Table of Contents
126.96.36.199 Fundamentals of the Damage-Relevant Operating Behavior of Combustion Chambers
The static pressure level in the combustion chamber is the highest in the entire engine. This, combined with the temperature levels around the combustion chamber, puts especially high operating stress on the housings (Fig. "Temperature variation at the combustion chamber outlet"). The intensive cooling with air films carried by the combustion chamber walls that enclose the flame creates high localized temperature gradients and corresponding thermal strain. In these zones, typical damage mechanisms such as thermal fatigue (see Chapter 188.8.131.52) and plastic deformation (Fig. "Combustion chamber deformation") have a pronounced effect. The high overall temperature levels can quickly lead to overheating damage if the cooling system fails (Fig. "Typical combustion chamber damages").
Combustion is always accompanied by pressure vibrations. These normal vibrations put dynamic stress on the combustion chamber walls and can generally be satisfactorily controlled. The intensity of the pressure vibrations is considerably increased in combustion with a high volume of excess air. Excess air is necessary for environmental reasons, as it minimizes nitrous oxides (low NOx). The safe prevention of unallowable vibrations evidently requires extensive development work.
The damaging loads on a combustion chamber are clearly dependent on the composition of the fuel (Fig. "Fuel hydrogen content influencing combustion chamber life"). Even apparently minor deviations can be extremely costly.
The outer combustion chamber housing is a pressure cooker with relatively thin walls under powerful mechanical and thermal loads. This is clear from the typical damages (Fig. "Combustion chamber housing failure consequences") that take the form of an explosion-like expansion of hot gases with extensive consequences.
If hot gases escape from the side of the combustion chamber liner due to damage and strike the combustion chamber housing, it can rapidly overheat and fail in an explosive manner (Volume 2, Ill. 9.3-3).
The main shaft system of an engine is usually protected from hot gas contact and overheating with the aid of a tubular inner housing called the inner combustion chamber liner, which is parallel to the axis. This housing is subject not so much to stress from pressure differences, but from forces that attached components, such as the turbine inlet stator, direct into it. Damage mechanisms such as dynamic fatigue due to the excitement of combustion chamber vibrations, fretting, or erosion due to abrasive particles in the combustion chamber (labyrinth wear products, etc.; see Volume 1, Chapter 5.3.1) can determine the life span of this part. If this engine part supports a main bearing or if the turbine stator behind the combustion chamber is attached to it, then it necessitates considerably higher safety requirements with regard to loads such as thermal strain and external forces (bearing forces, axial gas forces, etc.).
The injection system must guarantee optimal combustion conditions with proper distribution of the fuel, even over long operating times. Erosion (Volume 1, Chapter 5.3.2) caused by the overheating of fuel ahead of the nozzle and/or blocking or plugging of the nozzle must be prevented, or the flame may escape sideways from the engine.
Figure "Fuel hydrogen content influencing combustion chamber life" (Ref. 184.108.40.206-4): The durability/life span of hot parts such as combustion chambers and high-pressure turbines is heavily influenced by changes to the composition of the fuel and its properties, if these affect the thermal radiation of the flame and hot streaks. The influence of particles such as soot and deposits (erosion and corrosion, see Volume 1 Chapters 5.3.2 and 5.4.5) created by the combustion process.
Hot streaks are inhomogeneities in the gas flow. If a hot gas streak comes into contact with the combustion chamber wall, localized overheating and shortened life spans are to be expected. If the streaks continue into the turbine, they especially affect the turbine guide vanes. Even though the designer strives to prevent hot streaks, they can be an unavoidable consequence of fuel characteristics. Fuels with high viscosity compromise the spraying process and the distribution of droplet size. This affects combustion (also see Ill. 220.127.116.11-4).
A more frequent problem is undesirable changes in the spray pattern due to a change near the fuel nozzle. Erosion of the nozzle mouth due to cracking products in overheated fuel and external coke deposits near the nozzle mouth and/or the air injection can affect the spray pattern.
The thermal radiation of the flame heats up the combustion chamber walls, especially in the hot primary zone (Fig. "Combustion chamber components and processes"). The turbine blades are less affected because their line of sight to the flame is very small, and the radiation must first pass through the cooler dilution zone. Understandably, the radiation of the flame in reverse flow combustion chambers does not affect the turbine blades. Radiation primarily heats up the inlet edges of the first turbine stator vanes. If the life span of these blades is determined by the thermal fatigue at the exit edges, then the effect of the radiation on their life spans is correspondingly minor. High-pressure turbine rotor blades in the second stage are covered and protected from the radiation by blades in the first stage.
