The Centrifugal-Flow Compressor.

The centrifugal compressor consists basically of an impeller and a diffuser manifold. Other components such as compressor manifold may be added to direct the compressed air into the combustion chamber. As the impeller revolves at high speed, air is drawn in at the eye. Centrifugal force provides high acceleration to this air and causes it to move outward from the axis of rotation toward the rim of the rotor where it is ejected at high velocity and high kinetic energy. The pressure rise is produced in part by expansion of the air in the diffuser manifold by conversion of the kinetic energy of motion into static pressure energy.

The centrifugal compressors can be manufactured in a variety of designs including single-stage, multiple-stage, and double-sided types. The centrifugal compressor has a number of features to recommend its use in certain types of gas turbine engines. Chief among its attributes are its simplicity, ruggedness, and low cost. Because of its massive construction, it is much less susceptible to damage from the injection of foreign objects. The centrifugal compressor is capable to relatively high compressor ratio per stage. Above 80 percent efficiency may be reached with a compression ratio of 6 or 7 to 1. Above this ratio, efficiency drops off at a rapid rate because of excessively high impeller tip speeds and attending shock-wave formation. This rules out this type of compressor for use in larger engines since high compression ratios are necessary for low fuel consumption. Some centrifugal-flow engines obtain somewhat higher ratios through the use of multistage compressors. Although the tip speed problem is reduced, efficiency is again lost because of the difficulty in turning the air as it passes from one stage to another. Double-entry compressors also help to solve the high-tip-speed problems, but this advantage is partially offset by the complications in engine design necessary to get air to the rear impeller, and by the requirement of a large plenum or air chamber, where the air from the inlet duct is brought to a slower speed for efficient direction change and higher pressures. The plenum chamber acts as a diffuser by which means the rear impeller can receive its air.

Because of the problems inherent in this type of design the centrifugal compressor finds its greatest application on the smaller engines, where simplicity, flexibility of operation and ruggedness are the principle requirements rather than small frontal area, and ability to handle high air flows and pressures with low loss of efficiency.

Post-Reading

I. Match these English words with their Russian equivalents:

II. a) Match verbs from the text with their definitions.

- to put smth with another thing or group of things

- to make goods in large quantities in a factory

- to deliberately let smth fall

- to balance the effect of smth, with the result that there is no advantage or disadvantage

- to stop considering smth as a possibility

b)Choose any 5 words from the blue banner and explain them.

III. Complete the following sentences using the ideas from the text.

1. The centrifugal compressor consists basically of …

2. The pressure rise is produced in part by …

3. The centrifugal compressors can be manufactured in a variety of designs including …

4. Because of its massive construction, it is much less susceptible to …

5. … may be reached with a compression ration of 6 or 7 to 1.

6. High compression ratios are necessary for …

7. Although the tip speed problem is reduced, efficiency is again lost because of …

8. … help to solve the high-tip-speed problems.

9. The plenum chamber acts as …

10. The centrifugal compressor finds its greatest application on…

IV. Match parts of the sentences in columns A and B.

VI. a) Answer the following questions.

1. What does a centrifugal compressor consist of?

2. What does centrifugal force provide?

3. What is the pressure rise produced by?

4. What types of design of a centrifugal compressor exist?

5. What are the advantages of a centrifugal compressor?

6. Why is a centrifugal compressor much less susceptible to damage from foreign object?

7. When does the efficiency drop off at a rapid rate?

8. By means of what do some centrifugal-flow compressors obtain higher ratios?

b) Think of 3 more questions to the text.

VII. Retell the text.

Language in Use

I. Find pairs of synonyms among the words.

II. Find the antonyms among the words.

III. a) Match the adjectives with their definitions.

b) Reproduce the context where they are used.

IV. Fill in the gaps using the words from the oval. Mind, there is one extra word! Translate the sentences into Russian.

1. Centrifugal compressors, (sometimes referred to as radial compressors) are a special class of radial-flow work-absorbing turbomachinery that ­­­­­­­­­­­__________ pumps, fans, blowers and compressors.

2. The earliest __________ of these dynamic turbomachines were pumps, fans and blowers.

3. What differentiates these early turbomachines from compressors is that the working fluid can be considered __________ thus permitting accurate analysis through Bernoulli's equation.

4. In contrast, modern centrifugal compressors are higher in speed and analysis must __________ with compressible flow.

5. For purposes of definition, centrifugal compressors often have density increases __________ than 5 percent.

6. Also, they often __________ relative fluid velocities above Mach 0.3 when the working fluid is air or nitrogen.

7. In __________, fans or blowers are often considered to have density increases of less than 5 percent and peak relative fluid velocities below Mach 0.3

8. In an idealized sense, the dynamic compressor achieves a pressure rise by adding kinetic-energy/velocity to a continuous flow of fluid __________ the rotor or impeller.

