Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGGAS-GENERATOR SECTIONPOWER TURBINESECTIONCOMBUSTIONCHAMBERCOMPRESSORSHAFT-L P ELEMENTiiTURBINEPOWERTURBINEPROPULSIONPOWERCOUPLINGLP ELEMENTSHAFT-H P ELEMENTH.P ELEMENTSTARTERHPELEMENTLPELEMENTLX3LOFigure 23-3.— Schematic diagram showing relationship ofparts in twin-spool gas turbine engine.147.137atmospheres. Part of the compressed air, calledprimary air , enters directly into the combustionchamber where it is mixed with the atomizedfuel so that the mixture can be ignited and burned.The remainder of the air, called secondary air ,is mixed with the gases of combustion. The purposeof the secondary air is to cool the combustiongases down to the desired turbine inlettemperature.Both axial-flow compressors and centrifugal(radial-flow) compressors are currently usedin gas turbine engines. There are several possibleconfigurations of these basic types, someof which are in use and some of which are inexperimental phases of development.In the axial-flow compressor the air is compressedas it flows axially along the shaft.An axial-flow compressor of good design mayachieve efficiencies in the range of 82 to 88percent at compressor pressure ratios up to8:1. At higher pressure ratios, the efficiencytends to decrease. Axial flow compressors maybe of the single-spool type, previously illustratedin figures 23-1 and 23-2, or of the twin-spooltype, as shown in figure 23-3. The gas turbineengine shown in figure 23-4 has an axial-flowcompressor of the single-spool type. Figure23-5 shows an axial-flow single-spool compressorremoved from its engine. Where twinspoolaxial-flow compressors are used, a separateturbine drives each spool, as shown infigure 23-3.The centrifugal (radial-flow) compressorpicks up the entering air and accelerates itoutward by means of centrifugal force. Thecentrifugal compressor (fig. 23-6) may achieveefficiencies of 80 to 84 percent at pressureratios of 2.5 to 4 and efficiencies of 76 to 81percent at pressure ratios of 4 to 10.The advantages of the axial-flow compressorinclude high peak efficiencies; a relatively smallfrontal area for any given air flow; and onlynegligible losses between stages, even whena large number of stages are used. The598