Table of Contents
11.2.2 The Combustion Chamber
In the combustion chamber, energy is transferred to the engine through burning of the fuel. Combustion chambers can be divided into several zones according to hot gas development (Fig. "Combustion chamber components and processes"). Combustion occurs in the primary zone. Air is mixed with the gases in the next downstream region (dilution zone). If there is still fuel present, then further burning can occur. In this case, an additional dilution zone attaches without burning.
There are two primary types of combustion chamber, with different design principles. Older engine types are usually outfitted with can type combustion chambers. These are made up of several axially oriented flame tubes configured symmetrically in relation to the engine axis. Each chamber has its own liner for airflow. Advantages of this design (Ref. 11.2.2-2) include separate replacement of individual combustion chambers (repair, inspection), easier control of the fuel/air ratio, and resistance to deformation. Drawbacks include the relatively large structural volume (large length and diameter increase the total engine weight).
Modern engines are outfitted with annular type combustion chambers. These have the advantage of smaller diameters and a shorter structure, better efficiency (less pressure losses), and a better connection to the compressor and turbine. Problems include their susceptibility to deformation (thermal strain), the large, thin combustion chamber walls, and the fact that the entire combustion chamber must be removed for repair (the shaft system passes through the inside, preventing easy removal!).
A combined can-annular type combustion chamber has not gained wide acceptance. This is a can type combustion chamber without individual air ducts. Instead, the combustion chambers are arranged inside a ring-shaped liner with air flowing through it. The relatively large axial structural length is a drawback with this combustion chamber type. Another design that is seldom found in new engines is one which resembles a can type combustion chamber, but where each combustion chamber is designed like a small annular type combustion chamber with several burners. The diameter of the complete assembly and the complexity of the system were probably both considered too great.
Figure "Combustion chamber components and processes" (Ref. 11.2.2-2): The safe, economical, and least environmentally damaging burning of fuel in the entire operating range (flight envelope) requires controlling many different processes and functions, including:
- Stable combustion demands “soft” burning (e.g. no unallowed flickering) in all operating states (Ill. 18.104.22.168-4) and no unintentional extinguishing throughout the greatest possible operating range (Fig. "Combustion chamber development for low emission").
- High combustion efficiency.
- Small size (diameter, axial length).
- Minimal pressure losses (low flow resistance)
- Even temperature profiles (Fig. "Temperature variation at the combustion chamber outlet") at the exit to the high-pressure turbine (“G”).
- The aspect of pollutant emissions is continually gaining importance. The development of modern engines demands great efforts to minimize emission of pollutants such as NOx, CO, soot, and unburned hydrocarbons.
- Low production costs.
- Good ignition properties and reliable re-ignition behavior (Fig. "Combustion chamber development for low emission").
- Long operating life (Fig. "Chamber wall temperature determines overhaul intervals").
- Economical and simple maintenance and overhauls.
Combustion chambers consist of several typical components: the inner combustion chamber liner (“A”), which encloses the flame. The pressure- and load-bearing outer combustion chamber housing (“B”). The inner housing (also known as inner combustion chamber mantle “C”). The combustion chamber dome (“E”), which protects the combustion chamber from the airflow. The head of the injection system with the inflow of premixed air (Swirler, “F”) and the actual fuel nozzle (“D”). The igniter is not depicted in the diagram.
A typical combustion chamber can be divided into several zones by their functions:
In order to ensure optimal burning, a diffusor slows down the speed of the airflow into the combustion chamber. Most of the burning occurs in the primary zone (aka reaction zone or burning zone). An often overlooked, yet extremely important function of the primary zone is sustaining the flame. Without sufficiently slow flow speeds of the air/fuel mixture, the flame would be blown out of the combustion chamber. Discrete, directed air jets create air vortices that act against the direction of the air/fuel flow and are essential for stabilizing the flame (see Fig. "After burner problem zones"). Therefore, this is referred to as an aerodynamic flame holder in the primary zone.
Any remaining combustion occurs in the dilution zone or intermediate zone. Additional air can be added for this. Another dilution zone can be added. Here, further air is added to attain the desired low gas temperature (see diagram). In combustion chamber zones with high temperature levels, considerable NOx creation can be expected.
