15.5 and 15.6 Combustion section and turbine section — NGVs, blade types, stress and creep

Scope

• 15.5 Combustion section — A 1, B1 2: constructional features and principles of operation.
• 15.6 Turbine section — A 2, B1 2: different turbine blade types; blade-to-disc attachment; nozzle guide vanes; causes and effects of blade stress and creep.

Combustion chamber requirements

The combustor must burn fuel completely and stably over a very wide range of air–fuel ratios and flow conditions, with minimal pressure loss, an even temperature profile at the turbine entry, low emissions (NOx, smoke, CO), reliable relight, and long life — in a small space.

Airflow division

Air leaving the compressor is slowed in the diffuser (so the flame is not blown out) and enters the flame tube (liner):

• Primary (combustion) air — enters at the front through swirl vanes around the fuel nozzle and primary holes; creates a recirculating toroidal vortex that anchors the flame. Mixture here is near stoichiometric (roughly 15:1 air–fuel by mass).
• Secondary air — enters through holes along the liner to complete combustion.
• Dilution (tertiary) and cooling air — enters further downstream and through louvres/cooling rings, forming a cooling film on the liner wall and reducing gas temperature to an acceptable level for the turbine. FAA: secondary air cools the combustion gases from about 3,500 °F to near 1,500 °F (roughly 1,900 °C to 800 °C).

Overall air–fuel ratio is very lean (typically several tens to one) because most of the air is used for cooling and dilution.