Flow Through Irregular Orifices

[DonyWane]

I need to estimate compressible fluid flow through an irregular orifice (i.e., a thin, circular ring or gap as opposed to a circular hole). I would think that an upper bound would be provided by modeling my flow area as a circular hole with an equivalent flow area to my "gap" area. However, I need a more accurate estimate...a conservative upper bound will not help me much. Does anyone have any correlations for this sort of orifice/flow path or know where to point me to? If it helps, the pressure drop across the orifice will be on the order of 10 to 25 psi.

Thanks!

 

[saxon]

Dony, Orifices come in four flavors, annular, concentric, eccentric, segmental. I think you're looking for the annular flavor. Check in Perry's Chemical Engineer's Handbook.

Hope this helps.
saxon

 

[TD2K]

There was a long discussion on this a few months ago when someone was trying to calculate the flow through an annular area. You might want to try a keyword search.

 

[hacksaw]

such calculations are usually performed on the basis of the wetted perimeter of the orifice. the flow equations work fine in gap type flows.

 

[DonyWane]

Hacksaw,

When you say the "wetted perimeter of the orifice," do you mean I need to choose a standard "hole" orifice with an equal perimeter and run the flow equations?

I could calculate the annular diameter...or is that what you meant?

There was some discussion in a MechE forum that I found (and I believe that you participated in). The result of that discussion..for incompressible flow...was that an article by Lenkei could provide some insight. Are you familiar with that article, and if so, would it provide insight for compressible fluid flow?

 

[DonyWane]

I think that it would also be helpful to redefine the problem just a bit. Instead of a traditional annular flow, I am faced with to cylinders sitting on top of each other, separated by about .7 inches. The cylinders are about 3 feet in diameter. The flow is going up through the cyliders, but then it can also escape out of the "gap". It, in many ways, is more similar to flow through a slit or rectangle cut into a piece of plate than it is like flow through an annulus. But, perhaps, the two can be calculated similarly.

 

[hacksaw]

Two issues. Irregular orifices can be dealt with on the basis of an hydraulic radius (i.e. an effective flow area) regardless of the shape. This is approximate.

As far as the flow through your 0.7" slot, you know the area and you either know the pressure drop or you can estimate the fluid velocity. Either way you can come up with a flow estimate. Exact, hardly, but by the same token not to bad.

you are describing a common estimate made in civil hydraulics in regard to storm intercept leakage and or inflows. it is do able

 

[DonyWane]

Estimating the velocity is quite tricky, no? I am dealing with compressible fluid flow. I could assume sonic velocity as a bounding case. But, now I have that velocity through a very large slot area. If I were to find the hydraulic radius of the slot (ratio of flow area to perimeter), then I get a much smaller diameter - and corresponding area. Sonic velocity through each of these flow areas (full slot area and area of a circle with radius equal to the hydraulic radius of the slot) would result in very different volumetric flow rates. Are you recommending calculating the area from the hydraulic radius? Do I worry about discharge coefficients?

Thanks again for the discussion.

 

[hacksaw]

if you know the area, and you acknowledge that the flow is compressible, then knowing the static pressure you will be able to ascertain if sonic flow conditions are achieved and thus the fluid velocity.

this is not rocket science, just estimate it and be done with it

 

[javan]

use "ISO 5167" guide