NCEES 2016 thermal & fluid prb 507

[nirvick]

Can anyone have any idea why I don't get the NCEES answer for this problem?Actual answer is 91.428 psi

thanks

 

[Ramnares P.E.]

Use a few more significant figures in your lbm/ft^3 to slug/ft^3 conversion and you'll be almost identical to the answer.  I  used 1.939 slug/ft^3 and the resultant answer is 91.466 psi.  Just a matter of sig figs.

 

[Audi Driver P.E.]

Using the exact figures you provide in red through all the calculations, I get 91.488

 

[nirvick]

Thank you, I use 1.939 and get 91.466.

 

[Mikep]

[No message]

 

[P-E]

That book is great for hands on combustion problems. 

 

[cvanwy02]

I'm a little lost as to why gravity must be converted to slugs/ft^3.  It seems that you do not have to mess with this with all the other formulas in the MERM.  Where does the formula for this show up in the MERM?

 

[Ramnares P.E.]

Looking at the problem I don't think the conversion is necessary.  Perhaps the thread author is simply more comfortable working in those units >shrug<

 

[cvanwy02]

Some sort of conversion is necessary.  If you just used units of gravity and density, you would end up with an answer of Lb/ft-Sec^2.  

 

[Ramnares P.E.]

Enter the infamous g/gc.  

 

[cvanwy02]

Please forgive my ignorance, but I thought g/gc was equal to 1?

That's the way I've been studying the NCEES 2016 Practice Exam and this is the only problem I cannot get to workout.  

 

[Ramnares P.E.]

g/gc is a common fluids ratio used when mass and force and involved, i.e. lbm, lbf, or involving work of a fluid.

The MERM should have a discussion of this, if not, your Fluid Mechanics text will.  

 

[cvanwy02]

Ok, I figured it out. 

If you use the the U.S. Customary version of the Bernoulli equation as shown in the MERM, it simplifies to P1=p1h1 + (p2v2^2/2g) where P=pressure and p=density.

1) Density = 62.4 * 1.03= 64.3 lb/ft^3

2) Convert knots to ft/sec to obtain 50.634 ft/sec

3) Convert 14.7 psi to psf by multiplying by 144 to obtain 2116.8 psf.

4) Plug and Chug