NAVWEPS O(L8OT-80  AIRPLANE PERFORMANCE  energy to drive the propeller in addition to the  compressor  and accessories.  maintain a nucleus of combustion in the com-  bustion chamber. In rhe normal combustion  process,  the speed of flame propagation is quite  low and, if the local velocities are too high at  the forward end of the combustion chamber,  poor combustion will result and it is likely  rhar the flame will blow out.  The secondary  air-or cooling flow-is introduced downstream  from the combustion nucleus to dilute the com-  bustion products and lower the discharge gas  temperature.  The combustion chamber delivers high en-  ergy combustion gases  to the turbine section at  high pressure  and tolerable temperature. The  turbine nozzle vanes are a row of stationary  blades immediately ahead of the rotating tur-  bine.  These blades form the nozzles which  discharge the combustion gases as high ve-  locity jets onto the rotating turbine.  In this  manner, the high pressure  energy of the com-  bustion gases is converted into kinetic energy  and a pressure and temperature drop takes  place. The function of the turbine blades  operating in these jets is to develop a tangen-  tial force along the turbine wheel thus extract-  ing mechanical energy from the combustion  gases. This is illustrated in figure 2.8.  The fuel nozzle must provide a finely  atomized, evenly distributed spray of fuel  through a wide range of flow rates. Very  specialized design is necessary to provide a  nozzle with  suitable characteristics. The  spray parrern and circulation in the combustion  chamber must make efficient use of the fuel by  complete combustion. The temperatures in  the combustion nucleus can exceed 1,700” to  1,SW’ C but the secondary air will dilute the  gas and reduce the temperature to some value  which can be tolerated in the turbine section.  A pressure drop will occur through the com-  bustion chamber to accelerate  the combustion  gas rearward. In addition, turbulence and  fluid friction will cause a pressure  drop but this  loss must be held to the minimum incurred by  providing complete combustion. Heat trans-  ferred through the walls of the combustion  chamber constitutes a loss of thermal energy  and should be held to a minimum.  Thus, the  combustion chamber should enclose the com-  bustion space with a minimum of surface area  to minimize heat and friction losses. Hence,  the “annular”  typ: combustion chamber offers  certain advantages over the multiple “can”  type combustion chamber.  The form of the turbine blades may be a com-  bination of two distinct types. The imp&c  type turbine relies upon the nozzle vanes to  accomplish the conversion of combustion gas  static pressure to high velocity jets. The  impulse turbine blades are shaped to produce  a large deflection of the gas and develop the  tangential force by the flow direction change.  In such a design, negligible velocity and pres-  sure drop occurs with the flow across the tur-  bine rotor blades. The reaction  type turbine  differs in that large velocity and pressure  changes occur across the turbine rotor blades.  In the reaction turbine, rhe stationary nozzle  vanes serve only to guide the combustion gas  onto the turbine rotor with negligible changes  in velocity and pressure. The reaction tur-  bine rotor blades are shaped to provide a pres-  sure drop and velocity increase across the  blades and the reaction from this velocity in-  crease provides the tangential force on the  wheel. Generally, the turbine design is a  form utilizing some feature of each of the two  types.  The tur6inc section  is the most critical element  of the turbojet engine. The function of the  turbine is to extract energy from the combus-  tion gases and furnish power to drive the com-  pressor and accessories. In the case of the  turboprop engine, the turbine section must ex-  tract a very large portion of the exhaust gas  The turbine blade is subjected to high  centrifugal stresses  which vary as the square  of the rorative speed. In addition, the blade  Revised January 1965  113 