Motor center correction factors - how to calulate and apply...

[Niklas]

[color=#000000]I'm building a one-way type flowbench with one hole for intake- and one hole for exhaust testing. In exhaust mode the bench will function as a "motor center" bench like the SF-110. Does anybody have the correction factors from the SF-110 manual?
My gut feeling about this is that it should be as simple as using the quote of (the absolute temperatures after the motors) divided by (the absolute temperatures before the orifices)
Example: lets say we have an ambient temp of 20

[Thomas Vaught]

If someone else doesn't answer your question, I will try and find my info on the 110 bench.

Tom V.

[gofaster]

Nicklas,

Multiply the test flow by the temperature difference factor to get the corrected test flow. Your example does not agree with Superflow's chart.

Factors in degrees Fahrenheit

5, In .996, Ex 1.004
10, In .992, Ex 1.008
15, IN .988, Ex 1.012
20, In .984, Ex 1.016
25, In .981, Ex 1.019
30, In .977, Ex 1.023
35, In .973, Ex 1.027
40, In .970, Ex 1.030
45, In .966, Ex 1.034
50, In .962, Ex 1.038
55, In .958, Ex 1.042

I hope this helps. Maybe one of the Excel masters on the forum can make a spread sheet of this.

[Niklas]

[color=#000000]Ok thanks,
I've tried the "port flow analyser" software and tried to use different values for test temp. and flow temp. to see if that could help in determining the correction factor. Also, as from SF's numbers, it seems that the important parameter is the difference between test temp. and flow temp., much to my surprise as all I've learned from thermodynamics and such suggests that the factor should somehow be based on the quote of "[absolute flow temp] divided by [absolute test temp]".
So I put the data into excel and made an attempt to use its statistical function to see if I could create an equation that gives the correcton factor based on temperature. (As I'm writing my own software I want to have an equation so the factor gets calculated in "real time". See attached image. As we can see, excel found second degree equations that fit the given fixed factors with a very good accuracy (R^2=.999... best you can get is 1.0 wich is perfect fit) The superflow data looks very "jumpy" though that could perhaps be explained with the use of (only) three decimals in the factor (ex. 1.042) so it could be a round-off error.
By using the excel equation: factor=6.203666E-07*dTF^2 + 7.471047E-04*dTF + 1.000000E+00
(based on Performance trend runs) for calculating the factor, and comparing with SF's factors, we get this table:
Diff

[86rocco]

Judging by your previous post, you seem fairly comfortable with complicated math so maybe this'll help, I think I've posted sufficient information on these forums to derive a proper correction equation (I've never actually attempted it so I'm not 100% sure). You probably already realize that the temperature correction is in fact just an air density correctiom, the air picks up heat from the motors and becomes less dense. In the spreadsheet that I posted in there's an air density calculation and if you read the explanation page in the spreadsheet contained in you can see the effect air density has on air flow through an orifice.

[Niklas]

OK, I did some quick checks and it seems you were on the right track regarding the density 86rocco,
Using the quite complicated orifice flow calculations outlined in ISO 5167-1:1991/AMD.1:1998 and comparing flows at different temps it seems we get a correction factor table resonably close to what we got from superflow. Not exactly however, if and when I have the time to write a mathematically correct equation for this factor, I will post it here.
All for now,
/Niklas

[Tony]

I don't know the answer to this, but....

We know that if we place two identical orifice plates in series, and the mass flow through both is identical (same air) then the pressure drops will be the same too. But the first orifice will have a higher entering air pressure and a higher air density. The second orifice will be operating in less dense air flowing at higher velocity, and there will even be in theory a small temperature drop (Joule Thompson effect). Operating conditions will be considerably different between the two orifice plates, but the developed differential pressure always remains exactly the same. Fascinating stuff !

What we are actually doing with a blower centered bench is fairly similar, except there will be a real temperature increase involved as well. The mass flow still remains identical.

Heating the air will definitely have an effect. Jet engines and turbochargers have essentially the same mass flow at both ends, but the exhaust gas expands and has a much higher enthalpy, (or internal energy). But how exactly that ties in with our problem I really don't know.

I can ask a friend that is the professor of mechanical engineering at the university. He absolutely loves thermodynamics. I will go and see him after the summer vacation break, but that will not be for a few weeks.