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6. Exhaust
6.1 Principles of Operation
6.2 Gas Flow Characteristics
6.3 Manufacture & Materials
6.4 Afterburners: Introduction
6.5 Afterburners: Operation
6.6 Afterburners: Control System
6.7 Afterburners: Design & Manufacture
6.8 Afterburners: Performance
6.1 Principles of Operation
All jet engines have an exhaust system with a varying degree of complexity to guide the exhaust gases from the turbine to the atmosphere at a given velocity and direction. After all, it is the velocity and pressure of the exhaust gases that generate the thrust that propels the aircraft forward. This is not true for turboprops where the majority of the energy from exhaust gases is used up to drive the propeller. The design of the exhaust section is seemingly simple but it can have a significant effect on the overall performance of an engine.
The temperature of the gases entering the exhaust can be anywhere between 550°C and 850°C, depending on the type of the engine. Turboprops and turbofans with high bypass ratios have the cooler exhaust gas flows. On the opposite end of the spectrum, if an afterburner is used, the temperature in the jet pipe can reach and exceed that of 1500°C. Although the way combustion is carried out and the advanced cooling design protect the liner and exhaust casing from being exposed to such high temperatures, the material selection and manufacturing are crucial in avoiding thermal fatigue and cracking.
The temperature of the gases entering the exhaust can be anywhere between 550°C and 850°C, depending on the type of the engine. Turboprops and turbofans with high bypass ratios have the cooler exhaust gas flows. On the opposite end of the spectrum, if an afterburner is used, the temperature in the jet pipe can reach and exceed that of 1500°C. Although the way combustion is carried out and the advanced cooling design protect the liner and exhaust casing from being exposed to such high temperatures, the material selection and manufacturing are crucial in avoiding thermal fatigue and cracking.
Commercial jet engines have a fixed exhaust nozzle that slightly converges to promote the jet formation (i.e. acceleration of the exhaust gases). This is very simple in principle, but in reality, noise reduction features add to the complexity of the geometry and manufacturing. However, if an afterburner is used, the exhaust nozzle must be capable of switching between two positions or be fully variable to enable efficient operation across the range of exhaust gas volumes. This adds a significant amount of complexity to the design, assembly and operation of the exhaust section. In other cases, notably where high bypass ratios are used, the exhaust section may incorporate flow mixing features.
6.2 Gas Flow Characteristics
Exhaust gases leave the turbine at a speed typically between 200-350 m/s, which is high enough for significant friction losses along the jet pipe to arise. As such, it is desirable to slow down the flow which is achieved through the exhaust cone, which serves two functions; it gradually increases the cross-sectional area, thus slowing down the flow, and it also prevents the flow from recirculating at the end of the turbine, towards the centre of the disc. The velocity at this part of the exhaust is usually kept constant at around Mach 0.5, which is just under 300 metres per second, for the temperature of the gases (remember that the speed of sound increases as the temperature increases). Losses are also introduced by the slight swirl of the flow as the flow leaves the turbine, but the attachment of the cone to the casing is realised with aerofoil-shaped segments that also assist in correcting the direction of the flow.
Nozzle Geometry
Before the gases are released to the atmosphere, they go through a convergent section that raises the discharge velocity. Generally speaking, the discharge velocity is subsonic only when the thrust levels are intentionally low. In normal operating conditions, the flow reaches sonic levels for a given exit temperature. At that point, it can be described as choked, i.e. it is not possible to further increase the velocity of the flow unless the temperature is increased. This gives rise to a pressure at the end of the nozzle that is higher than the atmospheric pressure and the exchange of potentials results in a 'pressure thrust'.
A simple convergent exhaust nozzle wastes energy as the exiting gases do not expand quickly enough to the atmospheric pressure level. Depending on the type of engine and aircraft, a convergent-divergent (de Laval) exhaust nozzle may be used to recover some of the energy potential from the elevated pressure levels and lead to a further acceleration of the flow.
A simple convergent exhaust nozzle wastes energy as the exiting gases do not expand quickly enough to the atmospheric pressure level. Depending on the type of engine and aircraft, a convergent-divergent (de Laval) exhaust nozzle may be used to recover some of the energy potential from the elevated pressure levels and lead to a further acceleration of the flow.
