A Brayton Cycle with Regeneration using air as the working fluid has a pressure ratio of 7. The minimum and maximum temperatures in the cycle are 310 K and 1150 K. Assuming an isentropic efficiency of 75% for the compressor and 82% for the turbine and an efffectiveness of 65% for the regenerator, determine...
a.) The temperature of the turbine effluent.
b.) The net work output, in kJ/kg of air flowing through the system.
c.) The thermal efficiency of the cycle.
8 comments:
the awnser for part A of 700 degrees is the awnser that I get for temperature of the turbine effluent if we assume its reversable. Its diffrent once the isentropic efficancy is plugged in. It just seemed kinda coincedental and I was hoping you could check to see if the awnser is wrong. By the way my numbers gave me the other two parts of the problem and they match the posted awnsers.
which equation are you using for change in entropy? I can't seem to get either 1) or 2) using
dS=S2-S1-Rln(P2/P1). I was under the impression that equation took into account a possible change in Cp/v, by using the specific entropy tables from the back of the book. We are assumming air to be an IG, correct?
arbitrary:
I posted the wrong answer ! You are correct, I accidentally posted T5S (the temperature of the effluent from our hypothetical, isentropic compressor). The correct answer is T5 = 782 K. Sorry for the confusion.
Scattante:
You are correct. We are assuming air behaves as an ideal gas, but it does have variable heat capacities. The Ideal Gas Property Table for air uses the Shomate Eqn to take into account the fact that the heat capacity of the air is variable. IF we were treating the heat capacites as constants, then we WOULD have used an equation like: T5s/T4 = (P5/P4)^((gamma-1)/gamma) to determine T5S.
Why can't i get a thermal efficiency of 22%? My answer seems to be 53%....can anybody offer some advice or help or anything...
Rock:
I cannot offer much help based on what you have given me here.
The key to part (a) is to use isentropic efficiency of the turbine and the 2nd Gibbs equation to determine H5 and then interpolate on the Ideal Gas Property table for air to determine T5.
The key to part (b) is to use isentropic efficiency of the compressor and the 2nd Gibbs equation to determine H2 and then apply the 1st law to the compressor and the turbine to determine the specific shaft work for each. Wcycle is the sum of these two shaft work terms.
The key to part (d) is to use the regenerator effectiveness to determine H3 so we cancalculate Q34 = QH. Then, we can determine thermal efficiency from its definition using QH and Wcycle from part (b).
My stream numbers are from the figure on page 212.
Dr. B can you post the solution!
Hi Frank,
I already posted the solution to a very similar problem on my LearnThermo.com website. See for yourself !
http://www.learnthermo.com/examples/ch09/p-9f-1.php
Look around and see if you can find anything else useful.
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