Equivalent flow through orifice

[sajego]

I posted before about noise levels through an orifice containing about 150 small holes. Testing has confirmed that the measured dBA are not significantly higher with a single orifice. However the single orifice they used was just scrap and not designed specifically for our application. I need to design that orifice and want to keep pressure drop and flow through it equivalent to the multi-holed one.

I can calculate equivalent area, but I don’t know how to estimate the flow coefficient through something that has multiple passages. Any ideas for how to proceed?

 

[Flareman]

The orifice coefficient for a multihole arrangement will depend on the specific grouping of the holes relative to the incoming stream. To get a rough idea, treat each individual hole as a single orifice based on an incoming pipe size which has a diameter equal to the pitch of the holes. Where the holes are really close together the coefficient will be quite high. When they're well spaced it will be lower. Then there could be some sort of modification according to the size of the group and the diameter of the total incoming stream based on the same basic rules. It's my guess, when you juggle all these numbers around, that the multihole coefficient will be only slightly less than the single hole which replaces them in the pipe.

The noise benefit usually derived from multihole nozzles is related to a dilution of the noise generating, shear flow from the holes over a larger cumulative perimeter and by some noise cancellation within the total stream. It is most noticable when the orifices discharge to atmosphere and the noise is not attenuated by the equipment.


David