Gas Turbine Engine Theory (3)

Working cycle and airflow ( part-3)

Airflow

15. The path of the air through a gas turbine engine varies according to the design of the engine. A straight-through flow system (fig. 2-5) is the basic design, as it provides for an engine with a relatively small frontal area and is also suitable for use of the by-pass principle. In contrast, the reverse flow system gives an engine with greater frontal area, but with a reduced overall length. The operation, however, of all engines is similar. The variations due to the different designs are described in the subsequent paragraphs.

Fig. 2-5-1 Airflow systems.

Fig, 2-5-2 Airflow systems.

16. The major difference of a turbo-propeller engine is the conversion of gas energy into mechanical power to drive the propeller. Only a small amount of ’jet thrust’ is available from the exhaust system. The majority of the energy in the gas stream is absorbed by additional turbine stages, which drive the propeller through internal shafts (Part 5).

17. As can be seen in fig. 2-5, the by-pass principle involves a division of the airflow. Conventionally, all the air taken in is given an initial low compression and a percentage is then ducted to by-pass, the remainder being delivered to the combustion system in the usual manner. As described in Part 21, this Working cycle and airflow principle is conducive to improved propulsive efficiency and specific fuel consumption.

18. An important design feature of the by-pass engine is the by-pass ratio; that is, the ratio of cool air by-passed through the duct to the flow of air passed through the high pressure system. With low by-pass ratios, i.e. in the order of 1:1, the two streams are usually mixed before being exhausted from the engine. The fan engine may be regarded as an extension of the by-pass principle, and the requirement for high by-pass ratios of up to 5:1 is largely met by using the front fan in a twin or triple-spool configuration (on which the fan is, in fact, the low pressure compressor) both with and without mixing of the airflows. Very high by-pass ratios, in the order of 15:1, are achieved using propfans. These are a variation on the turbo-propeller theme but with advanced technology propellers capable of operating with high efficiency at high aircraft speeds.

19. On some front fan engines, the by-pass airstream is ducted overboard either directly behind the fan through short ducts or at the rear of the engine through longer ducts; hence the term ’ducted fan’. Another, though seldom used, variation is that of the aft fan.

SOURCE: The jet engine , Rolls-Royce 

Contents ( This Book ) ;
1 Basic mechanics 1
2 Working cycle and airflow 11
3 Compressors 19
4 Combustion chambers 35
5 Turbines 45
6 Exhaust system 59
7 Accessory drives 65
8 Lubrication 73
9 Internal air system 85
10 Fuel system 95
11 Starting and ignition 121
12 Controls and instrumentation 133
13 Ice protection 147
14 Fire protection 153
15 Thrust reversal 159
16 Afterburning 169
17 Water injection 181
18 Vertical/short take-off and landing 187
19 Noise suppression 199
20 Thrust distribution 207
21 Performance 215
22 Manufacture 229
23 Power plant installation 243
24 Maintenance 251
25 Overhaul 263
Appendix 1; Conversion factors 277