Exercise 27. Make up the dialogue using the key-words of exercise 25 and the conversational phrases.

s? Exercise 28. Ask questions

1. …?

The gas turbine engine is essentially a heat engine.

2. …?

The gas turbine engine consists of a rotary air compressor with an air intake, one or more combustion chambers, a turbine, and an ex­haust outlet.

3. …?

The compressors are driven by the engine turbine.

4. …?

The compressors are coupled direct to the turbine shaft.

5. …?

The function of the compressor is to accelerate the air, to diffuse it and to produce the required pressure rise.

7. …?

The turbine has the task of providing the power to drive the compressor and accessories.

8. …?

The turbine blade tips may rotate at speeds over 1, 500 feet per second.

9. …?

The exhaust systempasses the turbine discharge gases to

atmosphere to provide the resultant thrust.

€  Exercise 29. Do the two-way translation.

A.

Instructor: Historically, there are three types of compressors, used in aviation jet engines. These are centrifugal compressors, axial flow com­pressors and hybrid compressors. Each of these types has its own advantages and disadvantages.

Nick: Up to now we have discussed only axial flow compressors. Does it mean that they are the most efficient type?

Instructor: Yes, it does. Axial flow compressors are the most efficient and the most complex type of compressors, and they are used in most jet engines. On the other hand, centrifugal compressors are very sim­ple and robust, very easy to manufacture, but, unfortunately, their efficiency is low.

Nick: Are the axial compressors mounted on the turbine shaft?

Instructor: Yes. The axial compressors force the air straight backwards by means of a series of multi-blade fans mounted one behind the other.

B.

Instructor: This time we are going to discuss the central part of the jet engine - its combustion chamber. This is the place where fuel is mixed with the air from the compressor and ignited.

Peter: It must be quite complex!

Instructor: On the contrary, you'll be surprised by its simplicity! Just imagine two tubes, one inside the other. The inside one, called the flame tube, is fitted at the front with a perforated flare, located behind an entry snout. In the center of the flare there are a number of swirl vanes.

Nick: Up to now, it's quite simple.

Instructor: The air comes from the compressor at high velocity of one or two hundred meters per second...

Peter. It can surely blow out the flame.

Instructor: Very definitely it can. Besides, there is more available air than is needed for combustion. And this is the reason why there are two tubes: only the quantity of air needed for combustion goes into the inner tube, the rest passes outside it.

Nick: The flare clearly helps to decrease the velocity of the air entering the flame tube...

Peter: And also to raise its temperature and pressure.

Instructor: You're both right!

Peter: I don't see why the compressor should be made to deliver so much air when only part of it is needed for combustion.

Instructor: The temperature of the gases leaving the combustion zone may be as high as 2000 degrees Centigrade. This is too hot to feed into the turbine. About half of the excess air is needed to dilute and cool these exhaust gases.

C.

Instructor: The basic principles of gas turbine design are easy to understand, but their practical application is very complicated, to say the least. First, what does the turbine do?

Peter: It drives the compressor, of course.

Instructor: Anything else?

Nick: I suppose it drives various accessories, too. Fuel pumps, electric generators, and things like that.

Peter: In turbo-prop aircraft it drives the propeller shaft, in helicopters it drives the rotor shaft, both through reduction gears.

Instructor: You're both well in the picture! I am very impressed!

Now let's see how a turbine operates. The hot exhaust gases from the combustion chamber first pass fixed nozzle guide vanes.

Peter: They look like stator vanes in a compressor, don't they? Instructor: Yes, in a way. But actually they are quite different. Firstly, they have complex cooling ducts inside. Secondly, they are monocrystallic that is produced from a piece of metal, grown as a single crystal.

Nick: What substance is used as a cooling agent?

Instructor: As a cooling agent is used common air, bled from a compressor stage. Immediately behind the fixed nozzle guide vanes are the rotating blades of the turbine. These are the most critical items of the whole jet engine. In modern jet engines they have to withstand temperatures of up to 1400 degrees Centigrade and strains of up to three or four tons.

Peter: I think that design and manufacture of such blades must be quite a problem!

Instructor: This is the field of what is known as very high technologies and top secret know-how.

Nick: How many turbines may modern high bypass ratio engines have?

Instructor: Modern high bypass ratio jet engines may have three gas turbines, and three concentric shafts, running independently. Such engines are very fuel efficient and also extremely quiet.

¨ Exercise 31. Translate the texts.