NCEES 2016 thermal & fluid prb 507

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nirvick

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

thanks

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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.

 
That book is great for hands on combustion problems. 

 
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?

 
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<

 
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.  

 
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.  

 
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.  

 
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

 
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