Table of Contents
11.2.4.1 Fundamentals of Damage-Relevant Operating Behavior of Afterburners, Tailpipes, and Thrust Reversers
The exhaust gas zone behind the turbine is subjected to high thermal and mechanical loads. The unique flow conditions in the afterburner and tailpipe create special effects such as pressure vibrations due to unstable combustion. This causes specific damages to the affected engine parts.
Damage to and problems with afterburners/augmentors: Ignition (Fig. "After burner triggered compressor surge") and stable combustion (Figs. "Instabilities in combustion chambers and afterburners" and "Excitement of combustion vibrations") are problematic in the entire flight envelope. Reliable ignition can be achieved in various ways. Common measures are devices in the afterburner pipe that resemble a type of small combustion chamber (torch igniter, Fig. "Damages at after burner 'torch igniter'"). The torch igniter is lit with a spark plug or a catalytic (platinum) igniter. The afterburner can also be ignited with the aid of a brief darting flame from the combustion chamber through the turbines (Ref. 11.2.4-9). In this process, fuel is briefly sprayed into the combustion chamber from a small pressure accumulator. It can be assumed that this may also harbor the potential danger of damage to the turbine blading. The ignition system itself can be damaged in ways that compromise the ignition process (similar to combustion chambers, Fig. "Sensitiveness of 'Spark plugs' to poor design"). In addition, problems with the injection system can prevent correct ignition. The flame may also go out again after ignition if, for example, it is blown out of the afterburner pipe (Fig. "Excitement of combustion vibrations"). A pressure wave is created during ignition. If a cloud of fuel is still present in the afterburner pipe following a failed ignition, it can result in an especially powerful pressure wave. This can cause stalls in the fan area and in the compressor of the core engine (Example "Unfavourable afterburner conditions at high altitude", Fig. "After burner triggered compressor surge").
Stable combustion in the afterburner is not a matter of course (Figs. "Excitement of combustion vibrations" and "Influence on combustion vlbrations"). Due to the slow spreading rate of the kerosene flame, the flow speed of the fuel/gas mixture must be reduced considerably to prevent the flame being blown out of the afterburner with the mixture. This task is accomplished by the flame holder. The flame holder disturbs the flow so that zones with sufficiently low gas speeds are created (Fig. "Flame holder in afterburner"). All combustion processes have pressure pulses (sonic vibrations). These are usually within the design parameters of the engine. Since the early 1950s, researchers have been concerned with the phenomenon of dangerously powerful pressure vibrations in the high-frequency range of 300 to several 1000 Hz (screech, Fig. "Excitement of combustion vibrations"). Military fan engines, the bypass flow of which can be mixed with the hot gas from the combustion zone of the afterburner in various configurations (mixed flow, Fig. "Excitement of combustion vibrations"), have an additional problem. Low-frequency pressure vibrations between 30-300 Hz (buzz, Fig. "After burner designs and 'buzz'") have come
under scrutiny. These self-inciting and self-increasing vibrations can dynamically (Fig. "Gas vibrations inafterburner cause stalls in the fan") and/or thermally (Fig. "Dmage by afterburner 'screetch'") overload parts of the afterburner, turbine, and even the fan in a matter of seconds (Example "Missing cold stream nozzle"). Buzz can also originate in the inlet duct ahead of the engine and then be further excited in the afterburner.
The mechanical loads on afterburner pipes are often underestimated. The heat emission from an afterburner during operation is large enough to require the use of heat shields (inner AB liner) that are cooled by an airflow along their outer surface. This cooling air flows through many small openings and creates a cooling air film on the inner wall. Low-frequency dynamic loads resulting from restricted thermal expansion (thermal fatigue) and bending loads from G-forces combine with high-frequency vibrations, which are caused by unstable burning. For this reason, cracks and fractures due to dynamic fatigue are not uncommon in afterburner liners. These damages are concentrated in typical problem areas such as roll seam welds, flanges, and overlapping metal (Fig. "Fracture of after burner mantle pipe" and Volume 1, Ill. 4.4-5).
