Combustion chambers within a gas turbine engine are called combustors; they play a vital position in countless operations of the engine for example: emission levels, fuel efficiency and transient response (which is the aircraft’s response to changing conditions i.e. air speed and fuel flow). This combustion process within an engine must be as efficient as possible because there has been a quick rise in commercial aircraft traffic. Due to this rise there is obviously more risk of pollution being emitted into the atmosphere which many of the public may find threatening; efficient combustion can lower these emissions dramatically.
There are four processes that the air goes through within an engine; SUCK, SQUEEZE, BANG, BLOW.
(SUCK)(Air intake) - The engine takes in large volume of air through the fan and compressor stages.
(SQUEEZE) - The compressor is in charge of sucking in the air as well as compressing it; the compressor fans are driven from the turbine by a shaft. Compressors can reach compression ratios in excess of 40:1; so the pressure of the fluid (air) at the end is over 40 times more than the air that entered the compressor. The air then continues moving towards the combustion chamber of the engine because the fans are rotating and pushing the fluid in that direction.
(BANG) -This process involves the combustion chamber; the fuel is mixed with the air which as a result produces a bang, then the gases that form expand and are exhausted through the rear of the combustion chamber. In most commercial engines, fuel is burnt in the chamber up to 2000 degrees Celsius. The metals used in this part of the engine melt at 1300 degrees, so sophisticated cooling techniques are required.
The combustion chamber is a very comprehensive part of an engine because it has numerous responsibilities to achieve, such as being able to burn great capacities of fuel which is delivered through the fuel spray nozzles, also with the assistance of the compressor that supplies the chamber with large volumes of fluid (air).
Combustion's chambers have to be capable of discharging the heat in a way that the fluid expands and accelerates in order to fulfill a smooth stream of hot gas at all the conditions needed by the turbine. It must also release maximum amount heat as efficiently as possible because it has a restricted amount of space; at the same time the chamber must keep pressure loss to a minimum.
The fluid brought in by the fan does not go through the middle of the engine and isn't used for combustion; this is 60% of the entire airflow. This 60% is brought in gradually into the flame tube which lowers the temperature within the combustor and cools the wall of the flame tube.
(BLOW) – The mixture of fuel and air that causes the expansion of gas is forced through the turbine, which drives the fan and compressor and blows out of the exhaust nozzle providing thrust.
The way the turbine drives the compressors and accessories is by removing energy from the hot gases, which is the product from the combustion system; these gases are expanded to a lower pressure and temperature.
In order for the turbine to generate enough driving torque, it must consist of several stages. Each stage has one row of moving blades, as well as one row of stationary guide vanes; these help direct the air onto the blades.
A high turbine inlet temperature is needed to create high engine efficiency, but this would cause other problems because the turbine blades would need to withstand higher temperatures that exceed the blades melting point. So in order for the turbine to operate under these conditions, there would need to be a method of cooling the blades. The turbine blades have small holes in which they force cool air out of, this prevents it from melting; you may be thinking this would have a huge effect on the efficiency of the turbine. But this cooling method has a minimal effect on the engines overall performance; nickel alloys are used to create the turbine blades as well as the nozzle guide vanes, this is due to the materials good heat resistant properties.
Here is the illustrations of the suck, squeeze, bang, blow process
There are four processes that the air goes through within an engine; SUCK, SQUEEZE, BANG, BLOW.
(SUCK)(Air intake) - The engine takes in large volume of air through the fan and compressor stages.
(SQUEEZE) - The compressor is in charge of sucking in the air as well as compressing it; the compressor fans are driven from the turbine by a shaft. Compressors can reach compression ratios in excess of 40:1; so the pressure of the fluid (air) at the end is over 40 times more than the air that entered the compressor. The air then continues moving towards the combustion chamber of the engine because the fans are rotating and pushing the fluid in that direction.
(BANG) -This process involves the combustion chamber; the fuel is mixed with the air which as a result produces a bang, then the gases that form expand and are exhausted through the rear of the combustion chamber. In most commercial engines, fuel is burnt in the chamber up to 2000 degrees Celsius. The metals used in this part of the engine melt at 1300 degrees, so sophisticated cooling techniques are required.
The combustion chamber is a very comprehensive part of an engine because it has numerous responsibilities to achieve, such as being able to burn great capacities of fuel which is delivered through the fuel spray nozzles, also with the assistance of the compressor that supplies the chamber with large volumes of fluid (air).
Combustion's chambers have to be capable of discharging the heat in a way that the fluid expands and accelerates in order to fulfill a smooth stream of hot gas at all the conditions needed by the turbine. It must also release maximum amount heat as efficiently as possible because it has a restricted amount of space; at the same time the chamber must keep pressure loss to a minimum.
The fluid brought in by the fan does not go through the middle of the engine and isn't used for combustion; this is 60% of the entire airflow. This 60% is brought in gradually into the flame tube which lowers the temperature within the combustor and cools the wall of the flame tube.