The most important influence on damages to inner combustion chamber walls is a change in the fuel that alters the thermal radiation of the flame(Fig. "Combustion chamber life dependence from flame radiant energy"). This thermal radiation is primarily caused by soot particles in the flame.
Soot radiates as a blackbody (Fig. "Combustion chamber life dependence from flame radiant energy") with a stoichiometric flame temperature between 2500-2600 °K and most strongly influences the heat absorption of the combustion chamber walls.
Soot is generally formed in the primary zone by a high fuel/air ratio. This soot is almost completely incinerated in the dilution zone, so that only about 1-2% leaves the engine as a soot trail.
The top left diagram shows the total radiation energy of a sooting flame and a colorless flame (non-luminous flame of CO2 and H2O). One can see that radiation increases along with rising combustion chamber pressure. This is related to the fact that high engine power creates high combustion chamber pressure, which constricts the cone angle of the fuel spray, concentrating the fuel and hindering proper dilution and combustion. These conditions promote soot formation.
Soot is created in hot, fuel-rich areas of the primary zone. As described, the most soot is created at high power levels due to the high combustion chamber pressure and fuel/air ratio (top right diagram, see Fig. "Combustion chamber development for low emission").
The hydrogen content of the fuel depends on the proportion of various hydrogen compounds (chains of various length, rings). The middle left diagram shows the considerable influence on the temperature of the combustion chamber liner (the hot gas-carrying combustion chamber wall). A decrease in the hydrogen content, which means an increase in carbon content (Fig. "Soot formation by fuel composition and gas pressure"), leads to a considerable increase in the wall temperature (about 50°C). This effect can be explained by an increase in the radiation energy due to the large number of radiating carbon particles in the flame (middle left diagram). In the primary zone, the bulk of the warmth is transferred into the wall through radiation rather than convection through contact with hot gases. Changes in the fuel composition can evidently have grave effects on the behavior of the combustion chamber wall. For example, decreasing the hydrogen content by about 2% by weight can increase the radiation energy by about 100%. If the temperature of the combustion chamber wall increases, the overhaul intervals are shortened and the repair costs rise considerably (Fig. "Chamber wall temperature determines overhaul intervals"). The bottom right diagram shows that a temperature increase in the wall of about 60°C shortens the combustion chamber life span by an order of magnitude. This corresponds well to the known relationship between temperature and creep rupture life (Chapter 12.5). With regard to the influence on thermal fatigue (LCF loads correspond to the bottom right diagram), a serious dependency on the engine type is clearly given. The operating application of an engine in helicopters or fighters is evidently of secondary importance. This indicates the special influence of the design of combustion chambers. Reducing the hydrogen content by about one percent increases the life span, i.e. increases the tolerable number of cycles under comparable damages several times over.
Figure "Chamber wall temperature determines overhaul intervals" (Ref. 18.104.22.168-4): The serious influence of the hydrogen content of fuel on the temperature of combustion chamber walls is an important economic consideration for the engine operator. If the combustion chamber does not determine the overhaul intervals, then a decrease of the LCF life (thermal fatigue) will naturally have little influence on the intervals (top left diagram). However, the necessary repair costs for the combustion chamber can increase considerably due to greater damage (e.g. cracking, deformations, see Fig. "Hot gas streaks as combustion chamber problem").
The top right diagram shows how various engine types react to changes in fuel composition.
The fatigue diagram of a typical combustion chamber material (bottom) shows how changes in temperature change the life span of the combustion chamber wall. Changes of about100°K (“A” and “B”) lead to thermal loads that can cause overproportionately large life span changes (load cycles to cracking) of up to an order of magnitude.
Remember: Changes to the fuel specifications and/or use of different fuels can considerably shorten overhaul intervals and dramatically increase repair costs. Therefore, even apparently minor changes must be carefully researched (e.g. appropriate, realistic test runs).