One must also mention afterburners in VTOL fighter aircraft that are still under development. In this case, the hot gas jet of the activated afterburner is redirected, and it must be assumed that new challenges regarding stable combustion and the loads on the adjustable mechanisms in the afterburner await the designers.
Tailpipe: Engines of commercial aircraft generally have relatively simple fixed conical tailpipes. However, reducing the noise levels requires a mixer with a complicated shape. Similar configurations can be found in the engines of military helicopters. In this case, the mixer serves primarily to minimize the helicopter`s infrared signature. This is intended to reduce the helicopter`s vulnerability to heat-seeking weapons. Even though there is no available literature on the subject, it can be assumed that these thin-walled, filigreed metal sheet constructions are sensitive to high-frequency vibrations.
Fighters usually have adjustable tailpipes. These are fairly complicated devices with multiple flap systems and adjustment mechanisms (joints, links), a power unit (mechanical, hydraulic), and a control system. Convergent-divergent tailpipes are even more complicated. This complexity will increase if adjustable tailpipes for jet redirection (thrust vector jets) reach the serial production stage. The mechanical and control systems increase the failure risk of these systems. An additional complication is that problems resulting in only minor deviations during the vertical takeoff and landing process can have catastrophic consequences. Mechanical adjustment systems will most likely shift the issue of gliding- and dynamic wear (Volume 2, Chapter 6 and Fig. "Adjustable thrust nozzle problem zones") into the forefront. Problems can also be expected from the complicated control system.
Thrust reversers: Thrust reversers in commercial aircraft are located in the bypass flow behind the fan (Fig. "Thrust reverser reliability"), and in fighters they are located in the hot gas flow (Fig. "Hot gas thrust reverser problem areas"). This means that they are subjected to very different temperatures, and their major damage mechanisms will vary accordingly. Thrust reverser malfunctions present a special danger to the aircraft if they fail to open or open unintentionally during flight (Fig. "Problems during thrust reverser activation", Example "Missing cold stream nozzle"). Example "Importance of thrust reverser locking system" (Ref. 11.2.4-23) shows that even thrust reversers in civilian fan engines, which operate in the cool bypass flow, can suffer damage with considerable consequential damages to the nacelle. In this case, the result was fracture and partial loss of the redirecting nozzles.
Figure "Instabilities in combustion chambers and afterburners" (Ref. 11.2.4-13): Certain areas of the flight envelope (right diagram) promote the occurrence of unstable combustion in the afterburner. Slow flight speeds at great altitudes have specific effects on combustion in the afterburner.
In this case, poor conditions for the functioning of the flame holder are:
- low temperatures
- low combustion pressure
- high Mach speeds in the flow.
Fuel droplets are stabilised under these conditions, increasing the probability of low-frequency vibrations (“buzz”). The behavior of combustion in the afterburner is similar to that in the combustion chamber (Ill. 11.2.2.1-4).
At high flight speeds at low altitudes, large amounts of heat are released by conditions such as
- high turbine temperature
- high fuel/air ratio
- increased blockage of the air flow by the flame holder
- high pressure.
This increases the probability of high-frequency combustion vibrations (“screech”).
Figure "Excitement of combustion vibrations" (Ref. 11.2.4-13): Theories regarding the development and increasing intensity of unstable combustion are evidently still not entirely clear and remain under discussion. The following probable relationships are given in the literature, but they are not completely free from inconsistencies. Because recent work with low-emission combustion chambers has contributed a great deal towards the understanding of combustion phenomena, comparable processes in combustion chambers are also mentioned (Ill. 11.2.2.1-4).
The excitement of high-frequency pressure vibrations (screech) is the interplay of pressure disturbances and the combustion process. It is assumed that a pressure wave increases the heat emission of a flame in phase with the pressure disturbance. Inside the resulting hot spot that is carried by the air flow, the gas volume increases with the temperature and increases the process. The inciting pressure disturbances are assumed to be in periodic swirl separations (bottom diagram) at the edge of the flame holder. These create a radially-acting wave (Fig. "Dmage by afterburner 'screetch'") that can also travel around the circumference.
