Orifice plate thickness to achieve critical flow
Orifice plate thickness to achieve critical flow
(OP)
When checking my orifice calculation method (after reading denniskb's thread) I came across a statement saying that choked/critical flow does not occur accross thin, sharp edged, orifice plates. For chocking to occur the thickness of the orifice plate must exceed the orifice diameter.
Is this true and why?
Is this true and why?





RE: Orifice plate thickness to achieve critical flow
you are correct, neither thin or thick plates exhibit critical flow characteristics because of the entrance losses.
Thick plates do offer a discharge coefficient that is nearly independent of reynolds number and tend to approach critical flow behavior better than a thin plate. You only approach critical flow in really thick plates i.e. T/d>>>D, because you achieve pipe flow prior to the final expansion, but you still have to deal with the entrance losses.
There are corrections for the "critical flow" behavior of thin/thick plates (T/d~1) in Millers Flow Handbook, but they only increase the complexity of the calcualtion rather than improve performance.
A flow nozzle does achieve true critical flow. Some of the meter manf. can supply them.
RE: Orifice plate thickness to achieve critical flow
As pointed out to me, the following link supports this idea:
http://www.optimal-systems.demon.co.uk/flow-orifice.htm
RE: Orifice plate thickness to achieve critical flow
For thin plates it only occurs as a limiting condition as acknowledged in the web-site you've referenced.
The issue raised by Mark is that critical flow does not generally occur with plates that fall in the thin-plate category. True critical flow conditions can occur only for true nozzles with a contoured inlet.
RE: Orifice plate thickness to achieve critical flow
RE: Orifice plate thickness to achieve critical flow
author A.J. Ward-Smith , Int J Heat Fluid V1 no 3 pp123-132 1979.
Basically the discharge coeficient is related to the ratio of the plate thickness divided by the orifice diameter. For a truly sharp edge t/d=0, the Cd=1.0 . For a t/d in the range 1<t/d<3, Cd=0.81 . For very thick plates t/d >7 the Cd is calculated using the Fanno relationships for frictionally choked flow in piping.