Figure "Combustion chamber life dependence from flame radiant energy": The diagram shows the distribution of the radiation energy of the flame of a combustion chamber. A colorless (blue, non-luminous) flame radiates little energy and therefore transfers a relatively small amount of heat into the combustion chamber wall (detail diagram at bottom). The most energy-rich are the wavelength bands that are radiated from the combustion products H2O and CO2 . If there is soot in the flame, the radiation is considerably greater (luminous flame). Corresponding to the radiation energy of a blackbody, the combustion chamber wall is subject to considerably greater thermal loads by a luminous flame (Fig. "Fuel hydrogen content influencing combustion chamber life" and Fig. "Chamber wall temperature determines overhaul intervals"). The radiation energy increases with the fourth power of the temperature (Stephan-Boltzmann`s Law). This makes the especially high radiation loads on the combustion chamber wall in the primary zone understandable.
Figure "Temperature variation at the combustion chamber outlet" (Ref. 22.214.171.124-7): The temperature distribution in the hot gases at the combustion chamber exit can vary locally in both circumferential and radial directions by up to several 100 °C (top right diagram). The fuel nozzles usually appear in the temperature distribution as hot spots (bottom diagram). For this reason, overheating damage to the stator vanes of the first high-pressure turbine stage can often be correlated to a corresponding circumferential distribution.
The temperature distribution is also determined by other influences in addition to the position of the fuel nozzles. The infeed of combustion air and cooling air (top left diagram) also plays a role. Therefore, overheating patterns can appear very uneven.
Figure "Sensitivity of air supply bores and slits": Combustion chamber walls usually have many bores for the admeasurement and inflow of combustion air, as well as for generation of a protective cooling air film (bottom left diagram). Experience has shown that minor changes to these bores can decisively compromise the functioning of the combustion chamber (Examples 126.96.36.199-1 and 188.8.131.52-2). These include sharp edges, roughness and ridges in the bore wall, and burrs (bottom diagram).
The top diagram shows typical edge types for bores and their effect on the throughflow. While rounded inlet edges allow a largely undisturbed throughflow, sharp edges cause noticeable flow constriction. This changes the flow resistance. Burrs oriented against the direction of flow have an especially drastic effect (top right diagram). The throughflow mass of this type of bore is only about half of that of a bore with a rounded inlet edge.
Remember: Alterations to air openings in combustion chamber must be done with the utmost caution. This is especially true for changes to production procedures (e.g. to lower costs). If in doubt, the permissibility of this type of apparently harmless alteration must be verified with suitable (sufficiently realistic) test runs before serial implementation.
Example "Cost-value optimization problems in combustion chambers I": The can-type combustion chambers of an older engine type had many rows of bores around the circumference, which formed a cooling air film that protected the inner combustion chamber wall. The chipping production process of these cooling air bores was elaborate and expensive. In order to lower costs, the bores were experimentally made by electrical discharge machining with no subsequent machining of the edges. The test of a batch of these combustion chambers revealed unacceptable operating behavior. The high roughness, conical shape, and sharp edges created by the bore production process caused an unacceptably high flow resistance (Fig. "Sensitivity of air supply bores and slits"). This compromised the air supply of the combustion chamber and the development of the cooling air film, and the resulting poor operating behavior precluded the use of this economical boring process.
Example "Cost-value optimization problems in combustion chambers II": Economically priced combustion chambers of a small helicopter engine from military stockpiles were offered for sale on the world market. Several of these combustion chambers were purchased. During the acceptance runs of engines outfitted with these combustion chambers, powerful vibrations occurred in several cases. These were evidently due to instable combustion. At first glance, there was no difference between the flawed and functioning combustion chambers. Thorough inspection of the affected combustion chambers showed that the only noticeable difference was that the air bores in the primary zone had relatively sharp edges, and some also had a tiny burr. The observed combustion chamber behavior can be plausibly explained by the reduced air throughflow caused by these characteristics (see Fig. "Sensitivity of air supply bores and slits"). Experience has shown that combustion chambers are extremely sensitive to changes in the air “household”. The affected combustion chambers were classified as unfit for the intended application and reworking them was ruled out.
