Coefficient of discharge

[Scott]

I struggled for a while to figure out this Discharge Coefficient stuff and am sure there are others out there that have struggled with it as well. I'm starting a post so that all you brainiacs can help come up with an explanation that us non-college guys can understand.

Here's my take, let the flaming begin,

Cd is basically the ease at which air will flow through a flow element. Flow Elements can be an Orifice, a Venturi, a piece of PVC. So a Cd of (1) means the air travels easy through it, and a Cd of (.62) means the air has a harder time flowing through

I did some interesting calculations, with the flow rate spreadsheet downloaded from this forum. If you take the area of a sharp edged orifice with a .62 Cd and multiply that area by .62, now take this area and come up with a diameter that its equal to it and load that diameter into the formula spread sheet with the Cd set at (1), you get the same exact flow rate.

I think the Cd is actually the "Vena Contracta" which is the point of highest velocity that occurs at distance of .5 times the orifice diameter past the orifice in the direction of air flow.

Let hear it fellas

RRBD :p

[larrycavan]

Think of it as the efficiency rating of the orifice that can be altered plus or minus by altering the edge of the orifice. It is one of several elements that will establish the flow rate. Diameter, Dp, thickness of the plate and any influence of other forces that affect how the air / fluid is introduced to the orifice. Vena Contracta is not Cd but rather phenomenon that affects the Cd.

When calibrating your bench you are in effect comparing your orifice to one with established flow rates. This allows you to determine the Cd of your orifice.

No flaming required on tractorsport...it is prohibited and those who stoop to such levels of discussion will be visited be a gentleman that goes by the name of Vinny :angry:. He's been known to be persuasive through various means of discomfort. In fact, it has been reported that after he visits, there is generally an increase in Cd and diameter of a common human orifice....

Larry C

[Tony]

Another point to watch here is the condition of the upstream air. For the expected flow coefficient, the upstream air must contain no turbulence or random movement, there must be a "natural" pressure gradient form upstream, which causes the air to move towards the hole.

But how much tolerance the flow coefficient has to slight upstream turbulence is strongly influenced by the differential pressure. The higher the differential pressure, the closer you will get to the magical 0.62

The result is that for example, measuring flow at 1" depression may be rather tricky, but measuring flow at 100" depression with exactly the same setup, it would be far more tolerant. In theory, a 100:1 pressure increase would increase the CFM airflow by exactly x10.

In practice the two measured flow figures may not be exactly 10:1, but the higher figure would be far more accurate, as well as easier to measure.

So the hot tip is, try to design your orifice bench with as high a calibration pressure as your available blower power will allow. Accuracy and repeatability will be better. The problem is that an internal measurement orifice can never be supplied with completely tranquil air, no matter how hard you try. But what you have will work better when there is a higher design pressure across the orifice.

Fascinating stuff...

[larrycavan]

[hollywood63]

So whats the proper way to find the coefficient of discharge of a test plate? ???

[Tony]

Hollywood, Mount it flush on the top surface of your bench with the upper face exposed to undisturbed room air.

Larry, I am in full agreement with what you say. A large settling chamber of some sort, and maybe something to break up eddies and swirls entering the measurement orifice can help a lot. Higher orifice calibration pressure helps too.

Making two identical test orifices is not difficult. Place one over the test hole, and use the other internally as the measurement orifice. The measured pressure drops SHOULD be absolutely identical. It is a very sensitive test. If there is a significant difference, it indicates that the internal flow dynamics of your bench have some sort of a problem.

Equal pressure drop is easy to achieve with small orifice sizes, and pretty damned difficult with larger orifice sizes, where flow will be at near maximum for your bench. Internal bench flow around corners and where the internal cross sectional area changes will set up flow instabilities that effect the flow through the internal measurement orifice.

It is an excellent test, and very easy to do.

[larrycavan]

[hollywood63]

Easy enough I will use the plates that are going around as my base. What is the Coefficient of these?
Art

[bruce]