Fuel Nozzles

Most gas turbine engines use a fuel nozzle to inject fuel into the combustion chamber. This nozzle's job is to produce a highly atomized, precisely formed spray of fuel that can quickly mix and burn with the primary airstream under a variety of fuel and airflow conditions. Either the single (simplex) or dual (duplex) nozzle is used by the majority of engines. Some small engines just employ one nozzle (simple).

Simplex Nozzle

As its name suggests, this Simplex nozzle has the benefit of having a simpler design than the Duplex nozzle. Its main drawback is that it cannot produce a spray pattern that is acceptable with the significant variations in fuel pressures and airflows found in larger engines.

Duplex Nozzle

The spray angle must be rather wide at low airflow and low rpm in order to improve fuel and air mixing and increase the likelihood of ignite. However, a tight pattern is necessary at higher rpm and airflow to protect the combustion flame from contacting the combustion chamber walls.

The modest fuel flow used at idle is driven through a tiny outlet created by the primary holes, breaking it up into a fine spray. Due to the higher fuel pressure, the secondary holes are bigger but still provide a fine spray at higher rpm. The duplex nozzle's main benefit is that it can produce good fuel atomization and a correct spray pattern at all airflow and fuel delivery rates without the need for excessively high fuel pressures.

A component known as a flow divider is required to divide the fuel into low (primary) and high (secondary) pressure sources in order for the duplex nozzle to operate. As with the single-entry duplex type, this flow divider can be built into each nozzle, or the entire system can employ only one flow divider.

Dual-entry fuel nozzles require a double fuel manifold, whereas single-entry duplex nozzles with an inbuilt flow divider only need a single fuel manifold. Dual-fuel manifolds may not always be obvious as such. For instance, the JT3 and JT4 series engines from Pratt & Whitney employ a concentric manifold arrangement.

The flow divider is often a spring-loaded valve designed to open at a specified rpm fuel pressure, whether it is self-contained in each nozzle or mounted in the manifold. The flow divider distributes fuel to the primary manifold and/or qr nozzle orifice to produce a wide-angle spray pattern when the pressure is lower than this value. In order to increase the fuel spray pattern, fuel is permitted to flow through manifolds and/or nozzle orifices when pressures are higher than this value. The spray angle narrows (because spray angle is inversely related to fuel pressure) when engine rpm/fuel pressure rises more until it is once again right.

The channels in the majority of current nozzles are drilled at an angle to discharge the fuel in a swirling motion that produces high flame speed and low axial air velocity.

Furthermore, a shroud of air that surrounds the nozzle cools the tip and enhances combustion by delaying the buildup of carbon deposits on the face. The shroud also contributes some air for combustion and aids in keeping the flame contained in the liner's centre. When handling, fixing, or cleaning the nozzles, extreme caution must be taken because even a single set of fingerprints on the metering components might cause an abnormally high fuel flow.

In place of injector nozzles, some AlliedSignal Lycoming T-53 and T-55 versions and other variants employ vaporising tubes. Essentially a U-shaped pipe, the vaporising tube exits upstream of the compressor airflow. With this setup, the fuel and air are perfectly blended.