Swirl separations are also suspected of inciting buzz (low-frequency gas vibrations). These separations can also form at splitters in the inlet of the bypass duct and at the edges of the flame holder rings. Periodically changing fuel flows, e.g. due to vibrations in the fuel system, can cause unstable combustion. Similar effects are to be expected if periodical flame blowout and reignition occur at the flame holder.
Figure "Influence on combustion vlbrations" (Ref. 11.2.4-13): Several effects promote combustion vibrations. These are primarily the evaporation rate of the fuel and the mixing of the fuel with the gas flow. Poor evaporation and insufficient mixing promote undesirable stabilization and distribution of fuel droplets in the hot gas, which promotes combustion vibrations.
Figure "After burner designs and 'buzz'" (Ref. 11.2.4-18): This depiction is based on the afterburner development of a certain OEM. However, one can assume that this design evolution has a certain universal relevance. The afterburners of bypass engines used in today`s fighters can be classified into three categories according to their design concepts. The characteristic factor is the mixing of the airflow from the bypass with the hot gas from the core engine:
“Mix-then-burn” afterburner: This early concept (top diagram) has a satisfactory operating behavior. Its design characteristic is the flow mixer between the turbine exit and the burning zone of the afterburner. The diameter of the mixer is relatively small compared with those of the engine and afterburner pipe. The length of the mixer is roughly the same as its diameter.
The drawback of this afterburner type is its length. This creates special problems for engines located in the nacelle (typical configuration for fighters). These problems are related to volume, weight, and mechanical loads due to G-forces.
A further, more serious drawback of this concept is revealed during operation. This is the tendency to low-frequency combustion instability (buzz, rumble) at intermediate and high thrust levels. These gas vibrations can lead to fatigue damage very quickly.
Without the afterburner, the good mixing of fuel and air increases (dry) thrust. During afterburner operation, however, the flow resistance of the mixer causes an increased loss of thrust. Therefore, this afterburner type is less suitable for fighters with frequent afterburner use (i.e. less cruising flight without afterburner use).
“Mix-and-burn” Afterburner: This intermediate concept was efficently only tested in a development phase. It has a shorter structure (middle diagram) than the above concept. Fuel is injected into the bypass flow through the slits of the mixer. The mixer slits direct the bypass flow radially inward at an angle. The hot gas flow from the core is directed diagonally outward and has fuel injected into it ahead of the flame holder. This creates an intensive mixing of the two gas flows. The efficiency and thrust of this system are close to those of the mix-then-burn type.
The specific drawback of this concept (also see Fig. "Conclusions from pressure vibrations in after burner pipe") is the fact that the fuel droplets do not evaporate sufficiently quickly in the bypass flow. The unevaporated fuel droplets concentrate around the central axis of the afterburner and enter the tailpipe, where they create a high fuel/air ratio. This explains the relative instability of this afterburner type at high thrust levels. This made the stoichiometric burning required for maximum thrust unattainable.
“Mix/burn” afterburner: The requirements for this concept were the greatest possible thrust increase with the smallest dimensions. The thrust should be available especially during takeoff. In order to improve fuel distribution and suppress buzz, the gas flows are not mixed in this design (bottom diagram). However, a certain amount of mixing due to diffusion of the gas flows is unavoidable (bottom diagram). An especially ingenious combined injector/flame holder system made this design possible. The system is different in the bypass and core. Fuel vaporizers with a unique design that ensures even fuel distribution in the wake zone are used.
The bottom left diagram shows the buzz behavior of the three described afterburner designs. One can see the varying influence of the fuel/air ratio in the bypass flow and engine core flow on the combustion instability. In the mix/burn design, as opposed to the other two designs, the largely unmixed gas flows mean that the combustion behavior is not determined by the reciprocal influence of the fuel amounts in the two gas flows on one another.
