Inlet Ducts
Air inlet duct is normally considered an airframe part and made by the aircraft manufacturer. During flight operations it becomes very important to the overall jet engine performance, and will greatly influence jet engine thrust output. The faster the airplane goes, the more critical the duct design becomes. Engine thrust can be high only if the inlet duct supplies the engine with the required airflow at the highest possible pressure. The inlet duct must operate from static ground run up to high aircraft Mach numbers with a high duct efficiency at all altitudes, attitudes, and flight speeds. To compound the problem, the amount of air required by a turbojet engine is approximately 10 times or more than that of a piston engine of comparable size. Inlet ducts should be as straight and smooth as possible, and should be designed in such a way that the boundary layer air (a layer of still, dead air lying next to the surface) be held to the minimum. The length, shape, and placement of the duct is determined to a great extent by the location of the engine in the aircraft. Not only must the duct be large to supply the proper airflow, but it must be shaped correctly to deliver the air to the front of the compressor with an even pressure distribution. Poor air pressure and velocity distribution at the front of the compressor may result in compressor stall. Another primary task a duct must do during flight operation is to convert the kinetic energy of the rapidly moving inlet stream into a ram pressure rise inside the duct. To do this it must be shaped so that the ram velocity is slowly and smoothly decreasing, while the ram pressure is slowly and smoothly rising. Inlet ducts are rated in two ways: the duct pressure efficiency ratio and the ram recovery point. The duct pressure efficiency ratio is defined as the ability of the duct to convert the kinetic or dynamic pressure energy at the inlet of the duct into static pressure energy at the inlet of the compressor without a loss in total pressure. It will have a high value of 98 percent if the friction loss is low and if the pressure rise is accomplished with small losses. The ram recovery point is that aircraft speed at which the ram pressure rise is equal to the friction pressure losses, or that airspeed at which the compressor inlet total pressure is equal to the outside ambient air pressure. A good subsonic duct will have a low ram recovery point (about 160 mph (280km/h)). Inlet ducts may be divided into two broad categories: subsonic ducts and supersonic ducts. It is interesting to note that the engine manufacturers rate their engines using a bellmouth inlet. This type of inlet is essentially a bell-shaped funnel having carefully rounded shoulders, which offer practically no air resistance. The duct loss is so small that it is considered zero, and all engine performance data can be gathered without any correction for inlet duct loss being necessary. Normal duct inefficiencies may cause thrust losses of 5 percent or more because a decrease in duct efficiency of 1 percent will decrease airflow 1 percent, decrease jet velocity ½ percent, and result in 1 ½ percent thrust loss. The decrease in jet velocity occurs because it is necessary to increase the area of the jet nozzle in order to keep the turbine temperature within limits when duct losses occur.
Post-Reading I. a) Fill in the diagram according to the content of the text
b) The blocks are jumbled. Put them into the right order according to the text. II. Find the English equivalents to the following words and word combinations in the text:
III. Complete the sentences using the ideas from the text.
1. The faster the airplane goes, the more critical…. 2. The inlet duct must operate from …. 3. The length, shape, and placement of the duct is determined …. 4. Inlet duct must be shaped correctly to deliver the air to …. 5. Inlet ducts are rated in two ways …. 6. The ram recovery point is …. 7. It is interesting to note that the engine manufacturers rate their engines …. 8. Normal duct inefficiencies may cause …. 9. The decrease in jet velocity occurs …. IV. a) Answer the following questions:
1. What is the significance of the inlet duct? 2. In what conditions must the inlet duct operate? 3. How is the inlet duct to be designed? 4. Where does the inlet duct deliver the air? 5. What is one of the primary tasks of the inlet duct? 6. How are inlet ducts rated? 7. Into what categories may inlet ducts be divided?
b) Think of 3 more questions and write them down.
Language in Use I. Match the equivalents.
II. Find pairs of synonyms among the words.
III. Find pairs of antonyms among the words.
IV. a) Match the verbs with their definitions.
b) Reproduce the context where they are used.
