJ/kg

h4s = 175.56 kJ/kg

Q1 = h1 – h4s = 3165.54 – 175.56 = 2990 kJ/kg

WT = h1 – h2s = 3165.54 – 2216.52 = 949 kJ/kg

Wp = 1.675 kJ/kg

cycle = Wnet/Q1 = 947.32/2990 = 0.3168 or 31.68%

Mean temperature of heat addition

Tm1 = (h1 – h4s)/(S1 – S4s) = 2990/(7.0833 – 0.5926) = 460.66 K = 187.510C.

5.3 PROBLEM ON GAS TURBINE EFFICIENCY

A gas turbine operates on pressure ratio of 6. The outlet air temperature of the compressor is 300 K and the air entering the turbine is at a temperature of 5770 C. If the volume rate of air entering the compressor is 240 m3 per second. Calculate the net power output of the cycle. Also compute it’s efficiency. Assume that the cycle operates under ideal conditions.

Sol: T2= T(P2/P1)= 500.81 K

T4=T3/(P3/P4)r=509.18 K

WC=201.81 KJ/KG

WT=342.5 KJ/KG

WN=140.72 KJ/KG

Q1=350.94 KJ/KG

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P1V1=NRT1

(P1/RT1 )=(M/V)= e1 =[(1 x 105)/(287 x 300)]= 1.61 KG/M3

Power output= 140.72 x 240 x 1.61= 39.21 MW

Efficiency= WN/Q1= (140.72/350.94)= 40.16%

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Conclusion:

The work presented in this report is to bring an overview of the various types of turbines, their operation and the manufacturing processes involved in making these turbines. We mainly concentrated on the manufacturing and working processes.

As of today, wind turbines are the most eco-friendly turbines, compared to the steam and gas turbines. The wind is free and with modern technology it can be captured efficiently. Once the wind turbine is built, the energy it produces does not cause green house gases or other pollutants. The operating and maintenance cost of these turbines is low, compared to the steam and gas turbines. As the natural resources are depleting, there is a need to switch to wind power and other renewable sources of energy.

Further research in the manufacturing area can reduce the overall cost and incorporating composites and alloy materials in the manufacturing area can increase the working efficiency of the wind turbines.

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References:

[1] Steam Turbines: A Book of Instruction for the Adjustment and Operation of the Principal Types of this Class of Prime Movers by Hubert E. Collins.

[2] Wind Energy Engineering by Pramod Jain. [3] Power Plant Engineering - A.K. Raja, Amit Prakash Srivastava. [4] Protective coatings for turbine blades- Y. Tamarin, ASM International. [5] Bharat Heavy Electrical Electricals Ltd.(BHEL) [6] Rolls-Royce Corporation. [7] www.siemens.com [8] www.nasa.gov [9] Vestas Wind Systems [10] Suzlon Energy [11] Bonus Energy [12] web.mit.edu [13] web.me.unr.edu [14] ecourses.ou.edu

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