Figure "After burner problem zones": Like every engine component, afterburners have typical construction-specific problem zones. The injection system influences the stability of combustion (Fig. "After burner designs and 'buzz'" ), and therefore also affects the dynamic and thermal loads on the afterburner wall (Fig. "Dmage by afterburner 'screetch'"). Similar problems can be expected in the injection systems of combustion chambers and afterburners, such as coking (Fig. "Damage by coke build up at the fuel nozzle"). In addition, the complicated and filigreed injector systems are susceptible to fuel leaks. Low- and high-frequency dynamic loads can cause fatigue cracks with serious consequences (Volume 1, Ills. 4.3-3 and 4.3-4). The flame holder is subjected to especially high, changing, local part temperatures. This leads to typical damages such as heavy oxidation (burning) and thermal fatigue. Fretting especially affects flexible constructs with socket connections for balancing thermal strain.
Experience has shown that torch igniters are subjected to high thermal loads and exhibit all types of temperature-dependent damage, such as burning, cracking, warping, and hot gas corrosion (Fig. "Damages at after burner 'torch igniter'").
Bending loads and vibrations stress the outer afterburner liner. This causes repeated fatigue damage to failure (Volume 1, Ill. 4.4-5). In extreme cases, this can result in separation of the entire afterburner (Fig. "Fracture of after burner mantle pipe").
The inner liner suffers under similar loads to a corresponding inner combustion chamber wall (Figs. "Typical combustion chamber damages" and "Thermal fatigue in combustion chambers"). It is subjected especially to cyclical thermal loads. Hot streaks play an important role in thermal fatigue, overheating, and warping (Fig. "Hot gas streaks as combustion chamber problem").
Regulation of the fuel flow must also be mentioned. It has a decisive influence on any ignition problems and combustion instability (Fig. "After burner designs and 'buzz'").
Figure "Flame holder in afterburner" (Ref. 11.2.4-12): The progress of a flame in a kerosene/air mixture is surprisingly slow at a few meters per second. If this mixture moves faster than the flame travels in the opposite direction, it carries the flame out of the afterburner with it. This means that special measures are necessary to keep the flame in the afterburner pipe. Flame holders are used to swirl the mixture in a way that creates localized stable zones with sufficiently low gas speeds. This allows the flame to stabilize behind the flame holders. Combustion chambers also require flame holders, albeit of a different type. The aerodynamic flame holders in combustion chambers direct jets of air into the primary zone, creating swirls in which the flame can stabilize (Fig. "Combustion chamber components and processes").
Figure "Adjustable thrust nozzle problem zones": An adjustable tailpipe is a complicated mechanical device consisting of many individual parts that move in relation to one another. This accordingly requires joints, guides (links, slide rings, rollers), and sealing surfaces. Surfaces that are directly exposed to the gas flow (inner flaps) are subjected to high dynamic and thermal loads. This promotes damage mechanisms such as thermal fatigue, gliding wear, fretting, and HCF dynamic fatigue. Damaged and/or malfunctioning tailpipe regulators and control systems can cause extensive consequences (Volume 1, Ills. 5.1.3-5 and 5.1.3-6).
Figure "Thrust reverser reliability" (Ref. 11.2.4-11): Thrust reversers behind the fan are typical for civilian engines and are usually part of the design responsibility of the nacelle manufacturer. These thrust reversers have moveable parts (flaps and their controls) that are influenced by the surrounding atmosphere of the outer air flow and convection while standing (Example "Missing cold stream nozzle", Ref. 11.2.4-23). They are subjected to a combination of the following problems and loads:
Operating loads:
- Stress from restricted thermal expansion (LCF)
- Overheating, aging (e.g. of fiber-reinforced synthetic parts)
- Vibrations (HCF, Examples 11.2.4-3 and 11.2.4-5)
- Corrosion
- Fretting
- Long life spans, even if the accumulated operating times are relatively short. This allows corrosion, especially, to have an effect
- Lightning strikes
Overhaul, maintenance, handling, ground activities:
- Damage from vehicles on the ground
- Fouling from operating materials and auxiliary materials (fuel, oil, lubricants, cleaning media)
- No early replacement
- Maintenance errors (e.g. affecting the electronic regulator or blocking)