(BLOW) – The mixture of fuel and air that causes the expansion of gas is forced through the turbine, which drives the fan and compressor and blows out of the exhaust nozzle providing thrust.
The way the turbine drives the compressors and accessories is by removing energy from the hot gases, which is the product from the combustion system; these gases are expanded to a lower pressure and temperature.
In order for the turbine to generate enough driving torque, it must consist of several stages. Each stage has one row of moving blades, as well as one row of stationary guide vanes; these help direct the air onto the blades.
A high turbine inlet temperature is needed to create high engine efficiency, but this would cause other problems because the turbine blades would need to withstand higher temperatures that exceed the blades melting point. So in order for the turbine to operate under these conditions, there would need to be a method of cooling the blades. The turbine blades have small holes in which they force cool air out of, this prevents it from melting; you may be thinking this would have a huge effect on the efficiency of the turbine. But this cooling method has a minimal effect on the engines overall performance; nickel alloys are used to create the turbine blades as well as the nozzle guide vanes, this is due to the materials good heat resistant properties.
Here is the illustrations of the suck, squeeze, bang, blow process
3 Main Combustors
Can Type Combustor
A Can type combustor is a self-contained cylindrical combustion chamber. Every can contains it’s own igniter, fuel injector, casing and liner; this particular combustor is organised in a way that air from the compressor enters each chamber through the adapter. Every chamber contains two cylindrical tubes, a liner, and an outer combustion chamber; the combustion process takes place within the liner.
The airflow into the combustor is controlled via small louvers/slits in the inner dome, and by elongated slits along the length of the liner. These louvers are used as an opening, so that the air flow going through the slits can be used in the process of combustion and cooling; also this air flow is utilized to prevent carbon deposits from forming in the inside of the liner. Carbon deposits can in fact block critical air passages and disrupt airflow along the liner wall, so it’s important to stop it from forming.
Cannular Combustor
Cannular combustors have distinct combustion zones that are contained in individual liners, each with their own fuel injectors. Contrasting to the can type combustor, every combustion zone shares a joint ring casing; Cannular combustors utilize the characteristics of both the annular and can type combustion chamber.
This combustor contains an outer shell, which has various cylindrical liners mounted about the engine axis; also the combustors are totally enclosed by the airflow which enters through numerous holes/louvers in the liner. This specific airflow is mixed with the fuel, which has in fact been sprayed under pressure via the fuel nozzles; this mixture of air and fuel is ignited via igniter plugs. The flame created by the igniter is carried through the tubes to the remaining liners.
Annular Combustor
Annular combustors don’t have separate combustion zones; they just have a continuous liner and casing in a ring. They have various advantages such as constant combustion, smaller compact size, and less surface area. Also annular combustors have uniform exit temperatures; out of all the 3 combustor designs, annular combustion chambers possess the lowest pressure drop; it’s the simplest design of the 3. It’s used in many engines such as the CFM international CFM56 engine; most modern engines use this design due to its light weight, and complete pressure equalization.
Can Type Combustor
A Can type combustor is a self-contained cylindrical combustion chamber. Every can contains it’s own igniter, fuel injector, casing and liner; this particular combustor is organised in a way that air from the compressor enters each chamber through the adapter. Every chamber contains two cylindrical tubes, a liner, and an outer combustion chamber; the combustion process takes place within the liner.
The airflow into the combustor is controlled via small louvers/slits in the inner dome, and by elongated slits along the length of the liner. These louvers are used as an opening, so that the air flow going through the slits can be used in the process of combustion and cooling; also this air flow is utilized to prevent carbon deposits from forming in the inside of the liner. Carbon deposits can in fact block critical air passages and disrupt airflow along the liner wall, so it’s important to stop it from forming.
Cannular Combustor
Cannular combustors have distinct combustion zones that are contained in individual liners, each with their own fuel injectors. Contrasting to the can type combustor, every combustion zone shares a joint ring casing; Cannular combustors utilize the characteristics of both the annular and can type combustion chamber.
This combustor contains an outer shell, which has various cylindrical liners mounted about the engine axis; also the combustors are totally enclosed by the airflow which enters through numerous holes/louvers in the liner. This specific airflow is mixed with the fuel, which has in fact been sprayed under pressure via the fuel nozzles; this mixture of air and fuel is ignited via igniter plugs. The flame created by the igniter is carried through the tubes to the remaining liners.
Annular Combustor
Annular combustors don’t have separate combustion zones; they just have a continuous liner and casing in a ring. They have various advantages such as constant combustion, smaller compact size, and less surface area. Also annular combustors have uniform exit temperatures; out of all the 3 combustor designs, annular combustion chambers possess the lowest pressure drop; it’s the simplest design of the 3. It’s used in many engines such as the CFM international CFM56 engine; most modern engines use this design due to its light weight, and complete pressure equalization.
This clip shows mixing of the gases inside the combustion chamber.