EXHAUST

In this section, you will look at the function of a gas turbine engine exhaust system. The exhaust system guides the exhaust gases from the rear of the turbine into the atmosphere but the main task of the exhaust system depends on the type of gas turbine engine. The exhaust system does not make thrust, because most of the gas energy has been absorbed by the turbine for driving the propeller.

On the APU the exhaust system guides the exhaust gases overboard because all the gas energy will be absorbed by the turbine. The APU exhaust system also reduces the noise of exhaust gases by the use of mufflers in the exhaust duct. So in a jet engine, the exhaust system releases gases into the atmosphere. The exhaust gas flow leaves the engine in the necessary direction and with the optimum velocity to make an efficient thrust. On some jet engines, the exhaust system has a tube named the exhaust duct or tailpipe. This is a conical tube that is supported at its forward end by the turbine exhaust case.

The exhaust nozzle at the aft end of the exhaust duct accelerates the exhaust gases which leave the engine.

The exhaust cone guides the discharge flow and prevents excessive turbulence of the gases. The cone also prevents the reverse flow of the exhaust gases into the hub of the turbine rear stage. On modern high-bypass turbofan engines, the two exhaust streams usually exhausted separately. The hot gas flow exhausts via the primary nozzle and the cold fan airflow exhausts via the secondary nozzle.

 separate hot and cold stream nozzles
separate hot and cold stream nozzles

 

On modern long-range aircraft the high-by-pass engines sometimes have a combined exhaust nozzle. This system mixes hot and cold gas flow to reduce the velocity of the exhaust gases. The common exhaust nozzle’s advantage is that it reduces the very high exhaust gas velocities of the hot gas flow by mixing of the two gas flows. This gives higher propulsion efficiency but the disadvantage of the common exhaust nozzle is that it increases the weight of the engine. Some engines have an extra particular exhaust gas mixer. This mixer improves the mixing of the hot and cold gas flow to get higher propulsion efficiency. But, as you would expect, these mixers also add weight to the engine.

The gas leaving the last stage of the turbine section still possesses pressure potential, kinetic and heat energies. The objective now is to produce the maximum possible acceleration in the gas by converting the pressure energy to kinetic energy. This is carried out, by expanding the gas through a convergent exhaust nozzle as the gas exits to the atmosphere. It will be this acceleration that produces the thrust reaction.

The problem is the gas has first to be ducted to the exit nozzle before the acceleration takes place.

The last thing needed acceleration inside a long parallel walled duct like a jet pipe. If this is allowed, friction between the gas and the jet-pipe wall would create heat. As heat is a form of energy it must come from somewhere and pressure energy will be stolen out of the gas to produce it.

This means there would be less pressure energy to convert to kinetic energy at the exit nozzle and less thrust would be produced as a result. The pressure loss in a gas due to duct wall friction with respect to duct length is proportional to the cube of the gas velocity. Double the gas velocity and pressure loss will rise six times. As the gas leaves the turbine section it is traveling at around Mach 0.8 and possesses a significant residual whirl. Left like this, the gas would spiral down the jet-pipe at high velocity.

 exhaust gas mixer
exhaust gas mixer

The whirl means that the gas would be in contact with a greater length of the duct wall than it would be if the gas flow was truly axial. The high velocity would also ensure that pressure loss due to friction would be high. So, it makes sense to straighten the gas flow and slow it down.

If the kinetic energy is reduced it converts to pressure energy.

The object is to conserve as much pressure energy in the gas as is possible so as to convert it kinetic energy in the convergent exhaust nozzle. There will be some pressure loss due to friction but good design should keep this to a minimum. If you study the illustration you will see that the gas leaving the turbine section passes into a conical exhaust unit containing an inner exhaust cone supported by wide chord aerofoil sectioned struts. The duct section between the exhaust cone and the exhaust unit wall is divergent. The gas velocity reduces and its pressure rises. The gas flow is straightened as it passes through the cone support struts. The exhaust cone also acts as a heat shield and prevents the hot exhaust gases from reaching the rear face of the turbine disc and the turbine-bearing chamber.

Ideally, the gas should now exit through a convergent exhaust nozzle without delay. A long jet-pipe merely creates friction and gas pressure loss. On some military aircraft, the engine is mounted in the fuselage close to the centre of gravity so a long jet pipe is unavoidable and thrust is lower as a direct result of this.

