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Old 28th Nov 2012, 05:16
  #141 (permalink)  
goarnaut
 
Join Date: Nov 2012
Location: Europe
Posts: 15
Supersonic Speeds Inside Engine...

Brian...yes the tip speed of the compressor and fan will almost always be supersonic in a modern turbine engine...typically from M1.2 to M1.7...even a turbocharger compressor will go supersonic at its higher pressure ratios...



The reason is that high wheel speeds are necessary to achieve high rates of work...remember kinetic energy is a function of speed squared...and compressor and turbine work are a function of rotational speed...so the higher our wheel speed the greater the work output per given mass flow...


The sonic shock waves do contribute to losses of course...although the shock energy actually helps to compress the air...the axial gas speeds through the compressor and fan will generally be ~M 0.4 to M 0.5...


The axial gas speed through the first turbine nozzle guide vane ahead of the turbine wheel will usually be choked...ie Mach 1...although this will typically increase to just slightly over sonic speed just aft of the NGV...about M1.1 or 1.2 at most...as the annulus increases in size...giving a bit of converging-diverging nozzle effect...


The turbine wheel tip speeds are generally subsonic...about M 0.8...or M 0.9...however since the speed of sound increases with temperature...and at hot section temps it will be more than double that of freestream speed of sound...the turbine actual speed in m/s (or ft/s) will be similar to the cold section...


It is really quite something to stop and consider the amazing power that modern gas turbines produce...a single turbine wheel of ~0.75 m diameter will make close to 40,000 hp...with a mass flow of about 100 kg/s through the engine core...about the size of a 50,000 lb thrust fan engine you would see on a widebody airliner...


Regards,


Gordon.


PS:One more thought about the intake thrust...the very first post said it best...with the intake making over 60 percent of thrust...the airplane is basically “sucking” its way through the air at M2...that's it in a nutshell...

Last edited by goarnaut; 28th Nov 2012 at 05:18.
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