With rising temperature and air pressure, the combustion chamber efficiency increases to almost 100%. The bottom diagram (Ref. 22.214.171.124-1) shows the typical efficiency levels of a combustion chamber dependent on the fuel/air ratio. This line can change in different combustion chamber configurations (Ref. 126.96.36.199-2). The degree of combustion chamber in most engines during takeoff at sea level is typically close to 100%. In modern fan engines during cruising flight, it is closer to 99% - 99.5%. The fuel/air ratio is greater at the high altitudes typical of cruising flight due to the lower external air pressure. This higher fuel/air ratio leads to higher gas temperatures and reduced gas density. Therefore, in order to maintain a constant mass airflow, the throughflow speed of the gases must increase, causing greater pressure losses. If the gas speed is increased beyond a certain point, then there is less time for the dilution and combustion processes. In this case, the combustion chamber efficiency worsens to about 98 %.
Figure "Combustion chamber development for low emission": The stability of combustion depends on the air throughflow, i.e. the flow speed and the fuel/air ratio (left diagram). Stable combustion is only ensured in a certain, combustion chamber-specific diagram region (Ref. 188.8.131.52-1). Outside of this zone, the flame may extinguish or experience instability, such as flickering accompanied by corresponding pressure fluctuations. Measures that reduce NOx especially compromise combustion. A lean fuel/air mixture in the primary zone reduces the flame temperatures and compromises the stability of the combustion (Ill. 184.108.40.206-4).
The danger of the flame extinguishing is especially high at low engine power, such as during landing approach or idle, combined with the simultaneous presence of unusually strong external factors such as rain and/or hail (see Volume 1, Chapter 5).
This is also related to problems with re-igniting a combustion chamber after the flame went out or was intentionally temporarily extinguished during flight. One example of this is fuel blipping to take the engine out of a lock-in surge. Related problems can also occur when igniting an afterburner (Fig. "After burner triggered compressor surge"). Re-ignition during flight requires much greater ignition energy than do ignition tests on fuel/air mixtures (Ref. 220.127.116.11-11). Tests have shown that, along with the typical conditions at greater altitudes (lower pressure and temperature levels), the cause for ignition problems in turbojet engines can be found in the pronounced turbulence in the primary zone of the combustion chamber. In addition, the average fuel droplet size of the spray cone is a factor (see Ill. 18.104.22.168-4). In some fighter aircraft, reliable ignition at great altitudes is ensured through simultaneous injection of pure oxygen, which is carried in pressurized containers.
The right diagram shows the flame temperature and the rate at which NOx is formed, depending on the fuel/air mixture. It is clear that high flame temperatures are found in the region of stoichiometric combustion. The available oxygen is just enough to completely burn the fuel that has been mixed in. This region also has the greatest NOx development. Lean mixtures, i.e. mixtures with too much air, create low flame temperatures and correspondingly small amounts of NOx. However, undesired CO develops even more rapidly since no complete oxidation occurs. Rich mixtures with too much fuel promote coke formation and unburned hydrocarbons.
Figure "Low-emission combustion problems": The introduction of technologies to minimize emissions in exhaust gases, especially of nitrous oxides (NOx), influences many other areas (top left diagram). The black region corresponds to the author`s estimate. If the properties are in the grey problem zone, one can expect drawbacks relative to engines with no emission-reducing measures. It seems that life span, costs, and flame stability are the most problematic areas. The right diagram shows the typical emission behavior, dependent on power levels, of a combustion chamber with no special measures to minimize pollutant emissions (Ref. 22.214.171.124-15). The flame temperatures rise along with the power levels, increasing nitrous oxide (NOx) development. Simultaneously, oxidation of CO and hydrocarbons (HC) increases, reducing their amounts. Soot creation increases with power levels due to the increased fuel percentage and combustion chamber pressure. Partial loads reverse this relationship, making the development of the important pollutants occur in the opposite direction. Technical compromises are necessary to reach an optimal level. There are several suitable methods of reducing the development of NOx in the combustion chamber:
Increased primary air inflow: The primary zone (Fig. "Combustion chamber components and processes") is supplied with more air, reducing the combustion temperature to a sufficiently low level. During a longer dwell time in the intermediate zone, more intense oxidation reduces the proportion of CO. The short construction of modern ring combustion chambers makes use of this method more difficult.