V. Fill in the gaps using the words from the oval. Mind, there is one extra word! Translate the text into Russian.
VI. Use the prepositions in the box where it is necessary to complete the sentences in the text. Translate the text in written.
There are basically two forms of shock waves: 1) Normal shock waves lie perpendicular _____ the direction _____ the flow. These form sharp fronts and shock the flow _____ subsonic speeds. Microscopically the air molecules smash _____ the subsonic crowd _____ molecules like alpha rays. Normal shock waves tend to cause a large drop in stagnation pressure. Basically, the higher the supersonic entry Mach number _____ a normal shock wave, the lower the subsonic exit Mach number and the stronger the shock (i.e. the greater the loss in stagnation pressure across the shock wave). 2) Conical (3-dimensional) and oblique shock waves are angled rearwards, like the bow wave _____ a ship or boat, and radiate _____ a flow disturbance such as a cone or a ramp. _____ a given inlet Mach number, they are weaker than the equivalent normal shock wave and, although the flow slows down, it remains supersonic _____. Conical and oblique shock waves turn the flow, which continues in the new direction, _____ another flow disturbance is encountered downstream. VII. Fill in the gaps with the suitable derivative of the word given in brackets. Translate the text in written.
More advanced supersonic intakes are designed to have a normal shock in the ducting downstream of intake lip, so that the flow at compressor/fan entry is always subsonic. However, if the engine is throttled back, there is a _______ (to reduce) in the _______ (to correct) airflow of the LP compressor/fan, but (at supersonic conditions) the corrected airflow at the intake lip remains constant, because it is determined by the flight Mach number and intake incidence/yaw. This _______ (discontinue) is overcome by the normal shock moving to a lower _______ (cross-section) area in the ducting, to decrease the Mach number at entry to the shockwave. This weakens the shockwave, improving the overall intake pressure _______ (recover). So, the absolute airflow stays constant, whilst the corrected airflow at compressor entry falls (because of a higher entry pressure). Excess intake airflow may also be dumped overboard or into the exhaust system, to prevent the conical/oblique shock waves being disturbed by the normal shock being forced too far forward by engine throttling.
Speaking Work in pairs and compare the subsonic and supersonic ducts from the point of view of their characteristics and configuration. Use the phrases below:
Writing Summarize the information given in the text “Inlet Ducts”. Use the key-patterns.
Turbine (Part I) Before you Begin
I. Discuss the following questions with your peer.
1. Can you give any explanation of the term turbine? 2. What are its main components? 3. How many turbines are produced? What are they?
II. Scan the text and choose the suitable title for it:
1. Turbine Operation 2. Turbine Construction 3. Types of Turbines
Reading
Read the text below then: a) answer which paragraph, A-F, tells you about:
1. Some operations the blades undergo before their certification. 2. Work and use of multistage turbine. 4. Shrouded and unshrouded blades. 5. The turbine wheel. 6. The turbine assembly. 7. Methods of cooling the disk and the blades.
