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It is seen that EGT are of the order of 600°C (What is a normal EGT range of a jet engine?) for aviation turbines and nearly the same for stationary turbines. Why can't high temperature exhaust gases are further harnessed like adding another stage in turbine section that extracts more energy?

In aviation turbines can't we just add another turbine stage like another low-pressure turbine mounted on another spool? For land-based gas turbines we can somewhat utilize these high temperature exhaust gases in Combined cycle but what about aviation engines?

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    $\begingroup$ Welcome to Aviation! A general rule about aviation questions: Almost any question you ask starting with "Why can't..." will be answered with "Well, you can but it isn't done because...". Your best bet is to just ask "Why isn't..." or "Why don't we..." $\endgroup$ – FreeMan Apr 21 '17 at 12:48
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It can be done, but the better specific fuel consumption does not compensate for the extra weight. We all know that aero engines have to be as light as possible. In land, stationary installations, where weight matters but little, regeneration, intercooling, and even combined-cycle (in electricity generation, using a gas turbine that drives an alternator, but with the exhaust gases generating steam for a steam turboalternator, for example...). The efficiency is very, very high, but the weight and bulk are considerable, too...

But it has been tried: https://airandspace.si.edu/collection-objects/allison-t78-2-turboprop-engine

https://www.flightglobal.com/pdfarchive/view/1963/1963%20-%201880.html

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    $\begingroup$ airandspace.si.edu article reads The basic principle of regeneration involved reclamation of turbine exhaust heat to increase the temperature of compressor discharge air, thus reducing the amount of fuel required for the most efficient engine operating temperature. — that's a different thing entirely $\endgroup$ – ymb1 Apr 21 '17 at 12:15
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    $\begingroup$ @ymb Different to what...? Part of the thermal energy of the exhaust gases is recovered and used to improve the sfc... It answers the question 'Why can't we extract more from exhaust gases in gas turbines?'... Of course we can, and there are engines where it's done... $\endgroup$ – xxavier Apr 21 '17 at 12:29
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    $\begingroup$ OP wants more turbines to absorb all the energy in the stream. $\endgroup$ – ymb1 Apr 21 '17 at 12:38
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    $\begingroup$ From Flying, June 1962 books.google.es/… $\endgroup$ – xxavier Apr 21 '17 at 12:43
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    $\begingroup$ Another scheme, from 'Flight': [img]i.imgur.com/txT9gig.jpg[/img] $\endgroup$ – xxavier Apr 21 '17 at 13:05
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(Source)

Can't we just add another turbine stage?

In a turbojet, slowing down the exhaust (by adding more turbine stages) defeats the purpose of a jet engine.

And if you mean to use all the core's energy to run the fan in a turbofan, the outer bypass flow will diverge into the now low-pressure zone, causing drag, and might even cause flow reversal, i.e., air will be ingested from the back—causing a surge.

Also—at max power—a fan in a turbofan already runs at its most efficient speed, any faster and the gains will turn into losses.

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  • $\begingroup$ I'm not sure removing heat (the question) is equivalent to removing pressure (your key point). Do you have any reference to support your answer and provide a value for the lowest valid turbine outlet pressure to prevent a surge? $\endgroup$ – mins Apr 21 '17 at 13:50
  • $\begingroup$ Enlarging the turbine would obviously be coupled with enlarging the fan and increasing the bypass ratio. The bypass ratio recently increased from 5–6 to 10–11. So what is the limiting factor for the bypass ratio? $\endgroup$ – Jan Hudec Apr 23 '17 at 20:23
  • $\begingroup$ Hi @JanHudec - If the aim is to accelerate more air, then more fuel will be added, whether it's to a larger engine or same engine with tougher materials. OP doesn't want that. On a related point, note the CFM56 variants, the higher the bypass for the same core, the lower the thrust. $\endgroup$ – ymb1 Apr 24 '17 at 4:36

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