A short jet-pipe is the aim.

The gas should enter the jet-pipe with a truly axial flow direction with its velocity reduced to around Mach 0.5. As the gas reaches the convergent exhaust nozzle it then expands and accelerates through the nozzle. The gas pressure and temperature fall as the kinetic energy increases. Ideally, the gas should exit the nozzle close to Mach 1 with its pressure and temperature reduced to ambient conditions.

If the jet-stream gas pressure is higher than ambient after exit the nozzle is said to be under-expanding the gas. Simply, there would be pressure left that could have been converted into velocity but is now wasted. Because the exhaust nozzle creates the acceleration in the gas it is referred to as the propelling nozzle.

Current civil high by-pass engines employ a short converging section tailpipe.

The cross-sectional area of the propelling nozzle is extremely important. If the area were too small for the volume of gas trying to pass through, the gas pressure in the jet-pipe would be too high. This would reduce the pressure drop across the turbines causing them to slow and create backpressure, which would result in a rise in the compressor diffuser pressure. The compressors would then move towards the stall and surge. The propelling nozzle may choke if the exhaust gas velocity reaches Mach 1, which would further aggravate the problem. The initial increase in thrust gained by accelerating the gas to Mach 1 would soon be lost if the engine were to become unstable.

If the propelling nozzle orifice area were too large, the jet-pipe pressure would drop to a very low value. This would increase the pressure drop across the turbines causing the engine to over speed. The nozzle would be over expanding and thrust would be reduced. The correct size of the propelling nozzle is essential to achieve the correct balance of gas pressure, temperature, and thrust. A fixed area propelling nozzle can only be efficient over a very narrow engine operating range. A variable nozzle is sometimes employed for example on the Concorde’s Olympus 593 engines. The increase in equipment weight precludes the use of variable nozzles on other civil engines. The nozzle area in these cases is selected to give rated Thrust at take-off whilst maintaining an efficient thrust under cruise conditions.

 

INNER EXHAUST CONE

This cone sometimes referred to as an exhaust plug and typically made from Inconel. The cone is supported in the outer exhaust unit cone by wide chord streamlined struts. In conjunction with the outer cone, the inner exhaust cone forms an annulus with a divergent cross-sectional area. The exhaust cone assembly acts as a diffuser, which reduces the gas velocity, increases the pressure and straightens the flow.

The inner cone also acts as a heat shield to the face of the rear turbine disc. A vent hole often positioned at the apex of the cone. This prevents low pressure forming inside the cone which could cause buckling. The axial position of the cone adjusted on some engine types. This usually achieved using spacers in the cone forward attachment ring. This allows test bed set up of the exhaust unit area to adjust the rated thrust output. The end cap of the cone may contain a detune mass to damp out the vibration of the cone or plug which could lead to cracking and possible detachment of the cone in flight.

EXHAUST CONE STRUTS

These support the inner cone and remove the residual whirl from the gas. This ensures that the gas flow entering the jet-pipe is truly axial. They normally fabricated from Inconel.

JET-PIPE

The jet-pipe fixed at the front end to the outer exhaust unit cone and supported at the rear end in a fashion that permits axial expansion. Its function is to conduct the exhaust gases to the propelling nozzle with minimum pressure loss whilst protecting the surrounding airframe structure from heat damage. Ideally, the jet-pipe should be as short as is practical and have the lowest practical diameter. This would reduce the gas to wall friction and conserve gas pressure. On longer jet-pipes, a swinging support link provided close to the center of gravity.

The rear support may consist of an expansion joint containing support rollers. The jet-pipe has heat insulation blankets fitted to prevent heat radiating to the airframe. On modern pylon mounted engines, the jet-pipe is very short and has a converging taper section. In conjunction with the exhaust plug, this forms the propelling nozzle. The high by-pass engines may eject the by-pass cold stream separate or, more recently, integrated with the hot core flow.

PROPELLING NOZZLE

This designed to have a convergent cross-section. The gas subjected to expansion as it passes through the nozzle. This raises the velocity of the gas as its pressure and temperature decrease. The ideal exit velocity of the gas will be close to Mach 1. If the gas velocity at the throat of a convergent propelling nozzle reaches Mach 1 the nozzle will choke. Most propelling nozzles operate in the choked condition as this gives the maximum obtainable propulsive thrust.

 

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