Jet Propulsion A Simple Guide to the Aerodynamics and Thermodynamic Design and Performance of Jet Engines

Introduction

The gas turbine has many important applications but it is most widely used as the jet engine. Many of the gas turbines used in land-based and ship-based applications are derived directly from aircraft engines. The gas turbines designed specifically for land use, most often as part of a combined cycle plant, are rather different, but the concepts for the cycle is similar to the jet engine and some of the technology is related to that for aircraft propulsion.

The attraction of the gas turbine for aircraft propulsion is the large power output in relation to the engine weight and size – it was this which led the pre-Second World War pioneers to work on the gas turbine. Most of the pioneers then had in mind a gas turbine driving a propeller, but Whittle and later von Ohain realised that the exhaust from the turbine could be accelerated to form the propulsive jet. Sometimes the gas turbine is still used to drive a propeller to form an efficient engine for relatively low-speed flight, the turbo-prop. However the gas turbine is used for aircraft propulsion, it is the high power output for a given weight is that makes it attractive.

Purists will object to this description of the gas turbine as a cycle. Strictly speaking a cycle uses a fixed parcel of fluid which in a gas turbine would be compressed, heated in a heat exchanger, expanded in a turbine and then cooled in a heat exchanger. The ideal gas turbine is sometimes called a Joule or Brayton cycle. The gas turbine ‘cycle’ we consider here takes in fresh air, burns fuel in it and then discharges it after the turbine: in other words it does not cycle the air. Here we are nevertheless adopting the standard terminology of the industry.

This chapter looks at the operation of simple gas turbines and outlines the method of calculating the power output and efficiency. The treatment is simplified by treating the working fluid as a perfect gas with the properties of air, but later some examples are discussed to assess the effect of adopting more realistic assumptions.