NAWEPS 00-8OT-80  AIRPLANE PERFORMANCE  at very high velocity and high kinetic energy.  A pressure rise is produced by subsequent ex-  pansion in the diffuser manifold by converting  the kinetic energy into static pressure energy.  The manifold then distributes the high pres-  sure discharge to the combustion chambers.  A double entry impeller allows a given diam-  eter compressor to process a greater airflow.  The major components of the centrifugal com-  pressor are illustrated in figure 2.7.  The centrifugal compressor can provide a  relatively high pressure  ratio per stage but the  provision of more than one or two stages is  rarely feasible for aircraft turbine engines.  The single stage centrifugal compressor is  capable of producing pressure ratios of about  three or four with reasonable  efficiency. &es-  sure ratios greater than four require such high  impeller tip speed that compressor efficiency  decreases  very rapidly.  Since high pressure  ratios are necessary  to achieve low fuel con-  sumption, the centrifugal compressor finds  greatest application to the smaller engines  where simplicity and flexibility of operation are  the principal requirements rather than high  efficiency.  The axial flow compressor  consists of altet-  nate rows of rotating and stationary airfoils.  The major components of the axial flow com-  pressor  ate illustrated in figure 2.7. A pressure  rise occurs through the row of rotating blades  since the airfoils cause a decrease  in velocity  relative to the blades. Additional  pressure  rise takes place through the row of stationary  blades since these airfoils cause a decrease  in  the absolute velocity of flow.  The decrease  in velocity, relative or absolute, eEeLts a com-  ptession of the flow and causes the increase in  static pressure. While the pressure rise pet  stage of the axial compressor  is relatively Jo%-,  the efficiency is very high and high pressure  ratios can be obtained efficiently by successive  axial stages. Of course, the eficient pressure  rise in each stage is limited by excessive gas  velocities.  The multistage axial flow com-  pressor is capable of providing pressure ratios  While the combustion chamber design may  .take various forms and configurations, the  main features  of a typical combustion chamber  ate illustrated by figure 2.8.  The combustion  chamber receives the high pressure discharge  from the compressor and introduces apptoxi-  mately one half of this air into the immediate  area of the fuel spray. This primary combus-  tion air must be introduced with relatively  high turbulence and quite low velocities to  The combustion  chamber  must convert the fuel  chemical energy into heat energy and cause a  large increase  in the total energy of the engine  airflow.  The combustion chamber will opet-  ate with one principal limitation:  the dis-  charge from the combustion chamber must be  at temperatures  which can be tolerated by the  turbine section. The combustion of liquid  hydrocarbon fuels can produce gas temperatures  which are in excess of 1,700 to 1,800° C.  However, the maximum continuous turbine  blade operating temperatures rarely exceed  NO0 to J,OOO”  C and considerable excess air  must be used in the combustion chamber to  prevent exceeding these temperature limits.  The axial flow compressor can provide  efficiently the high. pressure ratios necessary  for low fuel consumption. Also, the axial  compressor is capable of providing high air-  flow with a minimum of compressor  diameter.  When compared with  the centrifugal com-  pressor, the design and construction of the  axial compressor is relatively complex and  costly and the high efficiency is sustained over  a much narrower range of operating conditions.  For these reasons, the axial compressor finds  greatest application where rhe demands of  efficiency and output predominate over con-  siderations. of cost, simplicity, flexibility of  operation,  etc. Multispool compressors and  variable statot blades serve to improve the  operating characteristics of the axial com-  pressor  and increase  the flexibility of operation.  from five to ten (or greater) with efficiencies  which cannot be approached with a multi-  stage centrifugal compressor.  111  Revised Januwy 1965 