Figure "Hot gas streaks as combustion chamber problem": Hot streaks are a typical damage-causing phenomenon in combustion chambers. They can develop in various ways:
Poor distribution of the fuel/air mixture (Ref. 184.108.40.206-5): Combustion chambers do not have an even fuel/air mixture around the circumference, due to the individual fuel nozzles and the albeit apparently minor differences in the air inflow (geometric tolerances of the location and diameter of cooling air bores, position and fuel distribution of the nozzles, form tolerances of the walls, etc.). This leads to large temperature gradients in the gas flow (Fig. "Factors worsening the compressor behavior") with heat exchanges that are correspondingly varying combinations of convection and radiation processes. If the fuel concentration is located close to the combustion chamber wall (bottom diagram), a temperature increase occurs in a streak in the direction of flow. If the cooling air film is overloaded, the typical consequence is overheating limited to the circumference. Buckling of the lip at the streaks are a sign of overheating. These plastic deformations are caused by high thermal strain and lead to cracking (thermal fatigue, see Chapter 12.6.2) and burning (serious oxidation). This type of damage compromises the cooling air film, which in turn promotes overheating. Therefore, this process is self-perpetuating.
Deformation of the structure that creates the cooling air film (Ref. 220.127.116.11-6): The cooling air is usually directed into the combustion chamber wall through bores that are generally oriented axially. Inside, a lip helps distribute the air as a cooling film that travels along the inside of the wall and protects it from direct contact with hot gases. The lip absorbs considerably more heat than the bores, resulting in plastic compression of the lip, which can cause it to buckle (Fig. "Combustion chamber deformation"). When the plastically buckled area cools, it is subject to high tensile stress and, in extreme cases, cracking, as would be expected from this type of thermal fatigue process. In this case, as well, the described self-increasing effect of heavy oxidation (burning) causes additional weakening of the cooling air film.
Figure "Combustion chamber soot problems" (Ref. 18.104.22.168-8): Smoke is undesirable for various reasons. It is an unhealthy environmental pollutant. In military aircraft, smoke is also a signature that makes the aircraft more easily detectable for enemies (top diagram). In addition, a smoking flame has a damaging effect on combustion chamber walls, due to its intensive thermal radiation. This results in short overhaul intervals and high repair costs (Fig. "Fuel hydrogen content influencing combustion chamber life" and Fig. "Chamber wall temperature determines overhaul intervals"). Smoke is created by fine carbon particles that are created by the combustion process and are distributed in the gas. Even a small amount of these particles can create a considerable smoke cloud. Depending on the size of the particles, the same amount of particles can have very different visual appearances. Soot is primarily created by rich fuel/air mixtures, even if it only occurs in localized areas. The primary zone is usually the area where soot/smoke develops (bottom diagram). In the rear area of the combustion zone, and in the dilution zone, on the other hand, the soot is broken down through oxidation. If fuel is premixed with air, soot is created in the flame only when the fuel/air ratio is clearly above the stoichiometric ratio. Smoke development does not give any indication of the efficiency of the combustion, i.e. the specific fuel consumption. Smoke development generally increases with rising pressure in the combustion chamber, and decreases as the combustion chamber exit temperature rises (Ref. 22.214.171.124-9). Higher air inflow temperatures into the combustion chamber increase soot development.
Figure "Emission characteristics operation depending" (Ref. 126.96.36.199-8): The many different emissions can be categorized according to specific states of operation. The states of operation corresponding to increasing engine power are on the abscissa. One immediately sees that HC and CO are emitted especially at low engine power (idling, landing approach). NOx and smoke/soot develop more at higher power levels, i.e. during takeoff and climbing flight.
Figure "Soot formation by fuel composition and gas pressure" (Ref. 188.8.131.52-10): The typical diagram for hydrocarbons at top shows the soot development in a flame with premixed air, using a special fuel (Zyclohexan) as an example. As shown, the soot development begins above a certain threshold value, depending on pressure and the fuel/air ratio. This value is about half as great as the upper theoretical threshold value at chemical equilibrium. Unburned hydrocarbons are created, and they break down in various ways, depending on the temperature and time. High flame temperatures when soot development begins will result in dry amorphous hydrocarbons. Rich mixtures at low pressures lead to a combination of tar and carbon.
