the fuel spray, which results in better atomization of the fuel,  better burning, and efficiency. The swirl vanes function to  provide two effects imperative to proper flame propagation: layer of air along the inside wall of the liner. This layer of air  also tends to control the flame pattern by keeping it centered  in the liner, thereby preventing burning of the liner walls.  Figure 1-55  illustrates the annular combustion chamber liner. Some provision is always made in the combustion chamber  case for installation of a fuel nozzle. The fuel nozzle delivers  the fuel into the liner in a finely atomized spray. The more the  spray is atomized, the more rapid and efficient the burning  process is. Two types of fuel nozzle currently being used in the various  types of combustion chambers are the simplex nozzle and  the duplex nozzle. The construction features of these nozzles  are covered in greater detail in Chapter 2, Engine Fuel and  Fuel Metering Systems. The swirl vanes greatly aid flame propagation, since a high  degree of turbulence in the early combustion and cooling  stages is desirable. The vigorous mechanical mixing of the  fuel vapor with the primary air is necessary, since mixing by  diffusion alone is too slow. This same mechanical mixing  is also established by other means, such as placing coarse  screens in the diffuser outlet, as is the case in most axial- flow engines. The spark igniter plugs of the annular combustion chamber  are the same basic type used in the can-type combustion  chambers, although construction details may vary. There are  usually two igniters mounted on the boss provided on each  of the chamber housings. The igniters must be long enough  to protrude from the housing into the combustion chamber. The can-annular combustion chambers also must have the  required fuel drain valves located in two or more of the  bottom chambers, assuring proper drainage and elimination  of residual fuel burning at the next start. The flow of air through the holes and louvers of the can- annular chambers, is almost identical with the flow through  other types of burners.  [Figure 1-56]  Special baffling is used  to swirl the combustion airflow and to give it turbulence.  Figure 1-57  shows the flow of combustion air, metal cooling  air, and the diluent or gas cooling air. The air flow direction  is indicated by the arrows.  The burners are interconnected by projecting flame  tubes which facilitate the engine-starting process as  mentioned previously in the can-type combustion chamber  familiarization. The flame tubes function identically to those  previously discussed, differing only in construction details. The can-annular combustion chamber is not used in modern  engines. The forward face of each chamber presents six  apertures, which align with the six fuel nozzles of the  corresponding fuel nozzle cluster.  [Figure 1-56]  These  nozzles are the dual-orifice (duplex) type requiring the use  of a flow-divider (pressurizing valve), as mentioned in the  can-type combustion chamber discussion. Around each  nozzle are preswirl vanes for imparting a swirling motion to  Figure 1-56. Can-annular combustion chamber components and  arrangement.  Figure 1-55. Annular combustion chamber liner.  Rotor shaft Fuel nozzle cluster Spark igniter Flame tubes Outlet ducts 1-46 1  High flame speed—better mixing of air and fuel,  ensuring spontaneous burning. 2  Low air velocity axially—swirling eliminates overly  rapid flame movement axially.