In a convergent-divergent nozzle, the exhaust gases are accelerated up to the sonic point at the throat (the narrowest point) and are fully supersonic in the divergent section up until exiting the nozzle. This rapid expansion also results in a net thrust on the walls of the divergent section, further increasing the total thrust generated.
The size of the nozzle is very important in achieving the balance between pressure, temperature and thrust requirements. A smaller nozzle area leads to an increase in all these variables but also increases the risk of compressor surge. Increasing the area lowers these values, but it might do so to inefficient levels. Even in engines where the external nozzle walls are fixed, it is possible, or even desirable, in some cases to do small adjustments to the cross-sectional area of the nozzle using movable flaps or plugs for more efficient operation. This is so that a slightly larger area is used during lower engine speeds (and subsequently temperatures) and is decreased when the conditions allow for maximum thrust.
The size of the nozzle is very important in achieving the balance between pressure, temperature and thrust requirements. A smaller nozzle area leads to an increase in all these variables but also increases the risk of compressor surge. Increasing the area lowers these values, but it might do so to inefficient levels. Even in engines where the external nozzle walls are fixed, it is possible, or even desirable, in some cases to do small adjustments to the cross-sectional area of the nozzle using movable flaps or plugs for more efficient operation. This is so that a slightly larger area is used during lower engine speeds (and subsequently temperatures) and is decreased when the conditions allow for maximum thrust.
Bypass Mixing
As has been discussed previously, jet engines with a bypass give rise to a cold stream of bypassed air and a hot stream of core flow. In some cases, the two streams are introduced to a common segment of the exhaust for more efficient mixing. In others, and particularly those where high bypass ratios are used, the mixing may be external as each exhaust cross-section is optimised for the specific flow stream.
6.3 Manufacture & Materials
It is apparent that the exhaust must be able to cope with the high temperatures. This is achieved using a combination of heat resistant alloys and efficient cooling using bleed and bypass air. However, unlike the turbine, the exhaust assembly is in closer proximity to other parts of the aircraft and, in order to protect these, heat insulation is used. The outer layers of the heat insulation are often made of textured steel to increase the surface area and to accommodate larger deformations without cracking. In addition to thermal insulation, noise-reducing layers may also be used. This can take many different forms, but a lightweight honeycomb structure sandwiched between steel sheets is a common option.
For very high exhaust gas temperatures, the jet pipe typically consists of two concentric cylinders with a gap between them. The inner one is called the liner and a cool stream of air flows between the liner and the casing. This flow is essential in keeping the temperature of the liner to acceptable levels and also crucial in carrying away heat from the core flow. The liner is perforated to allow a controlled introduction of cool bypass air into the hot core section.
The exhaust cone and the aerodynamic mounts are also designed in such a way that allows for thermal expansion and alleviation of the thermal stresses that develop during operation.
This consideration extends to other parts in the exhaust - the circumferential and axial thermal expansion can be significant and the mounting techniques are such that should allow for this expansion and contraction without excessive deformations or cracking.
The exhaust cone and the aerodynamic mounts are also designed in such a way that allows for thermal expansion and alleviation of the thermal stresses that develop during operation.
This consideration extends to other parts in the exhaust - the circumferential and axial thermal expansion can be significant and the mounting techniques are such that should allow for this expansion and contraction without excessive deformations or cracking.
6.4 Afterburners: Introduction
Afterburning is a method of significantly increasing the thrust of a jet engine for a short period of time which can be useful in particular circumstances such as the need to depart very quickly or with a short take-off length. Many combat jets are equipped with engines with afterburners; to put the cost of afterburning in context, even in this generously funded industry, the shear fuel consumption rate and damage debit to the engine limit the use of the afterburner to cases where its necessary to do so.
Afterburning is realised by injecting fuel in the space between the turbine and the exhaust nozzle to combust with the remaining air. This process increases the velocity of the flow and subsequently the attainable thrust. The temperature in the core of the jet pipe can be as high as 1700°C, but this extremely hot stream of air is concentrated in the centre of the pipe, while a cooler 'boundary' flow keeps the liner from overheating. As discussed above, the exhaust nozzle area should be increased when the afterburner is being used to avoid an excessive rise in pressure that would negatively affect the operation of the engine as a whole.
Conversely, when an afterburner is installed but not in use, the area at the end of the exhaust is slightly smaller than if the engine had no afterburner installed at all. This is to account for the slightly different arrangement of the exhaust components and the added weight.
