The combustion section houses the combustion process, which raises the temperature of the air passing through the engine. This process releases energy contained in the air/ fuel mixture. The major part of this energy is required at the turbine or turbine stages to drive the compressor. About 2/3 of the energy is used to drive the gas generator compressor. The remaining energy passes through the remaining turbine stages that absorb more of the energy to drive the fan, output shaft, or propeller. Only the pure turbojet allows the air to create all the thrust or propulsion by exiting the rear of the engine in the form of a high-velocity jet. These other engine types have some jet velocity out the rear of the engine but most of the thrust or power is generated by the additional turbine stages driving a large fan, propeller, or helicopter rotor blades.
The primary function of the combustion section is, of course, to burn the fuel/air mixture, thereby adding heat energy to the air. To do this efficiently, the combustion chamber must:
- Provide the means for proper mixing of the fuel and air to assure good combustion,
- Burn this mixture efficiently,
- Cool the hot combustion products to a temperature that the turbine inlet guide vanes/blades can withstand under operating conditions, and
- Deliver the hot gases to the turbine section.
The location of the combustion section is directly between the compressor and the turbine sections. The combustion chambers are always arranged coaxially with the compressor and turbine regardless of type, since the chambers must be in a through-flow position to function efficiently. All combustion chambers contain the same basic elements:
- Perforated inner liner
- Fuel injection system
- Some means for initial ignition
- Fuel drainage system to drain off unburned fuel after engine shutdown
There are currently three basic types of combustion chambers, variations within type being in detail only. These types are:
- Can type
- Can-annular type
- Annular type
The can-type combustion chamber is typical of the type used on turboshaft and APUs. [Figure 1-52] Each of the can-type combustion chambers consists of an outer case or housing, within which there is a perforated stainless steel (highly heat resistant) combustion chamber liner or inner liner. [Figure 1-53] The outer case is removed to facilitate liner replacement.
Older engines with several combustion cans had each can with interconnector (flame propagation) tube, which was a necessary part of the can-type combustion chambers. Since each can is a separate burner operating independently of the other cans, there must be some way to spread combustion during the initial starting operation. This is accomplished by interconnecting all the chambers. As the flame is started by the spark igniter plugs in two of the lower chambers, it passes through the tubes and ignites the combustible mixture in the adjacent chamber, and continues until all the chambers are burning.
The flame tubes vary in construction details from one engine to another, although the basic components are almost identical. [Figure 1-54] The spark igniters previously mentioned are normally two in number, and are located in two of the can-type combustion chambers.
Another very important requirement in the construction of combustion chambers is providing the means for draining unburned fuel. This drainage prevents gum deposits in the fuel manifold, nozzles, and combustion chambers. These deposits are caused by the residue left when the fuel evaporates. Probably most important is the danger of afterfire if the fuel is allowed to accumulate after shutdown. If the fuel is not drained, a great possibility exists that, at the next starting attempt, the excess fuel in the combustion chamber will ignite and exhaust gas temperature will exceed safe operating limits.