b) Think of your own heading for parts G and H. A. The turbine wheel is one of the most highly stressed parts of the engine. Not only must it operate at temperatures of approximately 1800ºF (982ºC), but it must do so under centrifugal loads imposed by high rotational speeds of over 60,000 rpm for small engines to 8,000 rpm for the larger ones. Consequently, the engine speed and turbine inlet temperature must be accurately controlled to keep the turbine within safe operating limits. B. The turbine assembly is made of two main parts, the disk and the blades. The disk or wheel is a statically and dynamically balanced unit of specially alloyed steel usually containing large percentages of chromium, nickel, and cobalt. After forging, the disk is machined all over and carefully inspected using X-rays, sound waves, and other inspection methods to assure structural integrity. The blades or buckets are attached to the disk by means of a “fir tree” design to allow for different rates of expansion between the disk and the blade while still holding the blade firmly against centrifugal loads. The blade is kept from moving axially either by rivets, special locking tabs or devices, or another turbine stage. C. Some turbine blades are open at the outer perimeter, whereas in others a shroud is used. The shroud acts to prevent blade-tip losses and excessive vibration. Distortion under high loads, which tend to twist the blade toward low pitch, is also reduced. The shrouded blade has an aerodynamic advantage in that thinner blade sections can be used and tip losses can be reduced by using a knife-edge or labyrinth seal at this point. Shrouding, however, requires that the turbine run cooler or at a reduced rpm because of the extra mass at the tip. On blades that are not shrouded, the tips are cut or recessed to a knife edge to permit a rapid “wearing-in” of the blade tip to the turbine casing with a corresponding increase in turbine efficiency. D. Blades are forged from highly alloyed steel and are passed through a carefully controlled series of machining and inspection operations before being certified for use. Many engine manufacturers will stamp a “moment weight” number on the blade to retain rotor balance when replacement is necessary. E. The temperature of the blade is usually kept within limits by passing relatively cool air bled from the compressor over the face of the turbine, thus cooling the disk and blade by the process of convection. This method of cooling may become more difficult, as high Mach number flights develop high compressor inlet and outlet temperatures. F. Some gas turbine engines use a single-stage turbine, whereas other employ more than one turbine wheel. Multistage turbines are used where the power required to drive the compressor would necessitate a very large turbine wheel. Multistage wheels are also used for turboprops where the turbine has to extract enough power to drive both the compressor and the propeller. When two or more turbine wheels are used, a nozzle diaphragm is positioned directly in front of each turbine wheel. The operation of the multistage turbine is similar to that of the single stage, except that the succeeding stages operate at lower gas velocities, pressures, and temperatures. Since each turbine stage receives the air at a lower pressure than the preceding stage, more blade area is needed in the rear stage to assure an equitable load distribution between stages. The amount of energy removed from each stage is proportional to the amount of work done by each stage. G. Most multistage turbines are attached to a common shaft. However, some multistage turbine engines have more than one compressor. In this case, some turbine wheels drive one compressor and the remaining turbines drive the other. H. The wheel is subjected to both high speed and high temperature. Because of these extreme conditions, blades can easily deform by growing in length (a condition known as creep) and by twisting and changing pitch. Since these distortions are accelerated by exceeding engine operating limits, it is important to operate within the temperature and rpm points set by the manufacturer.
Post-reading
I. Find the English equivalents to the following words and word combinations:
II. a) Have a look at these three figures. What does each of them illustrate? Fig.1
b) name all the components of the device in fig.1? Fig.2 Fig.3
c) What do you know about the device in fig.2?
d) What part of the text does Fig. 3 correspond to? Prove your answer.
III. Complete the sentences. If necessary refer to the text.
1. The engine speed and turbine inlet temperature must be accurately controlled _______. 2. After forging, the disk is machined all over and carefully inspected using _______. 3. Blades are forged from _______. 4. The temperature of the blade is kept _______, thus cooling the disk and blade by the process of convection. 5. Multistage turbines are used where _______ would necessitate a very large turbine wheel. 6. _______, a nozzle diaphragm is positioned directly in front of each turbine wheel. 7. Since each turbine stage receives the air at a lower pressure than the preceding stage _______.
Language in Use I. Form adjectives from the nouns below. Use the suffixes that are in the clouds. Mind, more than one variant is possible.
II. a) Find the right prepositions for each verb. Watch out! There may be more than one combination.
b) Explain their meaning. c) Quote the sentences from the text to illustrate their use. d) For the rest of the combinations give examples.
III. Fill in the gaps using the words from the oval. Mind, there is one extra word!
Among the most _______ stressed components in a gas-turbine engine is the turbine wheel. The buckets of the wheel are subjected to high centrifugal stresses and to a fluctuating stream of _______ gases which _______ temperature and may, at the same time, introduce _______ stresses. The disk of the _______ is subjected to heat by conduction from the buckets and may, in some cases, be directly heated by the _______ gases. Temperatures of over 1500ºF are not _______ near the _______ of the first-stage turbine disk. Where there is more than one _______ disk in an engine, the second and third-stage disks do not _______ such high temperatures. Turbine disks are often cooled by means of air bled from the compressor section and directed through _______ passages to _______ around the turbine disk.