Pre-mix combustion chambers: The fuel and air are mixed thoroughly before entering the primary zone (in industrial gas turbines, called DLN = dry low NOx). The high amount of excess air ensures low temperature levels. There is no dilution zone. This technology must control its tendency to combustion chamber vibrations.
Figure "Self increasing gas vibration in a combustion chamber" (Ref. 126.96.36.199-3): Low-frequency pressure vibrations (rumble) at around 50-120 Hz are typical for combustion processes. In engine combustion chambers with fuel-injection nozzles, rumble is observed especially during idling. Gas vibrations cause various problems:
- Noise development that is very irritating to passengers
- Vibrations that can lead to dynamic fatigue of combustion chamber and turbine components
- Increased dynamic wear
- Efficiency decreases
- Extreme pressure fluctuations that can cause compressor stalls.
Although experience has shown that self-increasing pressure vibrations occur more frequently with certain combustion chamber and fuel nozzle configurations, they cannot be predicted with any certainty (Fig. "Influences at combustion chamber vibrations").
The following is a description of the excitement mechanism of gas vibrations in combustion chambers with fuel nozzles:
In modern engines, fuel is injected into the combustion chamber with the aid of nozzles. The compressor air is used to spray the fuel (e.g. the principle of an air spray nozzle, Ref. 188.8.131.52-1).
During idle, the pressure in the air- and fuel supply systems is relatively low. This causes the mass flows to be more sensitive to pressure fluctuations in the combustion chamber. Because the diffusion of the fuel is largely dependent on the fuel flow rate and the air speed, the pressure fluctuations also change the size distribution of the fuel droplets. A decrease in combustion chamber pressure accelerates the inflowing air and creates smaller droplets. The droplet size determines the time necessary for the vaporization of the fuel. This effect is especially pronounced at the relatively low air temperatures during idling (slower vaporization).
The rapid vaporization of smaller droplets causes them to increase combustion and intensify head development. As a result of the lower gas flow speed during idling, the dwell time becomes longer and more energy is given off in the dilution zone. Irregularities create hot spots that travel along with the gas flow. This type of combustion creates a pressure pulse that is caused by the increased resistance when the larger gas volume (greater flow speed) passes through the tight cross-section of the combustion chamber exit (Phase “1”). This pressure pulse decelerates the airflow in the combustion chamber, i.e. at the air spray nozzle, and creates a smaller spray cone with larger fuel droplets (Phase“2”). Combustion slows and a cold spot is created (“Phase “3”). If a cold spot leaves the combustion chamber and passes through the turbine stator, the smaller gas volumes cause the static pressure in the combustion chamber to decrease. The results are increased spray air speed, a larger spray cone, and small droplets that accelerate combustion (Phase “4”). The conditions for the formation of a hot spot are present again, and Phase “1” can recur. If pressure fluctuation conditions are right, the vibration can increase itself.
The bottom diagram depicts the described process as a regeneration diagram (Ref. 184.108.40.206-12).
The processes during combustion, especially in low NOx combustion chambers with air pre-mixing, is described in detail in Ref. 220.127.116.11-8.
Figure "Influences at combustion chamber vibrations" (Ref. 18.104.22.168-12): The many factors that influence the thermoacoustic vibration behavior of a combustion chamber make predicting unallowable vibrations difficult. These influences vary greatly, such as the flow resistance (acoustic resistance) of the turbine stator, the damping effect of a bored combustion chamber wall, or the sensitivity of the fuel inflow and air mixing process to pressure vibrations (Fig. "Self increasing gas vibration in a combustion chamber"). The turbine reflects the sound waves similar to a solid wall, and the combustion chamber walls act as dampers. The flame responds to the combustion chamber pressure fluctuations with heat-release fluctuations. The strength of these flame responses depends on the acoustic properties of the flame. The heat-release fluctuation is more intense in more compact combustion chambers since their energy density is greater. Because smaller combustion chamber walls have less damping qualities, their tendency to instability is greater. This means that combustion chambers of aircraft engines, with their typical high power concentrations, are especially prone to problems during development to reduce pollutant emissions. The following design characteristics of combustion chambers can reduce the tendency to combustion vibrations:
- Changing the flame response
- Increasing the damping of the combustion chamber walls
- Constant adjustment of the fuel throughflow with a control loop during the combustion process. This requires proper pressure transducers and a sufficiently fast fuel flow control system.