Airflow systems.

Fig. 2-5-1 Airflow systems

THE RELATIONS BETWEEN PRESSURE, VOLUME AND TEMPERATURE

THE RELATIONS BETWEEN PRESSURE, VOLUME AND TEMPERATURE 

Fig. 2-3 An airflow through divergent and convergent ducts.

7. During the working cycle of the turbine engine, the airflow or ’working fluid’ receives and gives up heat, so producing changes in its pressure, volume and temperature. These changes as they occur are closely related, for they follow a common principle that is embodied in a combination of the laws of Boyle and Charles. Briefly, this means that the product of the pressure and the volume of the air at the various stages in the working cycle is proportional to the absolute temperature of the air at those stages. This relationship applies for whatever means are used to change the state of the air. For example, whether energy is added by combustion or by compression, or is extracted by the turbine, the heat change is directly proportional to the work added or taken from the gas.

Working cycle and airflow

INTRODUCTION
1. The gas turbine engine is essentially a heat engine using air as a working fluid to provide thrust. To achieve this, the air passing through the engine has to be accelerated; this means that the velocity or kinetic energy of the air is increased. To obtain this increase, the pressure energy is first of all increased, followed by the addition of heat energy, before final conversion back to kinetic Energy in the form of a high velocity jet efflux.
WORKING CYCLE

Fig. 2-1 A comparison between the working cycle of a turbo-jet engine and a piston engine.

2. The working cycle of the gas turbine engine is similar to that of the four-stroke piston engine. However, in the gas turbine engine, combustion occurs at a constant pressure, whereas in the piston engine it occurs at a constant volume. Both engine cycles (fig. 2-1) show that in each instance there is induction, compression, combustion and exhaust.
These processes are intermittent in the case of the piston engine whilst they occur continuously in the gas turbine. In the piston engine only one stroke is utilized in the production of power, the others being involved in the charging, compressing and exhausting of the working fluid. In contrast, the turbine engine eliminates the three ’idle’ strokes, thus enabling more fuel to be burnt in a shorter time;hence it produces a greater power output for a given size of engine.

Rolls-Royce/Snecma Olympus

Rolls-Royce RB37 Derwent 1

A straight-through version of the reverse-flow Power Jets W2B, known as the W2B/26, was developed by the Rover Company from 1941 to 1943. Taken over by Rolls-Royce in April 1943 and renamed the Derwent, it passed a 100hr. test at 2000 lb thrust in November 1943 and was flown at that rating in April 1944. The engine powered the Gloster Meteor III which entered service in 1945.

A turbo-rocket engine.

Fig. 1-12 A turbo-rocket engine.

20. The turbo-rocket engine (fig. 1-12) could be considered as an alternative engine to the turbo/ram jet; however, it has one major difference in that it carries its own oxygen to provide combustion,
21. The engine has a low pressure compressor driven by a multi-stage turbine; the power to drive the turbine is derived from combustion of kerosine and liquid oxygen in a rocket-type combustion chamber. Since the gas temperature will be in the order of 3,500 deg. C, additional fuel is sprayed into the combustion chamber for cooling purposes before the gas enters the turbine. This fuel-rich mixture (gas) is then diluted with air from the compressor and the surplus fuel burnt in a conventional afterburning system.
22. Although the engine is smaller and lighter than the turbo/ram jet, it has a higher fuel consumption. This tends to make it more suitable for an interceptor or space-launcher type of aircraft that requires high speed, high altitude performance and normally has a flight plan that is entirely accelerative and of short duration.

A turbo/ram jet engine.

Fig. 1-10 Comparative propulsive efficiencies

16. At aircraft speeds below approximately 450 miles per hour, the pure jet engine is less efficient than a propeller-type engine, since its propulsive efficiency depends largely on its forward speed; the pure turbo-jet engine is, therefore, most suitable for high forward speeds. The propeller efficiency does, however, decrease rapidly above 350 miles per hour due to the disturbance of the airflow caused by the high blade-tip speeds of the propeller. These characteristics have led to some departure from the use of pure turbo-jet propulsion where aircraft operate at medium speeds by the introduction of a combination of propeller and gas turbine engine.
17. The advantages of the propeller/turbine combination have to some extent been offset by the introduction of the by-pass, ducted fan and propfan engines. These engines deal with larger comparative airflows and lower jet velocities than the pure jet engine, thus giving a propulsive efficiency (Part 21) which is comparable to that of the turbo-prop and exceeds that of the pure jet engine (fig. 1-10).

Mechanical arrangement of gas turbine engines.

Fig. 1-9-2

Basic mechanics

 

Fig. 1-9-1

Mechanical arrangement of gas turbine engines.

PRINCIPLES OF JET PROPULSION

Fig. 1-4 Hero’s engine - probably the earliest
form of jet reaction.

6. Jet propulsion is a practical application of Sir Isaac Newton’s third law of motion which states that, ’for every force acting on a body there is an opposite and equal reaction’. For aircraft propulsion, the ’body’ is atmospheric air that is caused to accelerate as it passes through the engine. The force required to give this acceleration has an equal effect in the opposite direction acting on the apparatus producing the acceleration. A jet engine produces thrust in a similar way to the engine/propeller combination. Both propel the aircraft by thrusting a large weight of air backwards (fig. 1-3), one in the form of a large air slipstream at comparatively low speed and the other in the form of a jet of gas at very high speed.
7. This same principle of reaction occurs in all forms of movement and has been usefully applied in many ways. The earliest known example of jet reaction is that of Hero’s engine (fig. 1-4) produced as a toy in 120 B.C. This toy showed how the momentum of steam issuing from a number of jets could impart an equal and opposite reaction to the jets themselves, thus causing the engine to revolve.

Fig. 1-5 A garden sprinkler rotated by the
reaction of the water jets.

8. The familiar whirling garden sprinkler (fig. 1-5) isa more practical example of this principle, for themechanism rotates by virtue of the reaction to thewater jets. The high pressure jets of modern