2.7. FUEL NOZZLES
On most gas turbine engines, fuel is introduced into the combustion chamber through a fuel nozzle that creates a highly atomized and accurately shaped spray of fuel suitable for rapid mixing and combustion. Most engines use either the simplex or the duplex nozzle. The exception to this is the Lycoming T53-L-11 engine which uses vaporizer tubes in place of fuel nozzles. Each type of nozzle is discussed in the following subparagraphs.
- Simplex nozzle. Figure 2.4 illustrates a typical simplex nozzle; as its name implies, it is simpler in design than the duplex nozzle. Its big disadvantage lies in the fact that a single orifice cannot provide a satisfactory spray pattern with the changes in fuel pressure.
Figure 2.4. Simplex Fuel Nozzle.
- Duplex nozzle. Because the fuel-flow divider and the duplex nozzle work hand in hand, the description of these units is combined. The chief advantage of the duplex nozzle is its ability to provide good fuel atomization and proper spray pattern at all fuel pressures. For the duplex nozzle to work, there must be a fuel-flow divider to separate the fuel into low (primary) and high (secondary) pressure supplies. Single-entry duplex nozzles have an internal flow divider and require only a single fuel manifold, while, as shown in figure 2.5, dual-entry fuel nozzles require a double fuel manifold. The flow divider, whether self-contained in each nozzle, or installed separately with the manifold, is usually a spring-loaded valve set to open at a specific fuel pressure. When the pressure is below this value, the flow divider directs fuel to the primary manifold. Pressures above this value cause the valve to open and fuel is allowed to flow in both manifolds. A fuel flow divider is shown in figure 2.6.
Figure 2.5. Dual Entry Duplex Nozzle.
Figure 2.6. Fuel Flow Divider.
In addition, an air shroud surrounding the nozzle, as shown in figure 2.7, cools the nozzle tip and improves combustion by retarding the accumulation of carbon deposits on the face. The shroud also helps to contain the flame in the center of the liner.
Figure 2.7. Air Shroud.
A word of caution; extreme care must be taken when cleaning or handling the nozzles, since even the acid on the fingers may corrode and produce a spray pattern which is out of tolerance.
- Vaporizing tube. Engines such as the Lycoming T53-L-11 use vaporizing tubes instead of injector nozzles. The vaporizing tube is a T-shaped, ceramic-coated pipe, whose exit faces upstream to the airflow. Figure 2.8 shows a vaporizing tube that is used on the T53-L-ll.
Figure 2.8. Vaporizing Tube.