The bottom diagram depicts the amount of soot created in a premixed flame relative to the proportion of hydrogen in the fuel by weight. The grey field indicates the scattering of measured values in the literature. There is a clearly recognizable trend to a linear relationship between decreasing hydrogen content (increasing carbon) and increasing soot development (Fig. "Combustion chamber soot problems")
Figure "Surrounding influencing the engine ignition" (Refs. 184.108.40.206-13 and 220.127.116.11-14): The ignition of a combustion chamber depends on the environmental conditions in many different ways. Low temperatures (bottom left diagram) and high altitudes (low atmospheric pressure, bottom right diagram) compromise ignition. The interaction of the ignition and starter is also affected. This is true both for high altitudes as well as for startup on the ground, especially if the fuel is very cold. The fuel type is also a factor (also see Volume 1, Ill. 5.1.5-3).
At low temperatures, the starter must overcome an especially large moment, slowing compressor acceleration. This is further complicated by the lower energy output of the electrical batteries. The starting moment is increased by the following influences at low temperatures:
- Power takeoff of the compressor due to the larger mass flow.
- Air friction at the compressor and turbine blades.
- Rubbing in bearings and gears.
- Rubbing in the auxiliary components.
- Oil viscosity
Fine fuel dispersion is necessary for reliable ignition. This requires a sufficient fuel inflow and a minimum pressure difference between the dilution air (at the compressor exit) and the combustion chamber pressure (pressure decreases in the compression chamber). These conditions are reached at a later time in the starting process at lower temperatures, due to the increased starting moment and the decreased battery power. This results in lower RPM with decreased pressurization, as well as longer start times to ignition. These influences, combined with the increased viscosity of colder fuel, result in larger fuel droplets that are slow-reacting and evaporate slowly at the low temperature. This hinders ignition and the subsequent spreading of the flame. A larger amount of fuel, on the other hand, results in finer droplets and makes ignition possible in the necessary time even at lower temperatures (bottom left diagram). Even if the flame has stabilized, combustion is less effective and the turbine power to the compressor is reduced. This compounds the acceleration problems of the compressor. If the designed ignition does not occur as it should, then the necessary compressor acceleration cannot take place and the turbine may overheat without being immediately noticed. Further possibilities include a hung start (top diagram, Fig. "Typical problems during start-up" and Fig. "Fragments blown into compressor from combustion chamber"), in which the engine cannot be further accelerated or takes a long time to ignite and/or reach idle RPM. These long starting times can damage the starter and/or the transmission.
In order to ensure safe ignition even in extreme environmental conditions in the designed flight envelope, certain constructive measures are necessary:
- Suitable configuration of the spark plugs in the fuel-rich primary zone of the combustion chamber (Fig. "Combustion chamber components and processes" and Fig. "Sensitiveness of 'Spark plugs' to poor design"). It must be ensured that the spark plugs are not covered with fuel, as this could damage them and unallowably cool the sparks.
- Sufficient fuel throughflow to guarantee fine dispersion at the point of ignition.
- Optimized boring in the combustion chamber
- Pressure atomization with fine droplets
The problem is insufficient atomization due to the fuel amount being too low during ignition. This can be solved by staging the fuel flow so the atomizers near the spark plugs receive a higher fuel throughflow during ignition.
Air blast atomization is superior to pressure atomization during operation. However, during engine startup, this sytem does not ignite as well due to the dependency of the atomization effect on a sufficient pressure gradient between the atomization air and the combustion chamber. One solution is a pilot fuel system. This type of system first burns finely atomized fuel from a special pressure jet (hybrid system). Unallowable coking and blocking of the jet can be prevented by properly positioning it in the combustion chamber.
18.104.22.168-1 Fa. Rolls Royce, “The Jet Engine”, issue 1996, ISBN 0 902 121 2 35, pages 35 to 43.
22.214.171.124-2 I.E.Traeger, “Aircraft Gas Turbine Engine Technology”, 2nd edition, Glencoe, ISBN 0-07-065158-2, pages 135-142.
126.96.36.199-3 M.Zhu, A.P.Dowling, K.N.CBray, “Self-Excited Oscillation in Combustors With Spray Atomizers”, ASME Paper 00-GT-108 of the “International Gas Turbine and Aeroengine Congress and Exhibition”, Munich, Germany, May 8-11, 2000.
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126.96.36.199-15 T.L.Dubell, S.A.Seyd, “Control of Aircraft Engine Emissions, Status and Future Directions”, Proceeding Paper 95-Yokohama-IGTC-133 of the “Yokohama International Gas Turbine Congress”, Japan, October 22-27, 1995, pages I-195 to I-202.