Exhaust system | EXHAUST GAS FLOW

Fig. 6-1 A basic exhaust system.

INTRODUCTION
1. Aero gas turbine engines have an exhaust system which passes the turbine discharge gases to atmosphere at a velocity, and in the required direction, to provide the resultant thrust. The velocity and pressure of the exhaust gases create the thrust in the turbo-jet engine (para. 5) but in the turbopropeller engine only a small amount of thrust is contributed by the exhaust gases, because most of the energy has been absorbed by the turbine for driving the propeller. The design of the exhaust system therefore, exerts a considerable influence on the performance of the engine. The areas of the jet pipe and propelling or outlet nozzle affect the turbine entry temperature, the mass airflow and the velocity and pressure of the exhaust jet.
2. The temperature of the gas entering the exhaust system is between 550 and 850 deg. C. according to the type of engine and with the use of afterburning (Part 16) can be 1,500 deg. C. or higher. Therefore, it is necessary to use materials and a form of construction that will resist distortion and cracking, and prevent heat conduction to the aircraft structure.
3. A basic exhaust system is shown in fig. 6-1. The use of a thrust reverser (Part 15), noise suppressor (Part 19) and a two position propelling nozzle entails a more complicated system as shown in fig. 6-2. The low by-pass engine may also include a mixer unit (fig. 6-4) to encourage a thorough mixing of the hot and cold gas streams.

Fig. 6-2 Exhaust system with thrust reverser, noise suppressor and two position propelling nozzle.

Fig. 6-3 Gas flow through a convergentdivergent
nozzle.

EXHAUST GAS FLOW
4. Gas from the engine turbine enters the exhaust system at velocities from 750 to 1,200 feet per second, but, because velocities of this order produce high friction losses, the speed of flow is decreased by diffusion. This is accomplished by having an increasing passage area between the exhaust cone and the outer wall as shown in fig. 6-1. The cone also prevents the exhaust gases from flowing across the rear face of the turbine disc. It is usual to hold the velocity at the exhaust unit outlet to a Mach number of about 0.5, i.e. approximately 950 feet per second. Additional losses occur due to the residual whirl velocity in the gas stream from the turbine. To reduce these losses, the turbine rear struts in the exhaust unit are designed to straighten out the flow before the gases pass into the jet pipe.
5. The exhaust gases pass to atmosphere through the propelling nozzle, which is a convergent duct, thus increasing the gas velocity (Part 2). In a turbojet engine, the exit velocity of the exhaust gases is subsonic at low thrust conditions only. During most operating conditions, the exit velocity reaches the speed of sound in relation to the exhaust gas temperature and the propelling nozzle is then said to be ’choked’; that is, no further increase in velocity can be obtained unless the temperature is increased. As the upstream total pressure is increased above the value at which the propelling nozzle becomes ’choked’, the static pressure of the gases at exit increases above atmospheric pressure. This pressure difference across the propelling nozzle gives what is known as ’pressure thrust’ and is effective over the nozzle exit area. This is additional thrust to that obtained due to the momentum change of the gas stream (Part 20).

Fig. 6-4 A low by-pass air mixer unit.

6. With the convergent type of nozzle a wastage of energy occurs, since the gases leaving the exit do not expand rapidly enough to immediately achieve outside air pressure. Depending on the aircraft flight plan, some high pressure ratio engines can with advantage use a convergent-divergent nozzle to recover some of the wasted energy This nozzle utilizes the pressure energy to obtain a further increase in gas velocity and, consequently, an increase in thrust.
7. From the illustration (fig. 6-3), it will be seen that the convergent section exit now becomes the throat, with the exit proper now being at the end of the flared divergent section. When the gas enters the convergent section of the nozzle, the gas velocity increases with a corresponding fall in static pressure. The gas velocity at the throat corresponds to the local sonic velocity. As the gas leaves the restriction of the throat and flows into the divergent section, it progressively increases in velocity towards the exit. The reaction to this further increase in momentum is a pressure force acting on the inner wall of the nozzle. A component of this force acting parallel to the longitudinal axis of the nozzle produces the further increase in thrust.
8. The propelling nozzle size is extremely important and must be designed to obtain the correct balance of pressure, temperature and thrust. With a small nozzle these values increase, but there is a possibility of the engine surging (Part 3), whereas with a large nozzle the values obtained are too low,
9. A fixed area propelling nozzle is only efficient over a narrow range of engine operating conditions. To increase this range, a variable area nozzle may be used. This type of nozzle is usually automatically controlled and is designed to maintain the correct balance of pressure and temperature at all operating conditions. In practice, this system is seldom used as the performance gain is offset by the increase in weight. However, with afterburning a variable area nozzle is necessary and is described in Part 16.

Fig. 6-5 High by-pass ratio engine exhaust systems.

10. The by-pass engine has two gas streams to eject to atmosphere, the cool by-pass airflow and the hot turbine discharge gases.
11. In a low by-pass ratio engine, the two flows are combined by a mixer unit (fig. 6-4) which allows the by-pass air to flow into the turbine exhaust gas flow in a manner that ensures thorough mixing of the two streams.
12. In high by-pass ratio engines, the two streams are usually exhausted separately. The hot and cold nozzles are co-axial and the area of each nozzle is designed to obtain maximum efficiency. However, an improvement can be made by combining the two gas flows within a common, or integrated, nozzle assembly. This partially mixes the gas flows prior to ejection to atmosphere. An example of both types of high by-pass exhaust system is shown in fig, 6-5.