However, it is quite complex in operation. Its operation is characterized by the necessity of flame stabilization or anchoring, that is the smooth continuous ignition process. In order to anchor the flame and secure complete combustion, a strong reverse flow of heated air and gas must be produced.
One of the advantages of having individual combustion chambers is that replacement of combustion chambers and liners is possible without complete engine disassembly.
Fig. II.3.2. The annular combustion chamber
Annular combustion chamber has a variety of configurations as designed by individual engine manufacturers. Basically, an annular combustion chamber is a single chamber completely surrounding the engine. Fig. II.2.2 illustrates this type of combustion chamber. Fuel is fed through fuel nozzles, which distribute the flame around the circumference of the chamber. The chamber outer case (liner) is punched with uniform rows of holes, which diffuse the compressed air and help to maintain an efficient flame pattern.
The annular chamber has important advantages: minimized friction between the gas and the combustor surfaces, improved annular pressure equalization and reduced weight, all adding to the efficiency of this combustor type.
Fig. II.3.3. The Cannular Combustion Chamber
The cannular (can-annular) combustion chamber is a combination of the “can” type and the “annular” type. Fig. II.2.3 illustrates a cannular combustor consisting of a number of combustion chambers or “cans” arranged evenly around the circumference of an annular chamber. Each separate chamber is in the form of a small chamber, in that there is an inner liner through which air flows to cool the inside of the chamber. Both the inner and outer liners are provided with holes for the entry of primary and secondary air.
Each combustion chamber is supplied with fuel through fuel nozzles mounted at the head of the chamber. The nozzles are attached to the fuel manifold. Primary air for fuel combustion enters the chambers through swirl vanes, arranged around the fuel nozzles, which contributes to good mixing of air and fuel and even burning. This combustor design secures complete burning of fuel, thus making it the most efficient combustor type.
Exercise24. Answer the questions on text 3.
1. What types of combustors are known to be mainly used for gas turbine engines? 2. Is a can-type combustor as simple in operation as in design? 3. What is the main characteristic feature of its operation?
4. What must be done to anchor the flame? 5. What advantages of the individual combustion chamber do you know? 6. Configuration of the annular combustion chamber depends on the choices of individual engine manufacturers, doesn’t it? 7. What kind of chamber is the annular combustor? 8. What is the function of fuel nozzles? 9. What do the holes, punched in the outer liner, serve for? 10. What makes the annular combustion chamber attractive for manufactures? 11. Which of the combustors is a combination of two types? 12. What is the peculiarity of its design? 13. How does the cannular combustion chamber secure complete burning of fuel and make it the most efficient combustor type?
Exercise 25. Say whether the following statements are true or false. Correct the false ones.
1. The design of the can-type or individual combustion chamber is rather complex, but it is quite simple in operation. 2. The main disadvantage of the annular combustion chamber is its very heavy weight and very high cost. 3. The annular chamber has minimized friction between the gas and the combustor surfaces. 4. Each separate chamber of the cannular combustion chamber is provided with inner liner through which heated gases flow to anchor the flame pattern. 5. The primary air for fuel combustion in the cannular combustion chamber enters the chamber through swirl vanes, arranged inside the fuel nozzles.
Providing a proper combustion process is very important as the fuel oxidation reaction is the only source of energy in the engine. Combustion takes place in a gas turbine with a stationary type of flame at approximately constant pressure.
The process of combustion consists of four steps, namely:
formation of the combustion mixture, which includes:
atomization of fuel
vaporization of fuel
provision of turbulence of the air flow;
ignition of the mixture;
flame movement or combustion propagation;
addition of the secondary air.
Introduction of the fuel and air into all types of combustors is similar. The compressed air entering the chamber is separated into the primary and secondary streams. The primary air will take part in the process of combustion. It will be mixed with fuel to provide a combustible mixture.
The primary or combustion air is fed into the chamber at low velocity. The turbulence of this air is provided by passing it through a series of holes. This turbulence helps in good mixing of air with fuel. The fuel is sprayed into the combustor through the fuel nozzles. The fuel nozzles are devices for injecting and atomizing the fuel. The number of them varies with the combustion chamber design. The fuel and air must be highly homogeneous. Before mixing can occur, the liquid fuel must be vaporized.
As the burning continues, secondary air is admitted to the combustor at increasing velocities to provide air for the completion of combustion and to reduce the temperature of the combustion products to a value permissible in the turbine. The completeness of combustion directly influences the heat release, and hence the fuel consumption of the engine.
