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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

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


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

look at this article

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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