If the engine has a bypass, the injection of fuel is carried out after the two streams have rejoined. In rare cases, the afterburning can be carried out in the two flows separately, whilst the mixing is done before the flow enters the nozzle. Even in the latter case, there is some interaction between flows to aide the combustion in the colder stream.
Afterburning is realised by injecting fuel in the space between the turbine and the exhaust nozzle to combust with the remaining air. This process increases the velocity of the flow and subsequently the attainable thrust. The temperature in the core of the jet pipe can be as high as 1700°C, but this extremely hot stream of air is concentrated in the centre of the pipe, while a cooler 'boundary' flow keeps the liner from overheating. As discussed above, the exhaust nozzle area should be increased when the afterburner is being used to avoid an excessive rise in pressure that would negatively affect the operation of the engine as a whole.
Conversely, when an afterburner is installed but not in use, the area at the end of the exhaust is slightly smaller than if the engine had no afterburner installed at all. This is to account for the slightly different arrangement of the exhaust components and the added weight.
If the engine has a bypass, the injection of fuel is carried out after the two streams have rejoined. In rare cases, the afterburning can be carried out in the two flows separately, whilst the mixing is done before the flow enters the nozzle. Even in the latter case, there is some interaction between flows to aide the combustion in the colder stream.
6.5 Afterburners: Operation
As was the case previously, the velocity of the exhaust gases leaving the turbine is very high. However, when an afterburner is used, this velocity is not only a source of frictional losses, but it also inhibits complete combustion. In a similar fashion, the cross-sectional area is increased to reduce the velocity before the fuel is injected. In some designs, this reduction would have not been adequate and would have resulted in the unburned fuel being effectively carried away by the flow towards the atmosphere. To prevent that, flame stabilisers are sometimes used immediately after the fuel injectors. The purpose of this is to introduce a slight swirl that inherently slows down the axial component of the flow and sustains an efficient combustion process.
The fuel injectors are uniformly distributed along a ring in the middle of the afterburner. Igniting the fuel and air mixture can be realised in a number of ways:
Even though it would be technically possible for self-ignition to take place due to the high temperatures already present between the turbine and the exhaust, this would only be the case at sea level and assuming the temperature is always 800°C or more. The change in pressure at high altitudes makes self-ignition impossible and as a result, a robust ignition system is required.
The fuel injectors are uniformly distributed along a ring in the middle of the afterburner. Igniting the fuel and air mixture can be realised in a number of ways:
- Through catalytic combustion that is the result of a chemical reaction as the air and fuel mixture is vaporised over a metallic element (typically made of platinum or rhodium)
- Through the spark from a spark plug positioned next to the fuel injector
- Through hot-shot ignition where a flame originating from the main combustion chamber also ignites the fuel in the afterburner
Even though it would be technically possible for self-ignition to take place due to the high temperatures already present between the turbine and the exhaust, this would only be the case at sea level and assuming the temperature is always 800°C or more. The change in pressure at high altitudes makes self-ignition impossible and as a result, a robust ignition system is required.
6.6 Afterburners: Control System
The efficient operation of the afterburner requires the coordination of the fuel injection system and the mechanical system controlling the variable exhaust nozzle. In earlier engines, the pilot was able to control the exhaust area based on the ratio between the pressure at the exit of the compressor and the pressure in the jet pipe. When the area was manually increased, the fuel was automatically increased. A control system based on these measurements ensures that the pressure ratios across the engine are stable regardless of whether the afterburner is on or off. In more modern engines, the afterburner is used at pre-defined settings and enabling each one of them automatically adjusts both the exhaust area and the fuel supply.
When afterburning is initiated, the fuel valve opens to inject fuel into the afterburner and its combustion leads to a rise in the pressure in the jet pipe (such as the ratio of pressures at the exit of the compressor and the jet pipe drops). In response, the petals of the exhaust open up to increase the cross-sectional area and to subsequently drop the pressure in the jet pipe (the pressure ratio is restored to the same levels as before the afterburner was used).
It is also worth noting that the fuel consumption of the afterburner is rather high and in order to avoid fluctuation in the supply of the combustion chamber, a separate fuel pump is typically used for the afterburner.
