A combustor is a component or area of a gas turbine, ramjet, or scramjet engine where combustion takes place. It is also known as a burner, burner can, combustion chamber or flame holder. In a gas turbine engine, the combustor or combustion chamber is fed high-pressure air by the compression system. The combustor then heats this air at constant pressure as the fuel/air mix burns. As it burns the fuel/air mix heats and rapidly expands. The burned mix is exhausted from the combustor through the nozzle guide vanes to the turbine. In the case of ramjet or scramjet engines, the exhaust is directly fed out through the nozzle.
A combustor must contain and maintain stable combustion despite very high air flow rates. To do so, combustors are carefully designed to first mix and ignite the air and fuel, and then mix in more air to complete the combustion process. Early gas turbine engines used a single chamber known as a can-type combustor. Today three main configurations exist: can, annular, and cannular (also referred to as can-annular tubo-annular). Afterburners are often considered another type of combustor.
Combustors play a crucial role in determining many of an engine's operating characteristics, such as fuel efficiency, levels of emissions, and transient response (the response to changing conditions such as fuel flow and air speed).
Fundamentals
The objective of the combustor in a gas turbine is to add energy to the system to power the turbines, and produce a high-velocity gas to exhaust through the nozzle. As with any engineering challenge, accomplishing this requires balancing many design considerations, such as: Completely combust the fuel. Otherwise, the engine wastes the unburned fuel and creates unwanted emissions of unburned hydrocarbons, carbon monoxide (CO), and soot. Low pressure loss across the combustor. The turbine which the combustor feeds needs high-pressure flow to operate efficiently. The flame (combustion) must be held (contained) inside the combustor. If combustion happens further back in the engine, the turbine stages can easily be overheated and damaged. Additionally, as turbine blades continue to grow more advanced and are able to withstand higher temperatures, the combustors are being designed to burn at higher temperatures and parts of the combustor need to be designed to withstand those higher temperatures. It should be capable of relighting at high altitude in an event of engine flame-out. Uniform exit temperature profile. If there are hot spots in the exit flow, the turbine may be subjected to thermal stress or other damage. Similarly, the temperature profile within the combustor should avoid hot spots, as these can damage or destroy a combustor from the inside. Small physical size and weight. Space and weight are at a premium in aircraft applications, so a well designed combustor strives to be compact. Non-aircraft applications, like power-generating gas turbines, are not as constrained by this factor. Wide range of operation. Most combustors must be able to operate with a variety of inlet pressures, temperatures, and mass flows. These factors change with engine settings and environmental conditions (i.e., full throttle at low altitude can be very different from idle throttle at high altitude). Environmental emissions. There are strict regulations on aircraft emissions of pollutants like carbon dioxide and nitrogen oxides, so combustors need to be designed to minimize those emissions. (See Emissions section below.)
Sources:Flack, p. 440.Mattingly, Heiser, and Pratt, p. 325.
History Advancements in combustor technology focused on several distinct areas; emissions, operating range, and durability. Early jet engines produced large amounts of smoke, so early combustor advances in the 1950s were aimed at reducing smoke production. Once smoke was essentially eliminated, in the 1970s efforts turned to reducing other emissions, unburned hydrocarbons and carbon monoxide (for more details, see ). The 1970s also saw improvement in combustor durability, new manufacturing methods improved liner lifetime by nearly 100 times that of early liners (see ). In the 1980s combustors began to improve their efficiency across the whole operating range; combustors tended to be highly efficient (>99%) at full power, but that efficiency dropped off at lower power. Development over that decade improved efficiencies at lower power levels. The 1990s and 2000s saw a renewed focus on reducing emissions, particularly nitrogen oxides. Combustor technology is still being researched and advanced, with much modern research focused on improving the same aspects.
Components
The case is the outer shell of the combustor, and is a fairly simple structure. The casing generally requires little maintenance.Henderson and Blazowski, pp. 119–120. The case is protected from thermal loads by the air flowing in, so thermal performance is of limited concern. However, the casing serves as a pressure vessel that must withstand the difference between the high pressures inside the combustor and the lower pressure outside. That mechanical (rather than thermal) load is a driving design factor in the case.Mattingly, Heiser, and Pratt, p. 378.
The purpose of the diffuser is to slow the high-speed highly compressed air from the compressor to a velocity optimal for the combustor. Reducing the velocity results in an unavoidable loss in total pressure, one of the design challenges is to limit the loss of pressure as much as possible.Mattingly, Heiser, and Pratt, p. 375. The diffuser must be designed to limit the flow distortion as much as possible by avoiding effects like boundary layer separation. Like most other gas turbine engine components, the diffuser is designed to be as short and light as possible.Henderson and Blazowski, p. 121.
