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

 

[Latexman]

What's the fluid?

Good luck,
Latexman

 

[Latexman]

What's the temperature?

Good luck,
Latexman

 

[CJKruger]

boost33,

You either have to allow the flow or inlet pressure to vary. At 200 kPag the orifice is choked, so outlet pressure is irrelevant.

Assuming air at 25 C and you allow the flow to vary, the results are (using Korf):

P in = 200 kPag
P out = 0 kPag (to atmosphere)
Flow = 0.25 kg/h

Regards

 

[CJKruger]

boost33, please note that I used 200 kPag instead of 200 Pa by mistake.

 

[boost33]

Yes it is air at around 25 deg C

Thanks for the replies.

Is the outlet orifice still choked at 200Pa inlet? I'll have to go back and check my test books re. choked flow.

What I have done is set the inlet pressure (200Pa) and flow (0.4L/min) (We can do this I believe). We then set the outlet as atmospheric pressure (no flow setting) we can get some results. The pressure drop across each orifice is then visible. Whether this is correct or not I am not sure.

 

[CJKruger]

boost,

1) No, it will not choke at 200 Pa. I used 200 kPa by mistake.

2) No, you cannot specify the inlet pres, flow, orifice size and outlet pressure. There is nothing left to calculate if you do it.

 

[Latexman]

A better description of the flow path may be needed. Is there an inlet orifice AND an outlet orifice (bleed off hole)? What's between them?

Good luck,
Latexman

 

[unclesyd]

If the fluid is air and the flow path is a capillary you might try to get this article.

http://links.jstor.org/sici?sici=0080-4630(19660809)293:1434<329:ATOGFT>2.0.CO;2-0#abstract

 

[boost33]

Thanks.

The flow path inludes one orifice in the inlet then a small chamber (the pressure in this chamber works on the diaghram) and the outlet/bleed hole also has an orifice.

The fluid is air.

What I have found:

Setting inlet Pressure and outlet pressure gives some results but the results seem to only depend on the relationship between the 2 orifice sizes. I.e. you can not get an absolute answer since the outlet is set to 0Pa (gauge). E.g. 2 same size orifices will give the same result as 2 small orifices yet the performance of the part and the flow will be different.

Thanks again

 

[CJKruger]

boost33,

So I assume the layout is:

200 Pa -> Orifice 0.4 mm -> Pres unknown -> Orifice 0.4 mm -> 0 Pa (g)

1) You still have to leave the inlet pressure, flow or orifice sizes unknown.

2) With this low pressure drop air behaves like a liquid and the intermediate pressure is simply half way between the inlet and outlet pressure (100 Pa in your case).

 

[Latexman]

With L/D = 3 mm/0.4 mm = 7.5, I'd treat this like pipe or tubing. CJKruger's #2 is right on. I'd calculate it as follows:

0.4 mm ID tubing
6 mm length (2 x 3 mm each)
2 entrances *
2 exits *

* Assumes the diaphragm chamber flow area is large compared to the "orifice" diameter.

At dP = 200 Pa, flow is laminar (N[sub]Re[/sub] = 115). I got 0.00234 kg/hr or 0.0328 L/min. I think 0.5 L/min is at least an order of magnitude off. The density I used was 1.19 kg/m[sup]3[/sup].



Good luck,
Latexman

 

[dscbob]

Can you provide any insight as to when to use incompresible or compressible flow option in CFD?

Thanks,
Bob