Speaking
I. Use the information from the text and exercise II of the Post-Reading section to make a diagram on the topic “Turbine Construction” II. Compare your and your partner’s diagrams. Choose the diagram which better reflects the topic. Fill it in. III. Basing on your diagram be ready to speak before the class. The following phrases may help you.
Writing In the text it is spoken about a fir-tree design by means of which the blades are attached to the disk. It is the most satisfactory and the most widely used attachment. Do a search on other types of the blades attachment. Compare them with the fir-tree design. Unit VII Turbine (Part II)
Before you Begin
I. Brainstorm all possible ideas related to the topic
II. Read this abstract and render its idea in Russian
The function of the turbine is to drive the compressor and accessories, and, in the case of the turboprop, the propeller, by extracting a portion of the pressure and kinetic energy from the high temperature combustion gases. In a typical jet engine about 75 percent of the power produced internally is used to provide the compressor. What is left is used to produce the necessary thrust. In order to furnish the drive power to compress the air, the turbine must develop as much as 100, 000 hp (74, 570 kw) or more for the large jet engines. One blade or bucket of a turbine can extract about 250 hp (185 kw) from the moving gas stream. This is equivalent to the power produced by a typical eight-cylinder automobile engine. It does all of this in a space smaller than the average automobile engine, and with a considerable advantage in weight.
Reading
Skim the text and try to guess the meaning of the words in bold. With a few exceptions, gas turbine manufacturers have concentrated on the axial-flow turbine although some manufactures are building engines with a radial inflow turbine. The radial inflow turbine has the advantage of ruggedness and simplicity, and is relatively inexpensive and easy to manufacture when compared with axial-flow type. On this type of turbine, inlet gas flows through peripheral nozzles to enter the wheel passages in an inward radial direction. The speeding gas exerts a force on the wheel blades and then exhausts the air in an axial direction to the atmosphere. These turbine wheels used for small engines are well suited for a lower range of specific speeds and work at relatively high efficiency. The axial-flow turbine is comprised of two main elements consisting of a set of stationary vanes and one or more turbine rotors. The turbine blades themselves are of two basic types, the impulse and the reaction. The modern aircraft gas turbine engine utilizes blades which have both impulse and reaction section. The stationary part of the turbine assembly consists of a row of contoured vanes set at an angle to form a series of small nozzles which discharge gases into the blade of the turbine wheel. For this reason, the stationary vane assembly is usually referred to as the turbine nozzle, and the vanes themselves are called nozzle guide vanes. Post-Reading
I. Name types of turbines mentioned in the text. II. Speak about their advantages and disadvantages. III. Schematically show the flow of gas in the radial-inflow turbine. IV. Make a list of all design features typical for each type of the turbine and its stationary part.
Language in Use
I. Give the Russian equivalents to the following word combinations and phrases:
II. a) Form words opposite in the meaning using the prefixes in the box.Consult the dictionary if necessary. Translate them.
b) Explain them.
III.Match the equivalents.
IV. a) Find the right word for the words in the box below. The first example is done for you.
b) Define what part of speech the marked words belong to. Translate the sentences.
1. By reason of the rapid expansion, the heated air and combustion products at increased velocity force their way through the only exit from chamber. 2. Thrust is an applied force tending to produce motion in a body or to alter the motion of a body. 3. These vanes direct the flow to the appropriate angle of attack for the blades on the periphery of the turbine wheel. 4. In that position the valve allows the oil to flow from the engine through the transfer rings. 5. Internal combustion engines use any fuels that can be combined with an oxidizer in the chamber. 6. Extensive use of titanium in the front part of the compressor and high-nickel steel alloys in the rear is made. 7. Daimler’s new engine set the basis for all car engines going forward. 8. The axial-flow turbine is comprised of two main elements consisting of a set of stationary vanes and one or more turbine rotors. 9. The pattern of sound from a jet engine makes the noise problem even more bothersome than that coming from other types of engine.
Speaking a) Which of these two graphs better reflects the content of the text. Discuss it with your partner. If nither of them, draw your own. b) Fill in the chosen graph with the words or phrases from the text. c) Compare the results with your peers from the other group. If necessary make some changes. 1.