The liner contains the combustion process and introduces the various airflows (intermediate, dilution, and cooling, see ) into the combustion zone. The liner must be designed and built to withstand extended high-temperature cycles. Liners tend to be made from superalloys like Hastelloy X. Even though high-performance alloys are used, the liners must be cooled by air flow.Mattingly, p. 760. Some combustors also make use of thermal barrier coatings. However, air cooling is still required. In general, there are two main types of liner cooling; film cooling and transpiration cooling. Film cooling works by injecting (by one of several methods) cool air from outside of the liner to just inside of the liner. This creates a thin film of cool air that protects the liner, reducing the temperature at the liner from around 1800 kelvins (K) to around 830 K. Other types of liner cooling, transpiration cooling, is a more modern approach that uses a porous material for the liner. The porous liner allows a small amount of cooling air to pass through it, providing cooling benefits similar to film cooling. The two primary differences are in the resulting temperature profile of the liner and the amount of cooling air required. Transpiration cooling results in a much more even temperature profile, as the cooling air is uniformly introduced through pores. Film cooling air is generally introduced through slats or louvers, resulting in an uneven profile where it is cooler at the slat and warmer between the slats. More importantly, transpiration cooling uses much less cooling air (on the order of 10% of total airflow, rather than 20–50% for film cooling). Using less air for cooling allows more to be used for combustion, which is more important for high-performance, high-thrust engines.Mattingly, Heiser, and Pratt, pp. 372–374.Henderson and Blazowski, pp. 124–127.
The snout is an extension of the dome (see below) that acts as an air splitter, separating the primary air from the secondary air flows (intermediate, dilution, and cooling air; see ).Henderson and Blazowski, p. 124. / The dome and swirler are the part of the combustor that the primary air flows through as it enters the combustion zone. (see ) Their role is to generate turbulence in the flow to rapidly mix air with fuel. Early combustors tended to use bluff body domes (rather than swirlers), which used a simple plate to create wake turbulence to mix the fuel and air. Most modern designs, however, are swirl stabilized (use swirlers). The swirler establishes a local low pressure zone that forces some of the combustion products to recirculate, creating high turbulence. However, the higher the turbulence, the higher the pressure loss will be for the combustor, so the dome and swirler must be carefully designed so as not to generate more turbulence than is needed to sufficiently mix the fuel and air.Flack, p. 441.
The fuel injector is responsible for introducing fuel to the combustion zone and, along with the swirler (above), it is responsible for mixing the fuel and air. There are four primary types of fuel injectors; pressure-atomizing, air blast, vaporizing, and premix/prevaporizing injectors. Pressure atomizing fuel injectors rely on high fuel pressures (as much as ) to atomizeWhile atomize has several definitions, in this context it means to form a fine spray. It is not meant to imply that the fuel is being broken down to its atomic components. the fuel. This type of fuel injector has the advantage of being very simple, but it has several disadvantages. The fuel system must be robust enough to withstand such high pressures, and the fuel tends to be heterogeneously atomized, resulting in incomplete or uneven combustion which has more pollutants and smoke.Henderson and Blazowski, p. 127.Mattingly, Heiser, and Pratt, p. 379.
The second type of fuel injector is the air blast injector. This injector "blasts" a sheet of fuel with a stream of air, atomizing the fuel into homogeneous droplets. This type of fuel injector led to the first smokeless combustors. The air used is some of the primary air (see ) that is diverted through the injector, rather than the swirler. This type of injector also requires lower fuel pressures than the pressure atomizing type.
The vaporizing fuel injector, the third type, is similar to the air blast injector in that primary air is mixed with the fuel as it is injected into the combustion zone. However, the fuel-air mixture travels through a tube within the combustion zone. Heat from the combustion zone is transferred to the fuel-air mixture, vaporizing some of the fuel (mixing it better) before it combusts. This method allows the fuel to be combusted with less thermal radiation, which helps protect the liner. However, the vaporizer tube may have serious durability problems with low fuel flow within it (the fuel inside of the tube protects the tube from the combustion heat).Henderson and Blazowski, p. 128.
Premixing/prevaporizing injectors work by mixing or vaporizing the fuel before it reaches the combustion zone. This method allows the fuel to be uniformly mixed with the air, reducing emissions. One disadvantage of this method is that fuel may auto-ignite or otherwise combust before the fuel-air mixture reaches the combustion zone. If this happens the combustor can be seriously damaged.Henderson and Blazowski, p. 129.