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

Introduction

The compressor, which raises the pressure of the air before combustion, and the turbine, which extracts work from the hot high-pressure combustion products, are at the very heart of the engine. Up to now we have assumed that it is possible to construct a suitable compressor and turbine without giving any attention to how this might be done. In this chapter an elementary treatment is given with the emphasis being to find the overall features of the compressors and turbines, including the number of stages, the suitable rotational speeds, the diameters and some indication of the flowpath. The details of blade shape will not be addressed. Further information is obtainable in a recent book by Dixon and Hall (2013) and at a more specialised level for compressors in Cumpsty (2004).

The description of turbomachinery is based on the fan, compressor and turbine for the engines of the NEA. To avoid unnecessary duplication, the design is restricted to the case with a fan pressure ratio of 1.5 at cruise. At this condition the stagnation pressure and temperature entering the engine are p02=35.6 kPa and T02=245.4K. From Exercise 7.4 the mass flow through the engine to give the required thrust is 392 kg/s and of this 32.1 kg/s goes through the core (bpr = 11.2 The turbine inlet temperature is 1500 K, the overall pressure ratio is 45 and the HP compressor pressure ratio is 18. From Exercise 7.6 the fan diameter is 2.66 m.

For the large engine that we are considering the most suitable compressor and turbine will be of the axial type. These are machines for which the flow is predominantly in the axial and tangential directions, and stand in contrast to radial machines for which the flow is radial at inlet or outlet.

Because the pressure rises in the direction of flow for the compressor there is always a great risk of the boundary layers separating, and when this happens the performance of the compressor drops precipitously and it is said to stall. To obtain a large pressure rise (or, as it is more commonly expressed, pressure ratio) the compression is spread over a large number of stages.