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I am doing a bit of research on Brayton Cycle and find this T-S diagram for the cycle in https://www.grc.nasa.gov/www/k-12/airplane/brayton.html and have learned that the area encloses by the curve representes the heat going into the system. And I know entropy represetentes the amount of energy that can't do work. So is decreasing the area would make the gas turbine more efficient? How, on the graph, should the decrease make (lower the compressor exit temperature, raise the turbine entry temperature...)?

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  • $\begingroup$ Can you provide a reference that states that entropy represents the amount of energy that can't do work. $\endgroup$ – Chet Miller Jan 24 '18 at 23:33
  • $\begingroup$ Hmmm ... language-wise I would suggest describing the enclosed area as the "net heat" rather than the "heat going into the system" to be explicit that you mean $Q_h - Q_l$ and not just $Q_h$ (which is a reasonable interpretation of the latter phrase). $\endgroup$ – dmckee Jan 24 '18 at 23:37
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In the case of the Brayton cycle, the cold sink temperature is more or less fixed (ambient at 10,000 meters) so efficiency improvements require raising the temperature at which heat is added at constant pressure in the burner cans. This is the primary reason for the improvements in aircraft turbine efficiency which have occurred over the last 50 years: the development of turbine blade materials which can withstand higher and higher operating temperatures.

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  • $\begingroup$ I mean when doing the compression stage, the air will turn hotter due to the gas law. When the gas is hotter, it would be harder to compress them. So if we remove the heat in the air inside the compressor, would it reduce the amount of energy taken out of the turbine for compression so more useful work could be output by the gas turbine $\endgroup$ – user39178 Jan 25 '18 at 1:01
  • $\begingroup$ the t-s diagram or the p-v diagram will tell you whether or not this proposition is true. for more insight, recommend you post on the aviation stack exchange, where the turbine experts reside. $\endgroup$ – niels nielsen Jan 25 '18 at 1:25

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