Table of Contents
3.2.1 Design layout
Two different basic designs of the combustion chamber are distinguishable. Old engines employ several circumferentially distributed, relatively long, axial-oriented, tubular combustion chambers, each possesses an injection nozzle for fuel, ignited through an joining encircling loop. New machines have so-called annular combustion chambers that exhibit very short axial length, where the combustion occurs in the encircling annulus. Many nozzles, distributed evenly at the circumference, inject their fuel into this annulus. The newest generation of engines that have been developed on the lines of lowpollutant -emission display two concentric arrangements of fuel nozzles ( "Ill. 3.2.2-2"). In the process of development, or already partly in service for the purpose of stationary implementation, are specifically pollutant-resistant emission designs which manifest a relationship to both kinds of combustion chambers: tubular, front combustion chamber sections of relatively extensive length which slant into the rear annulus. A frequently used basic form found in older engines is the tangential combustion chamber (see "Ill. 2.1-6.1" and "Ill. 3.2.1-2").
Derivates ( "Ill. 3.2.2-2") as well as gas turbines of the hevy frame type use nowadays pre mixing combustion chambers ( "Ill. 3.2.1-5.1") for the reduction of the NOx emission. The combustion chamber is surrounded by the pressure absorbing combustion chamber casing ( "Ill. 3.2.3-2"). The combustor is separated by a cylindrical casing towards the rotor that serves as an inner pressure chamber as well as means to prevent the prohibited heating up of the rotor. At the combustor exit, there is a fixing mostly in connection with the HPT guide vanes.
The combustor walls have several functions: the creation of combustion space, the control of combustion, directing hot gas into the turbine, supplying combustion air, producing a suitable temperature distribution in the hot gas at the exhaust ( "Ill. 3.2.3-2"). The combustion chamber walls must be protected against overheating through the use of suitable material: cooling air film is usual, produced almost parallel to the wall through rows of holes, gills or annular slots. The cooling air can be used more effectively through special arrangements, like porous (effusion cooling) or multiple wall, perforated, sheet metal structures. In order to save the cooling air, the walls of the inner side are often coated with ceramic thermal barriers . A special feature is the so-called shingle/tile design of the later engines ( "Ill. 3.2.1-4"), by which the combustor wall possesses numerous ‘shingles’ in the direction of the combustion area. These metallic tiles can be equipped with ceramic spray coatings. Currently, trials using full ceramic shingles are underway (Lit. 3.2-30). The shingles are cooled through air from the rear that flows in from the gaps between the tiles in the combustion area.
The shingle design has the advantage of separating the supporting function from the function of the heat shield, so that an optimization is possible. The tiles can, thanks to the gap, move against each other and in doing so balance out thermal expansions easily, largely avoiding one of the main difficulties of all combustors, that of thermal fatigue. The shingle design allows for cheaper repair costs of local damages of the combustor through an exchange of tiles. Additionally, there is also the danger that tiles get detached and damage the turbine through strong vibrating combustion chambers (e.g., low dry NOx, "Ill. 3.2.2-5").
"Illustration 3.2.1-1": A combustion chamber is not only expected to burn fuel. There are numerous other requirements, like how and under which conditions this is to happen in order to guarantee usage in a gas turbine (Lit. 3.2-24). This includes:
- Stable combustion (e.g., no illicit flickering, "Ill. 3.2.1-3" and 3.2.2-5) during all conditions of operation.
- A high degree of combustion efficiency.
- Low pressure drop, respectively, lower flow resistance.
- Even temperature profile ( "Ill. 3.2.3-2") at the outlet to the high pressure turbine (G).
- A minimum of pollutant emissions such as NOx, CO, smoke and unburned hydrocarbon ( "Ill. 3.2.1-3").
- Low cost of components.
- Good ignitability, respectively good start characteristic.
- Long operation life.
- Inexpensive and simple maintenance and overhaul possibilities ( "Ill. 3.2.1-2").
