Air at 13 psia and 20°F enters an adiabatic diffuser steadily with a
velocity of 600 ft/s and leaves with a low velocity at a pressure of 14.5
psia. The exit area of the diffuser is 5 times the inlet area. Determine...
a.) The exit temperature
b.) The exit velocity of the air
7 comments:
In the first iteration, using v2=0, I found a delta H value of -7.2btu/lbf. Taking the Hinitial value at 580R to be 47.136, My first T2 estimate is at 40btu/lbf, or 550R. V2 then works out to be 102 ft/sec. A second iteration increases my T2 value and decreases my V2 value? Plus, how do we even use the ideal gas property table for air at 510R, it only goes down to 536.67 R?
Anon 5:03 PM
My value of deltaH was PLUS 7.2 on the 1st iteration. If you find you made a sign error, everything else will work out nicely.
I dont get what form of the first law we are using here! Im trying to use DeltaH=deltaV^2/2Gc and im getting a crazy number, like 5644btu/lb How do i get this 7.whatever you guys are talking about?
I got the same number, but remember lb-ft isn't btu. Look up the conversion on the last page.
I agree with graham, I got T2 = 610R and v2 the listed answer. The key is wrong, I think.
yo soy 11:54 AM
This is a flow system, so I suggest you use the form of the 1st Law that has shaft work and deltaH.
The equation you gave is a siplified form of the correct form of the 1st Law, so you are good to go.
Whenever the problem uses AE units, you have to be very careful. Here I will be you are having a unit conversion problem between Btu's and ft-lbf.
Kevin 7:00 PM
I apologize, the answer listed on the website is wrong. When I changed the inlet T, I did not post the new, corrected answer. T2 ~ 609 degR.
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