2.
While discussing make use of the following phrases:
Writing
Do a research and write an account to your chief on the turbines mostly used today in aviation. Unit VIII * * * Before you Begin
I. Look at the picture and say what might be the topic of the text.
II. Name 10 words related to the topic. Reading
I. Read the text and give a title to it. II. While reading the text, write out the words and word combinations you don’t know. Try to guess their meaning from the context.
* * *
The nozzle guide vanes (diaphragm) have two principal functions. First, they must convert part of the gas heat and pressure energy into dynamic and kinetic energy, so that the gas will strike the turbine blades with some degree of force. Second, the nozzle vanes must turn this gas flow so that it will impinge upon the turbine buckets in the proper direction; that is, the gases must impact on the turbine blade in a direction that will have a large component force in the plane of the rotor. The nozzle does its first job by utilizing the Bernoulli theorem. As through any nozzle, when the flow area is restricted, the gas will accelerate and a large portion of the static pressure in the gas is turned into dynamic pressure. The degree which this effect will occur depends upon the relationship between the nozzle guide vane inlet and exit areas, which, in turn are closely related to the type of turbine blades used. The turbine nozzle area is a critical part of engine design. Making the nozzle area too small will restrict the airflow through the engine, raise compressor discharge pressure, and bring the compressor closer to stall. This is especially critical during acceleration when the nozzle will have the tendency to choke (gas flowing at the speed of sound). Many engines are designed to have the nozzle operating in these choked conditions. Small exit areas also cause slower accelerations because the compressor will have to work against the increased back pressure. Increasing the nozzle diaphragm area will result in faster engine acceleration, less tendency to stall but higher specific fuel consumption. The area of the nozzle is adjusted at factory or during overhaul so that the gas velocity at this point will be at or near the speed of sound. The second function, that of turning the gases so that they strike the turbine blades at the correct angle, is accomplished by setting the blades at a specific angle to the axis of the engine. Ideally, this angle should be variable, but in practice the vanes are fixed in one position. It should be noted that the auxiliary power unit (APU) for several turbine-powered ground vehicles is equipped with variable angle nozzle vanes.
Post-Reading
I. Explain the words in bold from the text. II. Find in the text: a) the definition of Bernoulli theorem b) the description of increasing the nozzle diaphragm area result c) English equivalents to the following words and word combinations:
III. a) Answer the questions:
1. What does the text deal with? 2. What are the functions of the nozzle guide vanes? 3. What job is done by utilizing the Bernoulli theorem? 4. Why is the turbine nozzle area a critical part of engine design? 5. What is the result of gas acceleration?
b) Put 3 more questions to the text.
Language in Use
I. Match columns A and B to find: a)synonyms, b) antonyms. Mind, there is one extra word in each part. a)synonyms
b) antonyms
II. Give the Russian equivalents to the following word combinations:
III. What do the words mean in the text? Choose the correct meaning. Mind, in some cases you may use more than one variant.
Speaking
Work in pairs: a) Finish drawing this diagram. b) Fill it in with the words or phrases from the text. c) Discuss it with your peer from the other group. d) Be ready to speak on the topic.
The following phrases will be of great help:
Writing
Rewrite the sentences that are jumbled in the correct order to make up a meaningful text on description and location of the nozzle diaphragm. (More than one alternative is possible).
1. The nozzle vanes are usually constructed of high-temperature alloy, and they must be highly heat-resistant. 2. In many engines, the nozzle vanes are hollow and are formed from stainless-steel sheet. 3. The nozzle diaphragm consists of a group of nozzle vanes welded between two shroud rings. 4. In the typical nozzle diaphragm, the inner and outer bands contain punched holes to receive the ends of the nozzle vanes. 5. When there is more than one turbine wheel, additional nozzle diaphragms are installed to direct the heated gases from one wheel to the next. 6. Second-, third- and fourth-stage nozzle vane are often constructed of solid steel alloy. 7. They are then welded and ground smooth before being installed between the shroud rings. 8. These may be either forged or precision-cast.
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