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-   -   in - pressure, out pressure, is that make sense? (http://www.cfd-online.com/Forums/cfx/65946-pressure-out-pressure-make-sense.html)

Marteusz July 1, 2009 15:01

in - pressure, out pressure, is that make sense?
 
Hi everyone,

I want to simulate a flow through Bosch Rexroth directional valve. I modeled geometry. I want to obtain nominal flow rate through valve.

I have found how companies determine it.

This quotation from Bosch rexroth documentation from their website.

" No norm exists for the term “nominal flow rate”. To obtain a clear numerical value, Rexroth has defined the following:

Inlet pressure: pabs = 7 bar
Differential pressure: ∆p = 1 bar
Medium temperature: TM = 293 K (20 °C)
Humidity: Φ 0 %

Flow rates are indicated in volumes per time unit. The volume refers to a
normal state valid for pneumatics according to ISO 8778, and this is as
follows:

Pressure: p0 = 100 kPa (1 bar)
Temperature: T0 = 20 °C
Relative humidity: Φ 65 %

In order to identify the flow rate q as a slack volume, the acronym ANR
(Atmosphère Normale de Référence) can be added after the unit.
Example: q = 12 dm3/s (ANR)"

According to this documentation, I have asssumed reference pressure 1bar, 6 bar pressure at inlet, and 5 bar at outlet.

Ansys CFX gave me mass flow at outlet q = 0.00743 kg/s and i divided it by 1,18kg/s (density in nominal state) and I multiplied it by 1000 and 60 to determine nominal flow in dm^3/min.

But unfortunately results are about 4 times less than from technical card about this directional valve.

Did I simulate this valve in proper way? Why there is so big difference?

Thank you for your help


Mateusz K.

ckleanth July 1, 2009 16:08

1) I didnt understand what you meant about the operating conditions that valve was tested. until thats clear (at least to you) i dont think you will ever find the solution
2) you said many words but nothing on how you modeled the valve in cfx (however find the answer to (1) first ) i presume it was steady state? did you assumed an isothermal state ?
3) read the "recommended configurations of boundary conditions" in the cfx manual.

Marteusz July 1, 2009 17:06

1) operating conditions, They assumed 6bar at inlet and 5bar at outlet
2) yes it is steady state analysis, and I applied compressibility of flow by assuming Ideal Gas

rogbrito July 1, 2009 20:28

Hi Sir,

is it possible to do an experimental analysis (only one Sir) with this valve? After that, u can do your (several) simulations with these CFD tools.

Rogerio. :rolleyes:

John July 7, 2009 11:28

Defining pressure BC on both inlet/outlets does not make sense to me. You need some other BC conditions to make the problem closed.

Timon July 7, 2009 11:32

Total pressure in, static pressure out is fine...

Marteusz July 15, 2009 14:33

Quote:

Originally Posted by Timon (Post 221795)
Total pressure in, static pressure out is fine...

look at this article
http://www.abcm.org.br/symposiumSeri...SSM3_IV_05.pdf

especially at the end of section 2.3 and look how they apply bc. I aware that static pressure at in and out is not appropriate.


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