Gas Turbines for Aircraft Propulsion

AIR CRAFT ENGINE

The turbojet engine consists of three main sections: the diffuser, the gas generator, and the nozzle. The diffuser placed before the compressor decelerates the incoming air relative to the engine. A pressure rise known as the ram effect is associated with this deceleration.

The gas generator section consists of a compressor, combustor, and turbine, with the same functions as the corresponding components of a stationary gas turbine power plant.The gases leave the turbine at a pressure significantly greater than atmospheric and expand through the nozzle to a high velocity before being discharged to the surroundings.

The working fluid is air modeled as an ideal gas. The diffuser, compressor, turbine, and nozzle processes are isentropic, and the combustor operates at constant pressure.

In an actual engine, there would be increases in specific entropy across the diffuser, compressor, turbine, and nozzle.

Process a–1 shows the pressure rise that occurs in the diffuser as the air decelerates isentropically through this component.Process 1–2 is an isentropic compression.Process 2–3 is a constant-pressure heat addition.Process 3–4 is an isentropic expansion through the turbine during which work is developed.Process 4–5 is an isentropic expansion through the nozzle in which the air accelerates and the pressure decreases.

Combined Gas Turbine–Vapor Power Cycle

A combined power cycle couples two power cycles such that the energy discharged by heat transfer from one cycle is used partly or wholly as the input for the other cycle.

The stream exiting the turbine of a gas turbine is at a high temperature. This high-temperature gas stream can be used by the combined cycle shown, involving a gas turbine cycle and a vapor power cycle.

The two power cycles are coupled so that the heat transfer to the vapor cycle is provided by the gas turbine cycle, which may be called the topping cycle.

The combined cycle has the gas turbine's high average temperature of heat addition and the vapor cycle's low average temperature of heat rejection, and thus a thermal efficiency greater than either cycle would have individually.

For many applications combined cycles are economical, and they are increasingly being used worldwide for electric power generation.

The thermal efficiency of the combined cycle is

is the net power developed by the gas turbine

is the net power developed by the vapor cycle

is the total rate of heat transfer to the combined cycle, including additional heat transfer, if any, to superheat the vapor entering the vapor turbine.