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omidiut April 28, 2013 10:26

simulation of orifice
 
Hi all

i'm trying to simulate flow in restriction orifice with this boundary condition:

mass flow rate: 5.71 kg/s
Inlet pressure : 151 bar
outlet pressure :8 bar

my problem is find size of orifice to achieve this pressure drop.
first i use total pressure as inlet boundary condition and static pressure as outlet boundary condition. the calculated mass flow rate is: 5.9 kg/s
Second i use total pressure as inlet boundary condition and mass flow rate 5.71 kg/s as outlet boundary condition to investigate how much is the pressure drop of this orifice but the outlet calculated pressure is: 93.7 bar.
what does it mean? finally this orifice can provide 143 bar pressure drop?have my simulation any problem?


Best regards.

ghorrocks April 28, 2013 18:57

If you are asking why is my results inaccurate have a look at the FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

For high velocity flows there will be a big difference between static and total pressures. Make sure you use them correctly or you will get rubbish answers.

omidiut April 29, 2013 00:25

Thank you for reply Dear ghorrocks

I use static pressure as inlet pressure and don't see any difference between results. I think there is two topic in orifice design:
1- there is a chocking range according to up stream and down stream pressure by Pressure inlet and pressure outlet boundary condition we can find maximum mass flow rate throw orifice
2- every orifice with any size of bore can provide distinctive pressure drop,
I think this orifice can't provide 153 bar pressure drop and results are true.

What do you think?Do you have any suggestion?


Best regards.

ghorrocks April 29, 2013 00:35

Of course orifaces choke with compressible flow at high enough pressure differences. That is basic compressible fluid flow. But an incompressible flow will never choke. The flow rate will just keep increasing as the pressure goes higher.

And pressure drop for oriface size is also basic fluid mechanics - most fluids textbooks will have a pressure drop versus flow rate curve for an oriface plate in them. If you are asking whether your results are correct you should look in fluid mechanics textbooks for some benchmarks to compare against.

omidiut April 29, 2013 01:02

Thank you Dear ghorrocks

It is a multi holes orifice. i want to study effect of some parameters such as bore size, number of holes, holes arangement on pressure drop and mass flow rate. It's seems to be distinct and simple problem.
what do you thing about objection or wrong definition of this problem in CFX?

Best regards.

ghorrocks April 29, 2013 01:26

As I said before, the general question on accuracy is a FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

oj.bulmer April 29, 2013 04:34

I would let the outlet pressure as zero and inlet as mass flow rate, this way you get the inlet perssure in result and you can get your pressure drop. You don't need to provide inlet pressure in boundary conditions.

Do not judge your results through pressure drop, but some empirical indication, like resistance coefficient which is a ratio of static and dynamic head.

OJ

omidiut April 30, 2013 05:36

Than you for your reply Dear OJ

I implement your suggestion for boundary condition . total pressure at inlet reach to 148 bar.

Now why with 150 bar as inlet pressure and mass flow rate as outlet pressure the pressure drop is very smaller ( P out= 93 bar) ? while this boundary condition seems to be more physically. what is the reason of this difference?

and why i don't need to provide inlet pressure in boundary conditions?

Best regards.

oj.bulmer April 30, 2013 06:32

I was just trying to suggest the methods that are typically used in finding pressure drops. As I mentioned earlier, I wouldn't worry about the over-constraining by specifyng inlet and outlet pressures along with the mass flow. How are you sure that the values you mention are right? If all you are interested in is pressure drop (which by comparison will provide the best design of all the configurations you have), you shouldn't be concerned with absolute values of pressures. In incompressible flow where any small perturbation will be felt everywhere in the domain, you shouldn't require so much information to enter the domain.

OJ


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