9. This kinetic energy is then converted to ­­­­__________ in static pressure by slowing the flow through a diffuser.

V. Use the prepositions in the oval to complete the sentences in the text. Translate the text.

Centrifugal compressors are used _______ industry because they have fewer rubbing parts, are relatively energy efficient, and give higher airflow than a similarly sized reciprocating compressor (i.e. positive-displacement). Their primary drawback is that they cannot achieve the high compression ratio reciprocating compressors ­­_______ multiple stages. Centrifugal fan/blowers are more suited _______ continuous-duty applications such as ventilation fans, air movers, cooling units, and other uses that require high volume ­­_______ little or no pressure increase. _______ contrast, multi-stage centrifugal compressors often achieve discharge pressures of 8,000 _______ 10,000 psi (59 MPa to 69MPa). One example _______ an application _______ centrifugal compressors is their use _______ re-injecting natural gas back _______ oil fields to increase oil production.

VI. Fill in the gaps with the suitable derivative of the word given in brackets. Translate the sentences.

Operating limits

Many centrifugal compressors have one or more of the following (to operate) limits:

• Minimum Operating Speed - the minimum speed for (to accept)operation, below this value the compressor may be controlled to stop or go into an "Idle" condition.
• Maximum (to allow) Speed - the maximum operating speed for the compressor. Beyond this value stresses may rise above prescribed limits and rotor vibrations may increase rapidly. At speeds above this level the (to equip) will likely become very dangerous and be controlled to slower speeds.
• Stonewall or Choke - occurs under one of 2 conditions. (Typical)for high speed equipment, as flow increases the velocity of the gas/fluid can approach the gas/fluid's sonic speed somewhere within the compressor stage. This (to locate) 0may occur at the impeller inlet "throat" or at the vaned diffuser inlet "throat". In most cases, it is generally not (detriment)to the compressor. For low speed equipment, as flows increase, losses increase such that the pressure ratio drops to 1:1.
• Surge - is the point at which the compressor cannot add enough energy to overcome the system (resist). This causes a rapid flow reversal (i.e. surge). As a result, high vibration, temperature increases, and rapid changes in axial thrust can occur. These (occur) can damage the rotor seals, rotor bearings, the compressor driver and cycle operation. Most turbomachines are designed to easily withstand (occasion)surging. However, if the turbomachine is forced to surge repeatedly for a long period of time or if the turbomachine is poorly designed, repeated surges can result in a (catastrophe) failure. Of particular interest, is that while turbomachines may be very durable, the cycles/processes that they are used within can be far less robust.

Speaking

Work in pairs and make up a dialogue discussing similarities and distinctions of an axial-flow compressor and a centrifugal-flow one. Touch upon such items as :

- components

- characteristics

- preferences

- drawbacks

- sphere of application

Use phrases in the box.

Writing

Summarize the information given in the text “Compressors”. Use the key-patterns.

Unit III

Compressor Stall

Before you Begin

I. Tick the words which come to your mind when you think of such part of an engine as “Compressor”. Add your own words and expressions. Make the sentences on the subject using as many words from the list as possible.

II. Try to answer the questions before you read the text.

1. What are the most stressed parts of a compressor?

2. What problems may be inherent to the compressor?

3. What are solutions for these problems?

III. Scan the text and check if your predictions were correct.

Reading

I. While reading try to devide the text into logical parts and make a plan of the text.

II. Read the text and highlight the ideas not mentioned in the discussion.

Compressor Stall

Since an axial-flow compressor consists of a series of alternately rotating and stationary airfoils or wings, the same rules and limitations which apply to an airfoil will apply to the entire compressor. The picture is somewhat more complicated than in the case for a single airfoil, because the blades are close together, and each blade is affected at the leading edge by the passage through the air of the preceding blade. This “cascade” effect can be more readily understood if the airflow through the compressor is viewed as flow through a series of ducts formed by the individual blades, rather than flow over an airfoil that is generating lift. The cascade effect is of prime importance in determining blade design and placement.

The axial compressor is not without its difficulties, and the most vital of these is the stall problem. If for some reason the angle of attack, i.e., the angle at which the airflow strikes the rotor blades, becomes too low, the pressure zones will be of low value and the airflow and compression will be low. (The angle of attack should not be confusedwith the angle of incidence, which is a fixedangle determined by the manufacture when the compressor is constructed). If the angle of attack is high, the pressure zones will be high and airflow and compression ratio will be high. If it is too high the compressor will stall. That is, the airflow over the upper foil surface will become turbulent and destroy the pressure zones. This is, of course, decrease the compression and airflow. The angle of attack will vary with engine rpm, compressor inlet temperature, and compressor discharge or burner pressure. Decreasing the velocity of airflow or increasing engine rotor speed will tend to increase the angle of attack.

