MERM chap 29, constant spec heat vs isentropic?

[Firefly]

In working problems in Chap 29 of the MERM 12th, I found that some ways of doing the problem depend on if you assume isentropic or constant spec heat.

In solutions to problem 7 they say to assume isentropic but problem 8 which is an additional portion of problem 7...you are to assume constant spec heat.

I am confused as to how I would know that and why the change between the two similar problems.

Any help would be appreciated.

Thanks,

 

[BrianC]

I am a bit unclear on how to choose between the two methods other than the specific wording of the problems.

For this instance, problem 7 says "In an air-standard gas turbine...", where air-standard is your clue. Constant specific heat is used when the problem assumes cold air-standard. I believe the MERM briefly touches on this in footnote 1 of page 39-2, but I was able to get more detail from my thermo book. Obviously, problem 8 says to assume constant specific heat.

I also (vaguely) remember reading that the customary U.S. solutions use varying specific heat as typical, but I was not able to re-locate that statement anywhere. I am guessing there is a more correct engineering answer to this, so hopefully someone else can enlighten us.

 

[John_NY]

I am doing those problems now.

In the MERM it says you can use Ideal Gas or Air Tables. Air tables account for changes in C, but are based on Air (not the Mixture). Ideal gas uses constant Cp (also not totally true).

So it says that both ways have inaccuracy built in, but the examples will show both methods (SI using ideal gas, US Cust using Air Tables).