Three different types of combustion chambers are used on gas turbine engines:
1. the can/multiple can type combustion chamber
2. the can-annular combustion chamber
3. the annular combustion chamber
THE CAN/MULTIPLE CAN TYPE COMBUSTION CHAMBER
Centrifugal compressors of old gas turbine engine have can-type combustion chambers.
These combustion chamber sustain with their own air supply duct, fuel nozzle and flame tube within their casing.
Can type combustion chambers can have single combustion chambers or multiple combustion chambers, for very small gas turbine engines like APU it is usually a single combustion chamber engine.
The installation of multiple-can combustion chamber follows around the combustion section of an engine.
They are connected to each other by inter-connector tubes.
Ignition flame follows through these inter-connector tubes from one chamber to the other.
They are necessary because only two combustion chambers have igniter plugs on these engines.
The inter-connector tubes also equalize the pressure in all the combustion chambers.
The advantage of can-type combustion chambers is that they are simple in design, they have very good structural strength and they can be removed and replaced individually.
However the disadvantage of these combustion chambers is, that they are heavy and need much space, they also need complicated air supply ducts from the compressor.
These ducts lead to very high aerodynamic losses.
Another disadvantage of multiple can-type combustion chambers is the difficult ignition from one chamber to the other.
THE CAN-ANNULAR/TURBO-ANNULAR TYPE COMBUSTION CHAMBER
The can-annular type combustion chamber is much simpler in design. It has a set of flame tubes installed around the combustor section. All the flame tubes have a common outer casing and inner casing.
The advantage of the can-annular combustion chamber is that it is smaller and lighter than a similar multiple can-type combustion chambers.
It does not need complicated air supply ducts from the compressor and it still has good structural strength.
The disadvantage of this type is that the aerodynamic losses are quite high and the ignition from one flame tube to the other is very difficult.
THE ANNULAR COMBUSTION CHAMBER
The annular combustion chamber is most commonly used on modern gas turbine engines. An annular combustion chamber consists of only one annular flame tube. An inner and an outer combustion chamber casing provides the support for such flame tube. The combustion chamber casings are part of the load carrying engine structure.
Together with the flame tube, they also make the air channel for the secondary air. In the maintenance documentation, these casings are often called diffuser case or compressor rear frame. Very often the flame tubes of annular combustors are called the combustion chambers.
The three major parts of them are the dome, the inner and the outer liner. The dome has openings for the fuel nozzles which are surrounded by the swirl vanes.
You will find a special design for an annular combustion chamber on small gas turbine engines and APU‘s. It is called a reserve flow combustion chamber, because of the airflow direction in the flame tube.
The annular type combustion chamber has many advantages compared with the other types.
- They are smaller and lighter at the same airflow.
- The large combustion area means that they have better efficiency, that the combustion is complete and smokeless, and that the flame can spread easily over the circumference of the flame tube.
- They also have a better thermal load distribution for the turbine.
The disadvantages of annular combustion chambers are:
- They are expensive to make.
- Removal is very difficult and time consuming.
These disadvantages, however, are compensated by a very high service life.