Combustion chamber
A combustion chamber is a container in which an exothermic reaction ("combustion") takes place, usually continuously, through the supply of an oxidizer (oxygen carrier, usually air) and one or more fuels . This reaction can optionally be accelerated by a catalyst .
Structure and way of working
A combustion chamber generally has a large inlet opening for the oxidizer and a large outlet opening for the combustion gases. Fuel (s) and, if applicable, the catalyst are usually introduced into the combustion chamber in regulated quantities via nozzles; the flow rate of the oxidizer is generally known, but is not (directly) regulated. In rocket engines , the oxidizer is also introduced into the combustion chamber in regulated quantities. Gaseous fuel is added directly, while liquid fuel is first atomized or evaporated. When the fuel is solid, it can be finely ground and blown into the combustion chamber.
An essential feature of a combustion chamber is that it is not closed cyclically during operation, as is the case with e.g. B. makes an internal combustion engine . If hypergolic ("self-igniting") reaction components are not used, an igniter is required at least to start the combustion chamber , e.g. B. a spark plug .
A combustion chamber is used in rocket engines, gas turbines or in heating systems. Combustion chambers can be quickly regulated in their performance. The reaction chamber of a solid rocket is also called a combustion chamber. The fuel and oxidizer are in a solid mass inside the chamber. The mass initially forms the combustion chamber wall. These combustion chambers cannot be regulated.
Loads and material
Due to their application, combustion chambers are subject to high mechanical and thermal loads. Depending on the application, the pressure in a rocket combustion chamber can be up to 200 bar and the temperature of the reaction products up to 3300 ° C. In addition, there are acceleration forces and vibrations .
For the inner wall of the combustion chamber, superalloys or chromium-nickel steels with melting temperatures from 1350 degrees Celsius to 2623 degrees Celsius (molybdenum) are used, which are designed with the addition of tungsten , titanium and molybdenum according to the heat and pressure requirements.
Cooling and heat shield
If the thermal load is very high, the combustion chamber wall must be cooled. This is done either by the fuel, which is first transported in tubes over the combustion chamber wall before it is injected into the combustion chamber and warms up accordingly, or, in the case of short-term use, can also be done by a suitable heat shield , e.g. made of graphite , tungsten or molybdenum. In the combustion chambers of gas turbines, air is generally used for cooling; it enters through small holes in the combustion chamber wall and thus forms a cooling film. In addition, there are increasing numbers of combustion chamber designs in which a ceramic layer is applied to the hot gas side. This material can withstand significantly higher temperatures and, thanks to its good insulation effect, leads to a strong temperature gradient towards the actual combustion chamber wall. Here, too, the combustion chamber must be cooled from the outside by air (often by impingement cooling), but the advantage over film cooling is that the cooling air does not get into the combustion chamber and influence the combustion process there.
An air-cooled combustion chamber requires a pressurized cooling air supply. For this purpose, the actual combustion chamber is surrounded by a pressure housing - the cooling air is guided between the combustion chamber wall and pressure housing. Often the cooling air is branched off from the combustion chamber supply air before the inlet of the combustion chamber.
Combustion chamber designs in gas turbines
In addition to the external design, combustion chambers are also differentiated according to gas routing and fuel supply .
Classification according to design
Single combustion chamber
Also called tubular combustion chamber or “can-type combustion chamber”. In the case of a single combustion chamber design, several cylindrical combustion chambers are usually arranged in a composite. Each combustion chamber has its own injection nozzle. The advantages of the individual combustion chambers are lower development costs and a very simple design of the individual parts. Disadvantages are the higher weight and the increased space requirement compared to other types of construction. The gases entering the turbine also have a very uneven temperature distribution in the circumferential direction, which greatly shortens the service life of the turbine and the control system. This design was only used in the early days of jet engines, as the development effort was the least. Today this design is only used in smaller gas turbines.
Tubular ring combustion chamber
engl. "Can-annular combustion chamber"
This is a hybrid of the classic single combustion chamber and the modern annular combustion chamber. The individual combustion chamber is designed in a ring shape and has several injection nozzles arranged in a ring. This design offers some advantages of the annular combustion chamber (higher energy density, better combustion), but all the disadvantages of the single combustion chamber design. This compromise between performance, weight, size and development effort was rarely used in the past, as the development of the annular combustion chamber made rapid progress.
Annular combustion chamber
engl. "Annular combustion chamber"
With the annular combustion chamber, instead of several individual combustion chambers, only one combustion chamber with an annular combustion chamber is required for a jet engine, which saves space and weight. In addition, the achievable energy density is considerably higher than with other designs (with the CF6-80 engine, with a combustion chamber volume of around 20 liters, up to 12,000 liters of kerosene per hour are burned). There are several (up to 30) individual injection nozzles evenly distributed, so that the exiting gases have a very even temperature distribution in the circumference. This design was only developed further at a late stage, since it requires correspondingly powerful combustion chamber test rigs to test the combustion chamber completely or in segments.
Classification according to fuel supply
DC injection
The fuel is fed in the same flow direction as the combustion air. Advantage: Technically easy to master, mostly used nowadays. Disadvantage: The mixing of fuel / air is not optimal, it has to be helped by vortex formation, which increases the pressure loss.
Countercurrent injection
The fuel is supplied against the combustion air . It is hoped that this will result in an improved mixture formation, but the nozzles coke heavily, which is why this design has been "in development" for over 30 years and has not yet been used to any significant extent.
Fuel evaporation
In this design, the fuel is "injected" under low pressure into a pipe heated by the flame . The fuel evaporates in the pipe, through which air is also passed at the same time. The mixture formation is the best of all three processes, the combustion is therefore very clean. The injection system can also be kept simple due to the low pressure. This design was used quite early, when the injection nozzles were still difficult to manufacture, but was then almost completely replaced by the rapid development of this technology. Nowadays this technology is almost only used in model making.
Centrifugal atomization
The fuel is fed through the hollow engine shaft and is very finely atomized by the centrifugal force. The mixture formation is relatively good, the injection system can be kept simple due to the low pressure, but the combustion is necessarily radial, which means that a special combustion chamber has to be used, which has a higher pressure loss due to the multiple diversions of the gas flow.
Classification according to gas flow
DC combustion chamber
The combustion chamber is flowed through linearly without any significant changes in direction. Simple, compact design with low pressure losses. This design is the most commonly used in jet engines.
Reversing combustion chamber
As a rule, the gas flow changes direction twice by 180 ° (but not in the combustion zone!), As a result of which good mixing and a uniform temperature at the outlet is achieved. As a result, the overall length can also be reduced, which is why this design is often used in small gas turbines. The main disadvantage is the higher pressure losses that occur when the gases are diverted.
Radial combustion chamber
This design is used exclusively for centrifugal atomization, is very complex in terms of construction (cooling air ducting) and is only used in smaller gas turbines. The multiple changes in direction of the gas flow also result in an increased pressure loss.
Many of the combustion chambers used are hybrid designs that have been developed to meet the exact requirements of the engine. This list is therefore not exhaustive; new construction methods are constantly being tested in order to improve emissions, performance, weight, etc.
literature
- Ernst Götsch: Aircraft technology. Introduction, basics, aircraft science . New edition Motorbuchverlag, Stuttgart 2009, ISBN 978-3-613-02912-5 .
- Egon Schesky, Milosch Kral: Aircraft engines. Piston and gas turbine engines, structure, mode of operation and operating behavior . Rhombos Verlag, Berlin 2003, ISBN 3-930894-95-5 .