In general, any action that decreases airflow relativeto engine speed will increase the angle of attack and increase the tendency to stall. The decrease in airflow may result from the compressor discharge pressure becoming too high, for example, from excessive fuel-flow schedule during acceleration. On compressor inlet pressure may become too low in respect to the compressor discharge pressure because of high inlet temperatures and distortion of inlet air. Several other causes are possible.

During ground operation of the engine, the prime action that tends to cause a stall is choking. If the engine speed is decreased from the design speed, the compression ratio will decrease with the lower rotor velocities. With a decrease in compression the volume of air in the rear of the compressor will be greater. This excess volume of air causes a choking action in the rear of the compressor with a decrease in airflow, which in turn decreases the air velocity in the front of the compressor and increases the tendency to stall. If no corrective design action is taken, the front of the compressor will stall at low engine speeds.

Another important causeof stall is high compressor inlet air temperatures. High-speed aircraft may experience an inlet air temperature of 250° F (121°C) or higher because of ram effect. These high temperatures cause low compression ratios (due to density changes) and will also cause choking in the rear of the compressor. This choking stall condition is the same as that caused by low compression ratio due to low engine speeds. High compressor inlet temperatures will cause the length of the airflow vector to become longer since the air velocity is directly affected by the square root of any temperature change.

Each stage of a compressor should develop the same pressure ratio as all other stages. But when the engine is slowed down or the compressor inlet temperature climbs, the front stages supply too much air for the rear stages to handle and the rear stages will choke.

Post-Reading

I. In the text find definitions for the following terms:

- angle of attack

- angle of incidence

- a choking action

II. Find the Russian equivalents to the following words and word combinations from the text:

III. Complete the following sentences using the ideas from the text.

1. This “cascade” effect can be more readily understood if...

2. The angle of attack will vary with...

3. ... will increase the angle of attack and increase the tendency to stall.

4. If the engine speed is decreased from the design speed, the compression ratio will...

5. High-speed aircraft may experience...

6. High compressor inlet temperatures will cause the length of the airflow vector to...

7. On compressor inlet pressure may become too low in respect to...

IV. Match the begining of the sentence with its ending using the ideas from the text.

IV. Explain the meaning of the words marked in the text in English.

VI. a)Answer the following questions according to the text.

1. What does an axial flow compressor consist of?

2. What is a cascade affect?

3. What is the vital problem of a compressor?

4. At what conditions does the compressor stall occur?

5. What is the prime action that tends to cause a stall during ground operation?

b) Think of 3 more questions to the text.

Language in Use

I. Match pairs of synonyms .

II. Match pairs of antonyms.

III. Use the prepositions in the oval to complete the sentences in the text. Translate the text with the help of a dictionary in written.

This characteristic has been called both " Surge " and " Stall " _____the past , but is more properly called SURGE when it is response _____ the entire engine. The word stall applies_____ the action occurring at each individual compressor blade. Compressor surge , also called Compressor stall , is a phenomenon which is difficult to understand because it is usually caused _____complex combination of factors . The basic cause of compressor surge is fairly simple . Each blade _____an axial flow compressor is a miniature airplane wing, which , when subjected _____a higher angle of attack , will stall just as an airplane stalls. Surge may be defined as results_____ an unstable air condition _____the compressor. The unstable condition of air is often caused _____air piling up _____the rear stages of the compressor. Surge may cause loud bangs and engine vibration. In most cases , this condition is of short duration , and will either correct itself or can be corrected _____ retarding the throttle or power lever to idle and advancing it again , slowly.

IV. Insert the words into gaps.

A method of _____ onset of a gas turbine condition, such as compressor stall, includes receiving data indicative of an operating parameter of a compressor of the gas turbine. The method also includes performing a wavelet transformation on the data to generate wavelet transformed data. The wavelet _____ is configured to affect a processing characteristic regarding a performance of the wavelet transformation. Features indicative of onset of the gas turbine condition in the wavelet transformed data are then

_____ to provide an indication for controlling the gas turbine to prevent compressor stall from occurring. A system for detecting onset of compressor stall in a gas turbine includes a _____ for providing data indicative of an operating parameter of the compressor and a _____ for performing a wavelet transform on the data to identify features of the optimized wavelet transformed data indicative of onset of stall.

Writing

Summarize the information given in the text «Compressor Stall». Use the key-patterns.

Unit IV.

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