[AR] Estimating Coefficient of Discharge (Cd)
- From: "" <dmarc-noreply@xxxxxxxxxxxxx> (Redacted sender "gnsortino@xxxxxxxxx" for DMARC)
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Sat, 13 Sep 2014 20:03:45 +0000
I was just performing some flow tests of my injector with water and I noticed
that the Coefficient of Discharge (Cd) surprisingly changed from 0.65 to 0.47
when the orifice diameter was also changed from 0.23 in to 0.035 in. In both of
these cases the downstream pressure was ambient, however, for the first test
the upstream pressure was about 412psi although for the second it was closer to
380 psi. Presumably the pressure difference is due to my regulator unable to
keep up with higher flow-rates.
I have 3 theories about what could possibly cause this difference:
(1) Cd is sensitive to the orifice diameter (eg. The larger the diameter the
lower the Cd)
(2) Cd is sensitive to the upstream pressure or pressure drop
(3) A combination of 1 and 2
My suspicion is that most of the difference is due to higher pressure drop.
Unfortunately, I don’t have an easy way of proving this without buying a new
regulator.
Does this this theory somewhat make sense? I also found 2 links to formulas
online (both formulas are fairly similar) that estimate Cd. Is this a viable
approach to do at least a first approximation of correct Cd values?
http://www.valvias.com/flow-equations-discharge-coefficient-c.php
http://en.wikipedia.org/wiki/Discharge_coefficient
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