A combustor consists of many typical components: the inner combustor wall (“A“) that surrounds the flame. The pressure absorbing and supporting outer combustor casing (“B“). The inner casing (“C“). The combustion chamber dome (“E“) that cuts off against the direction of air current. The head of the injection system with the inlet of pre-mixed air from the “swirl chamber“ (“F“) and the actual fuel nozzle (“D“).
A typical combustion chamber can be divided into several zones depending on its function . In front of the head a diffuser takes care that there is a sufficiently low current speed of the burning air.
A large part of the combustion takes place in the primary zone, also called reaction zone or burn zone.
The remaining combustion occurs in the mixing zone or the intermediate zone, as the case may be. Air can be supplemented for this purpose. There can be an additional mixing zone („dilution zone“) in which air can again be added in order to derive the desired gas temperature (see diagram). With noticeable NOx formation can be reckoned in combustion chamber zones that have a high temperature niveau (lower diagram in "Ill. 3.2.1-3").
"Illustration 3.2.1-2": The size relationship is made clear with the example of two radial combustors of engines of the uppermost (silo combustion chamber) and lowest power range. Characteristics such as possibility to inspect are decisively influenced, but also assembly, transport and the necessary space.
"Illustration 3.2.1-3": The stability of a combustion depends on the air flow rate, respectively, the speed of air flow and fuel/ air ratio (Lit. 3.2- 24). It (stability) is only given in a certain, combustion specific region (left diagram). Apart from this area, instabilities can occur, i.e., flickering flame and corresponding pressure fluctuations ( "Ill. 3.2.2-5"). This is highly valid for combustors with NOx reducing measures through lowering the flame temperatures. One drives with especially thin fuel/air mixture into the primary zone ( "Ill. 3.2.2-1") or injects water or steam ( "Ill. 3.2.2-3") but this destabilizes the combustion.
In the diagram at the right, the flame temperature and the speed of the NOx formation is applied in dependence on the fuel/air mixture. As one can easily see, high flame temperatures are to be found around the region of the stoichiometric combustion. Here, the available oxygen is just sufficient for a complete combustion of the additionally mixed fuel. In this diagram also, lies the strongest NOx origin. Meagre mixtures, i.e., mixtures with high air surplus, lead to low flame temperatures and corresponding little NOx. The formation of the undesired CO is that much stronger, because completed oxidation does not take place. In rich mixtures with high fuel excess exists the increasing danger of strong soot formation and the origin of the unburned hydrocarbons.
"Illustration 3.2.1-4": If one wants to increase the efficiency of the gas turbine and simultaneously minimize the NOx initiation, the turbine inlet temperature must be increased by a possibly lower combustion temperature. This is best achieved when such a type of homogeneous temperature in the combustor is present, equalizing both temperatures if possible. It follows that possibly little cooling air from the combustion walls gets into the hot gas flow. On the other hand, the temperature of the combustor wall should hardly differ from the gas temperature, i.e., it should be relatively high. These demands can be fulfilled ( "Ill. 3.2.2-2") by modern combustors in a so called shingle/tile design (left scetch, Lit.3.2-6 and Lit. 3.2-23). The different operation loads of the combustor walls are divided into two optimized structures: the supporting, relatively cool outer wall („A“) takes the forces from pressure differences and mechanical loads (fixing, respectively force transmission). As a protection against thermal overload, it usually carries an inner shingle structure („C“), detachable through a screw connection („B“).
To maintain the minimum cooling air flow, the tiles on the side of the combustion chamber can be provided with a thermal barrier coating (in the left figure).In the right picture, there is an arrangement with full ceramic tiles („A“) schematically represented, similar to combustors of engines in the upper performance range (Lit. 4.3- 30), prior to production implementation.