When afterburning is initiated, the fuel valve opens to inject fuel into the afterburner and its combustion leads to a rise in the pressure in the jet pipe (such as the ratio of pressures at the exit of the compressor and the jet pipe drops). In response, the petals of the exhaust open up to increase the cross-sectional area and to subsequently drop the pressure in the jet pipe (the pressure ratio is restored to the same levels as before the afterburner was used).
It is also worth noting that the fuel consumption of the afterburner is rather high and in order to avoid fluctuation in the supply of the combustion chamber, a separate fuel pump is typically used for the afterburner.
6.7 Afterburners: Design & Manufacture
The material selection for the afterburner is largely similar to that of the exhaust- the jet pipe is often made of alloy steel that can sustain high temperatures, but is slightly more demanding from a manufacturing point of view and requires more insulation than jet pipes of engines without an afterburner. This is due to the elevated temperatures and pressures, even momentarily. A liner is also used in most afterburner assemblies and, apart from its primary cooling purpose, it also acts as an acoustic and vibration barrier to the rest of the assembly.
The nozzle is typically manufactured and assembled separately before being bolted onto the jet pipe. The nozzle can either be a two-position nozzle, that moves only between an open and closed state, or a variable area nozzle. The former consists of two 'eyelids' that extend and retract depending on whether the afterburner is on or off. The latter consists of multiple hinged slates ('petals') which can be naturally pushed by the gas flow to open up. Hydraulic or electromechanical actuation is used to apply resistance if the nozzle shall remain at its minimum (smallest diameter) position.
The nozzle is typically manufactured and assembled separately before being bolted onto the jet pipe. The nozzle can either be a two-position nozzle, that moves only between an open and closed state, or a variable area nozzle. The former consists of two 'eyelids' that extend and retract depending on whether the afterburner is on or off. The latter consists of multiple hinged slates ('petals') which can be naturally pushed by the gas flow to open up. Hydraulic or electromechanical actuation is used to apply resistance if the nozzle shall remain at its minimum (smallest diameter) position.
The fuel injection system consists of one or more concentric arrays of injectors that are affixed to the afterburner casing. The mounting pillars also house the pipes or channels that carry the fuel to the injection points. The flame stabiliser is a V-shaped ring that is mounted immediately after the injectors.
6.8 Afterburners: Performance
Thrust Increase
The increase in thrust that can be delivered from the use of the afterburner is dependent on the increase of the gas velocity, which is dependent on the increase of the temperature at the end of the exhaust. The velocity ratio increase is equal to the square root of the temperature ratio increase. For example, if the temperature of the exhaust gases after the turbine is 650°C and after afterburning, the temperature is equal to 1250°C, then the temperature ratio increase is equal to:
\(\frac{T_2}{T_1}=\frac{1250+273}{650+273}\approx1.650\)
As such, the increase in the velocity ratio will be equal to:
\(\sqrt{1.650}\approx1.28\)
i.e. 28%. The static thrust of jet engines with a bypass can increase by up to 70% if an afterburner is used. When cruising at high speeds, the additional thrust can be much higher. Ultimately, and since the increase in thrust depends on the temperature increase, the thermal capabilities of the materials and components in an afterburner is what sets its peak potential.
\(\frac{T_2}{T_1}=\frac{1250+273}{650+273}\approx1.650\)
As such, the increase in the velocity ratio will be equal to:
\(\sqrt{1.650}\approx1.28\)
i.e. 28%. The static thrust of jet engines with a bypass can increase by up to 70% if an afterburner is used. When cruising at high speeds, the additional thrust can be much higher. Ultimately, and since the increase in thrust depends on the temperature increase, the thermal capabilities of the materials and components in an afterburner is what sets its peak potential.
Fuel Consumption
We have already mentioned that the use of an afterburner has a dramatic effect on the total fuel consumption. Even though the thrust gains can be significant, the thrust-specific fuel consumption (TSFC) is also significantly increased. The fundamental reason for this drop in fuel efficiency is that the combustion in the afterburner takes place at relatively low pressure and high velocity, both of which are not ideal.
This means that for an aircraft travelling at sea level and with a velocity of Mach 0.9, the TSFC is 2.2x higher when the afterburner is used. If we consider that the time saved to reach the cruising altitude is noticeably shortened, the use of the afterburner during take-off is not as prohibitive from a fuel consumption point of view. In cases like taking off from an aircraft carrier or a short runway at high altitude, the use of an afterburner is both justified and very helpful.