6MS TFS, Problem #67

[amats42]

I am working the 6 minute solutions problem 67 and I was wondering how they derived the equation for power. I knew to use the isentropic equation with P=mdot*work. I canceled out the densities and almost ended up with their equation.

Pth = (k/k-1)*Q1*p1*[(p2/p1)^k-1/k - 1]

I don't really need help with the solution method, I just need to understand how they got k/(k-1) as part of the isentropic work equation. MERM 24.118 shows it as 1/(k-1).

A previous post said to derive it, but I still wouldn't get k/(k-1)



If someone needs I can post a photo tomorrow, but again I just need to know what isentropic equation to use.

 

[amats42]

Massive brain fart!

Power = Wdot = mdot(h2-h1)

I see how they derived the equation with k on numerator.

 

[P-E]

You're probably wasting your time on this one.  The TFS S&M problems aren't very good or sometimes even relevant.  Better use of time at this point is to go back and review concepts needed in the PM.  Pm session was challenging.  

 

[amats42]

You're probably wasting your time on this one.  The TFS S&M problems aren't very good or sometimes even relevant.  Better use of time at this point is to go back and review concepts needed in the PM.  Pm session was challenging.  

Thanks, P-E. That's exactly what I did but I did notice I was being dumb and using closed system equations and not open system isentropic, the formula is right there in the MERM..

Regarding 6MS you're right. I started losing confidence which isn't good at this point. Just reviewed some more NCEES practice problems and some concept problems.

The breadth changed for this test, btw. The PM and AM sessions aren't broken up into general versus depth anymore. The TFS exam is purely TFS now with 80 questions split between two sessions. I will report back after the exam!

 

[P-E]

Best of luck.