Modelling flow through orifice - very small part - using CFDesign 9

[boost33]

Hi there,

I am trying to measure the pressure drop across a 0.4mm diameter 3mm long orifice.

This is a bleed of hole. The inlet is also a similar sized orifice.

Air enters at around 0.5L/min at approx 200Pa (gauge) into a chamber that acts on a diaphram with the bleed off orifice present in this chamber also.

The orifice size and length is critical to performance. We are trying to use CFD to save some experimentation and help us gain confidence in the software.

In the CF design model I have a simplified inlet into chamner and the outlet.

I have boundary conditions

Inlet: 0.5L/min and 200Pa (gauge).

Outlet: 0Pa (gauge) and 0.5L/min.

We are trying to measure the pressure drop so I don't know if this outlet boundary condition is correct. Since it always gives 0kPa in the last 1mm of the orifice/pressure dropping section.

If I leave the pressure at the outlet unknown the program gives erroneous results.

Although very small flow and pressure the performance of this medical device is sensitive to these parameters (i.e the orifice diameter and length). It is important to know the range of values that give the correct pressure drop/performance so that tolerances can be set.

Thanks a lot

 

[cowski]

Is the length of your flow cap at least 5 times the diameter?
Are you applying the boundary conditions at the ends of the flow caps (recommended) or right on the inlet/outlet faces?
You mention that the outlet is a bleed off hole, is there any other outlet(s)?
Only 1 inlet?
Are there any moving parts involved and if so are they allowed to move during the simulation? (you will need more than the basic Cfdesign license for this).
Are you using compressible or incompressible flow? (I assume fluid is air? - for 200Pa pressure you should use the incompressible flow option).
Is the outlet open to atmosphere? If so, I would eliminate the 0.5L/min boundary condition and just use the 0 pressure condition.