.Exercise 28. Speak on:
General characteristics of combustion chambers and their types.
Can-type combustion chamber.
Annular combustion chamber.
Cannular combustion chamber.
II.4. TURBINE SECTION Exercise 1. Memorize the active vocabulary.
T. Today we’ll speak about the third main section of any gas turbine engine, namely, the turbine. Can you remind us its location and function?
S. The turbine is located after the nozzle diaphragm and its function is to extract power from a gas stream.
T. And what is this power used for?
S. As far as I remember it drives the compressor and accessories.
T. Right. It converts the kinetic (кінетична / кинетическая) or thermal (теплова / тепловая) energy of flowing fluid into useful rotational energy. By flowing fluid we mean air, hot gas, steam (пара / пар) or water. And what flowing fluid was used in first developed turbines?
S. I think it’s water.
T. Yes, quite right. Then comes steam turbine designed by British engineer. He efficiently used the energy of high-pressure steam. The construction of a steam turbine is rather simple. It consists of a rotating drum (барабан / барабан) with series of vanes mounted on it and the stator vanes redirecting the steam in between the moving ones. Can we say that a series of vanes mounted on a rotating drum is a prototype of a set of blades mounted on the drive shaft?
S. Yes, absolutely.
T. It’s correct. And what turbines are used in thermal and nuclear power plants ?
S. If I’m not mistaken, they are steam turbines.
T. That’s right. And as to gas turbines they are widely used in airplanes.
Exercise 3. Read and translate text 1. Tell what new information you have acquired after reading it.
Text 1. Turbine
Turbine is a rotating part of an engine which extracts energy from the combustion chamber gases, converting the kinetic and/or thermal energy of a flowing fluid into useful rotational energy. The working fluid may be air, hot gas, steam or water. This either pushes against a set of blades mounted on the drive shaft (impulse turbines) or turns the shaft by reaction when the fluid is expelled from nozzles (or nozzle-shaped vanes) around its circumference (reaction turbines). Water turbines were the first to be developed. In the 1880s Charles Algernon Parsons (1854-1931), a British engineer designed the first successful steam turbine, having realized that the efficient use of high-pressure steam demanded that its energy be extracted in a multitude of small stages. Steam turbines thus consist of a series of vanes mounted on a rotating drum with stator vanes redirecting the steam in between the moving ones. They are commonly used as marine engines and in thermal and nuclear power plants. Gas turbines are not as yet widely used except in airplanes.
Exercise 7. Work with a partner. Answer the questions on text 1.
1. What part of an engine is a turbine? 2. What does a turbine extract energy from? 3. What energy is converted into useful rotational energy? 4. What is the turbine used for? 5. What may be used as the working fluid for turbine operation? 6. How does the impulse turbine work? 7. What type of turbine turns the shaft by reaction when the fluid is expelled from nozzles around its circumference? 8. How does the reaction turbine work? 9. What did Ch. Algeron Parsons have to realize to design a successful steam turbine? 10. What do steam turbines consist of? 11. What do stator vanes do? 12. Where are steam turbine used? 13. What turbines are used in airplanes?
Exercise 8. Complete the following statements.
1. Turbines are used for converting energy of a flowing fluid into … . 2. The working fluid may be … . 3. The first turbines to be developed were … . 4. Ch. Algeron Parsons realized that the efficient use of high-pressure steam demanded that … . 5. Steam turbines consist of … . 6. Steam turbines are used as marine engines and in … . 7. … are widely used in airplanes.
Exercise 9. Write a summary of text 1 using the following terms:
Convert energy, working fluid, rotational energy, a set of blades, turbine stage, drive shaft, nozzle-shaped vanes, extract energy, rotating drum, gas turbines, redirect in between the moving vanes.
Exercise 10. Translate the text in writing and entitle it.
The turbine has the task of providing power to drive the compressor and accessories and to provide shaft power for a propeller or rotor through a coaxial shaft. It does this by extracting energy from the hot gases released from the combustion system and expanding them to a lower pressure and temperature. The turbine may consist of several stages, each employing one row of stationary guide vanes and one row of moving blades. The exhaust system passes the turbine discharge gases to atmosphere at a proper velocity, and in the required direction, to provide the resultant thrust.
Exercise 11. There are three different pronunciations of “ed” ending in the II-nd and III-rd forms of regular verbs.
Distribute the following verbs into 3 columns according to their pronunciation.