[Averome]

Hi, everybody. Help to determine what boundary conditions should be set to calculate the pressure drop in the pipe with the throttle. The example shows the boundary conditions at the velocity inlet and at the static pressure outlet. This set of conditions leads to an increase in inlet pressure and a change in the mass flow rate.
The main task is to find a suitable hole diameter to provide a pressure drop from 0.8 MPa to 0.1 MPa. the fluid inside the pipe is air.

Model:



velocity:



press:




I doubt the results. What better to set the boundary conditions that would get the correct results?

thanks in advance for your help

[lorbekl]

My guess would be to try and set the inlet to a mass flow/total pressure and keep the outlet as static pressure

[piu58]

For this, you don't need any numerical simulation. You may calculate all the values with Hagen-Poiseuille law. If your sketch is to scale, you only need to calculate what happens inside the small diameter. The resistance in the large tubes is very lower and may be neglected.

[Averome]

Originally Posted by lorbekl

My guess would be to try and set the inlet to a mass flow/total pressure and keep the outlet as static pressure

Thank you. I tried such a specification of the boundary conditions, I get a lot of pressure on the input of the order of 10 ^ 7 degrees, which, as it does, does not fit with the idea

[Averome]

Originally Posted by piu58

For this, you don't need any numerical simulation. You may calculate all the values with Hagen-Poiseuille law. If your sketch is to scale, you only need to calculate what happens inside the small diameter. The resistance in the large tubes is very lower and may be neglected.

Thank you for your advice. I tried to calculate according to the law of Hagen-Poiseuille. Faced with a lack of understanding of the result. According to the formula it turns out that the pressure drop is almost not significant, which does not correspond to common sense. Now I try to calculate the pressure drop by Idelchik " Guide to hydraulic resistance." Perhaps the law of Hagen-Poiseuille is not applicable for this mode.

[FMDenaro]

Originally Posted by Averome

Thank you for your advice. I tried to calculate according to the law of Hagen-Poiseuille. Faced with a lack of understanding of the result. According to the formula it turns out that the pressure drop is almost not significant, which does not correspond to common sense. Now I try to calculate the pressure drop by Idelchik " Guide to hydraulic resistance." Perhaps the law of Hagen-Poiseuille is not applicable for this mode.

The pressure drop in streamwise direction depends on the viscosity and involves the mass flow rate, as explained in any fluid dynamics textbook.
How do you compute it from the H-P solution?

[Averome]

Originally Posted by FMDenaro

The pressure drop in streamwise direction depends on the viscosity and involves the mass flow rate, as explained in any fluid dynamics textbook.
How do you compute it from the H-P solution?

the pressure and temperature at the inlet are known to me from this, I find the density of the air, then I find the dynamic viscosity from the dependence of pressure and temperature. formula H-P is transformed to find the pressure at the output. you can not convert if you need a difference.

[flotus1]

Originally Posted by Averome

Perhaps the law of Hagen-Poiseuille is not applicable for this mode.

This. The flow through an orifice can not be approximated using Hagen Poiseuille.
There would be other methods more suitable like empirical Zeta-values for circular orifices from literature...

The main task is to find a suitable hole diameter to provide a pressure drop from 0.8 MPa to 0.1 MPa. the fluid inside the pipe is air.

In order to work this out you need another boundary condition: the mass flow rate.
What your simulation would definitely need is a better mesh resolution in the area of the orifice.

[FMDenaro]

Originally Posted by Averome

the pressure and temperature at the inlet are known to me from this, I find the density of the air, then I find the dynamic viscosity from the dependence of pressure and temperature. formula H-P is transformed to find the pressure at the output. you can not convert if you need a difference.

The parabolic profile requires 3 conditions. Two are zero velocity at the wall. The third is the condition on the area under the velocity profile, that is the volumetric flow rate. That is linked to the pressure drop. Temperature has no relevance.

[Averome]

Originally Posted by FMDenaro

The parabolic profile requires 3 conditions. Two are zero velocity at the wall. The third is the condition on the area under the velocity profile, that is the volumetric flow rate. That is linked to the pressure drop. Temperature has no relevance.

we talk about different things. As I understand you asked how I calculated the density and dynamic viscosity of air, I answered. Your remark I don't understand what it refers to.

[Averome]

Originally Posted by flotus1

This. The flow through an orifice can not be approximated using Hagen Poiseuille.
There would be other methods more suitable like empirical Zeta-values for circular orifices from literature...


In order to work this out you need another boundary condition: the mass flow rate.
What your simulation would definitely need is a better mesh resolution in the area of the orifice.

Yes, I did, but I get very high pressure at the entrance is normal, I can not understand it, you can tell what some literature or scientific work that is "chewed"?
in the picture in the attachment I set the input mass flow and the output static pressure.


not understanding the results is due to the fact that with a small mass air flow results are plausible.