Basically, turbine operation is no different from that of a compressor. While a compressor is adding energy to the airflow passing through it by converting mechanical energy to pressure energy, a turbine conversely absorbs energy from the gas flow to convert it into mechanical shaft power, or torque. In aero engines, the axial-type turbine is exclusively employed because of the higher flow rate it makes possible. The design of the axial flow turbine can be single or multi-stage. A turbine stage comprises two main elements: 1) a set of stationary nozzle guide vanes in a stator; 2) a set of rotating blades, which follow downstream the guide vanes.
The stator is formed as a ring of stationary radial vanes inside the turbine casing. The vanes are of aerofoil section with their leading edges facing the flow coming from the combustion section. In the narrowing flow path between adjacent blades, the hot gas is accelerated to high velocity. Because of the nozzle effect, the stationary vanes are called nozzle guides vanes, and their complete arrangement constitute the turbine nozzle. The rotating wheel that follows the turbine nozzle downstream is the actual turbine.
Factor affecting the number of stages are: 1) the number of compressor spools; 2) the amount of energy that must be extracted from the hot gas; 3) the rotational velocity (rpm); 3) the maximum permissible turbine diameter. High compression ratios typical of modern engines require multi-stage turbines. The question that may be asked is how it is that a gas expands in a turbine and compresses in a compressor. The answer lies in blade design. The flow cross-section between adjacent blades is designed to increased in the downstream direction; we call this a diffuser. The effect of the diffuser is to accelerate the flow and converts its kinetic energy into pressure energy.
In a turbine the condition is reversed. Because the flow cross-section between adjacent blades narrows in the downstream direction, a nozzle effect exists, which causes the flow to accelerate and make it ready to perform work.
