The turbine section of an engine serves the purpose of extracting energy from the gas flow, converting it into mechanical energy.
This converted mechanical energy drives the compressors, the auxiliary gearbox in the engine and the mounted components on the gearbox, overcoming the friction.
A turbo-jet engine’s turbines extract about 60% of the energy in the gas flow after it leaves the combustion section.
The remainder is converted into thrust at the propelling nozzle.
A turbo-prop or turbo-shaft engine’s turbines are much more efficient and extract almost 100% of the energy in the gas flow to meet the additional task of driving a propeller or a service load, a rotor gearbox for example.
The gas leaves the combustion section with kinetic energy, pressure energy, and heat energy.
The turbine section expands the gas reducing its pressure and temperature at each stage and extracting work in the process.
Although there are gas velocity changes through each stage of the turbine section the overall gas velocity only reduces from Mach 1 to around Mach 0.85.
The gas temperature reduces by about 200°C through each turbine stage.
The overall cross-sectional area of the turbine section increases from front to rear.
This accommodates the increasing volume of the gas as it expands through each turbine and prevents choking of the stages.
The increasing section also permits larger diameter turbines with longer blades used in successive stages.
The longer blades are, then able to produce sufficient work from the decreasing energy level of the gas.
There are two types of turbine:
• The axial flow
• The radial flow
The radial flow type not used on aircraft gas turbine main engines. But found in use in air producers or cold air units of air conditioning packs.
The turbines grouped and each group drives its dedicated compressor through a separate, concentric shaft or spool.
The high-pressure compressor always is driven by the first, or high pressure, turbine and this drives the outer shaft.
On a three-spool engine, for example, the next group would be the intermediate pressure turbines and the third group would contain the low-pressure turbines driving the inner shaft.
Two or more turbines, each with their nozzle guide vanes, connected to a single shaft to provide the torque required to drive its compressor.
The turbine discs connected to their shafts by bolted, self-centering curvic couplings.
It essential that the turbine matched to the requirements of the compressor it drives if high compressor efficiency. Low specific fuel consumption, and high engine performance achieved.
If the vane spaces are too small they will choke and the backpressure created will transmit back to affect the HP compressor outlet diffuser pressure and move the compressor operating line up the graph.
If the vane spaces are too large, the turbine efficiency will be reduced. One turbine stage consists of one row of static nozzle guide vanes (NGVs) and a row of rotating turbine blades.