Gas Turbine Engine Fundamentals and Applications

Gas Turbine Engine Fundamentals

Gas turbines are rotary internal combustion engines. The working fluid undergoes transformations: compression, combustion, and expansion. The fluid expansion takes place between the blades of a turbine to produce mechanical energy.

Applications of Gas Turbines

Applications of gas turbines can be classified into:

• Industrial
• Aeronautics

Industrial Turbine Uses

Industrial turbines are used in both fixed and mobile installations. Among the main uses of industrial turbines are:

• Generating electricity via generators
• Driving large compressors and pressure pumps
• Propelling ships

They are also used in combination with other thermo-mechanical systems for exhaust heat recovery.

Aviation Turbine Types

Aviation turbines apply to retrorockets, turboprop, turbofan (or turbofan), and turboshaft engines.

Energy Conversion in Aviation Turbines

In the industrial turbine, turboprop, turboshaft, and auxiliary power plants, most of the energy produced by combustion is converted into mechanical energy in the turbine rotor.

In the turbojet, the mechanical energy that drives the turbine is only that required to drive the compressor; the rest of the energy exits as a reaction producing jet thrust.

In the turbofan, a significant proportion of energy drives the shaft to power the compressor and fan; the remainder comes from the exhaust to produce jet thrust.

Classification of Gas Turbines

Based on Pressure Characteristics

• Constant Pressure Turbines: Components include an open compressor, combustor, and turbine.
• Turbines at Constant Volume (Explosion): Components include a closed compressor, combustor, and turbine with valves.

Thermodynamic Cycles

The cycle developed by constant pressure turbines is the Brayton cycle.

The turbines operating at constant volume develop an Otto cycle.

Constant Pressure Turbine Operation (Brayton Cycle)

In the constant pressure turbine, atmospheric air enters the compressor and exits at a higher pressure to enter the combustion chamber. Fuel is injected continuously into the chamber, and combustion occurs at constant pressure. Combustion is initiated electrically by an ignitor; then, it continues as fuel is supplied. The sharp increase in pressure and temperature caused by combustion drives the gases out of the chamber into the turbine.

The expansion of combustion gases occurs down to atmospheric pressure through the turbine rotor. The useful work developed by the gas is first absorbed by the turbine shaft to move the compressor. The remaining work is absorbed into the shaft (for industrial gas turbines, turboprop, turboshaft, etc.) or used as jet energy (turbojet).

Constant Volume Turbine Operation (Otto Cycle)

The constant volume, or explosion, turbine differs in that combustion takes place inside a closed chamber with intake and exhaust valves. Power delivery is intermittent, and the gases exit the chamber in pulses to influence the turbine. This configuration was used in the early stages of gas turbine development, as material science at that time did not allow for a compatible operating system with acceptable performance.