Ceramic tiles (right scetch) are especially elastic (e.g., springy) mounted, to keep heat expansion differences in an equilibrium. These extraordinarily temperature stable tiles can expand freely within themselves or against the support structure during heat expansion differences. They are cooled from the rear by an air flow („D“), which enters through the gap in the combustion chamber
"Illustration 3.2.1-5.1": (Lit. 3.2-27, Lit. 3.2-28 and Lit. 3.2- 9): To minimize the NOx formation at industrial gas turbines of the heavy frame type with pre mix combustion chambers prevail ( "Ill. 3.2.2-2"). One design is showed here. With it a meager combustion is achieved. So the flame temperature can be suitable reduced ( "Ill. 3.2.1-1" and "Ill. 3.2.3-10"). Typical is the bisection in a prechamber (plenum), in which the fuel, in this case gas, is mixed with air and then gets through a flow channel (pre mix line) into the real combustion chamber. Not until here the ignition and combustion takes place. Such a device has, besides the problem of an instable combustion ( "Ill. 3.2.2-5") which is also known at Dry-Low-NOx combustion chambers of aero engine derivates two more problems. It comes to a fast overheating of the structure around the pre mix line and a failure of the burner. In this process the heated, typically uncooled walls act self energizing.
One reason of the combustion in the pre mix line is self ignition. Due to high temperatures and pressures (lowering self ignition temperature and rise of the oxygen concentration) as well as unfavorable (slow) flow and local fuel concentration (‘stoichiometric bubbles’) the mixture can already ignite in the pre mix line. Thereby the ignition delay (time till the ignition occurred) is of great importance. The longer the ignition delay, the earlier the fast flowing gasair mixture passed the pre mix line before it ignites.
"Illustration 3.2.1-5.2": Flame backlash is a further possibility ( "Ill. 3.2.1-5.1") to ignite the Fuel-air-mixture already in the pre mix chamber. Reasons for flame backlashes in pre mix chambers ( "Ill. 3.2.1-5.2"):
- Unfavorable flow conditions promote a flame backlash from the combustion chamber into the pre mix line.
- Burning velocity: The flame can spread against the flow direction into zones in which the flow velocity is lower than the burning velocity (progression velocity of the flame). This is e.g. the case if during a compressor surge the flow velocity falls too much. Because the burning velocity also rises with the turbulence of the flow, a remedy by rising the flow velocity is not necessarily effective.
- Vibrations of the combustion chamber ( "Ill. 3.2.2-5"): if the intensity passes a certain limit this can trigger a flame backlash. Thereby superposes the low frequent pulsing gas fluctuation the gasflow. This leads to local lower periodic flow velocity and pressure drop. In this areas the flame can move upstream.
- Core of a twist flow: the radial pressure gradient creates a central backflow zone. If an adequate backflow velocity is reached the flame backlashs into the burner
- Boundary layer of the wall: Favored in untwisted, low turbulent flows (laminar boundary layer). Also here the low flow velocities promote a flame backlash.
After the backlash the rising burning velocity against the flow direction promotes the stabilization of the flame at the gas injection.
"Illustration 3.2.1-6": (Lit. 3.2-27): The time of the ignition delay, that means the time from reaching the ignition temperature till the ignition is essential for the risk of a self ignition in the pre mix zone. The longer this period the more unlikely is an ignition of the fuel-air-mixture in the premix zone. This already happened in the combustion chamber
In the low temperature region below about 700 °C like in the premix zone, the ignition delay time of pure methane is so long that self ignition can be ruled out.
There is a different situation with natural gas. Indeed its main component is methane. But the ignition time delay is remarkably shortend because a content of higher hydrocarbons. With this the chance gets smaller that the mixture reaches the combustion chamber before a self ignition occurred.
Kerosin or diesel oil also lower clearly the ignition delay time in the low temperature region. This lies in the order of two magnitudes compared with methane. So an increased danger of self ignition exists.
This shows how strong seemingly small changes in the fuel influence the risk of failures by self ignition.