CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > General Forums > Main CFD Forum

Pressure loss over a valve

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   November 11, 2018, 10:54
Default Pressure loss over a valve
  #1
New Member
 
Angelina
Join Date: Nov 2018
Posts: 3
Rep Power: 7
Unicorn91 is on a distinguished road
I tried to calculate the mass flow over a valve. First made a calculation by hand with Bernoulli:

(See image "Bernoulli.png")

Okay that was easy. Now I want to insert e pressure loss because of the valve geometry. In Literature it ist described this way:

(See image "Druckverlustbeiwert.png")

But now if zeta is < 1 my volume flow (bzw. mass flow) grows.
Is that possible?

Pipe contractions can have e zeta term of 0.5. Would that rise the mass flow?
In which situations is that possible?

Thanks for your help!
Attached Images
File Type: png Bernoulli.PNG (28.1 KB, 17 views)
File Type: png Druckverlustbeiwert.PNG (14.6 KB, 13 views)
Unicorn91 is offline   Reply With Quote

Old   November 11, 2018, 13:10
Default
  #2
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,760
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
As you know, Bernoulli cannot be applied when a dissipative process is acting, as happens for the viscous flow around a valve. Correction due to friction that casues pressure loss can be found in fluid mechanics textbooks, such as White. You find many exercises
FMDenaro is offline   Reply With Quote

Old   November 11, 2018, 17:49
Cool
  #3
New Member
 
Angelina
Join Date: Nov 2018
Posts: 3
Rep Power: 7
Unicorn91 is on a distinguished road
I was a bit confused as I realized that the definition of the pressure loss is based on Bernoulli as I described above.

https://de.wikipedia.org/wiki/Druckverlustbeiwert

First I startetd with the "first law of thermodynamics":
(See "first law of thermodynamics.png")

In my opinion I have to respect the pressure loss separately as I described. But I was not sure when I saw the definition of the pressure loss. And what happens when zeta is < 1?

@ FMDenaro: I've found the testbook from White. Wow almost 900 pages! Can you tell me which chapter or which exercise you mean?
Attached Images
File Type: png First law of thermodynamics.PNG (34.9 KB, 9 views)
Unicorn91 is offline   Reply With Quote

Old   November 11, 2018, 18:11
Default
  #4
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,760
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by Unicorn91 View Post
I was a bit confused as I realized that the definition of the pressure loss is based on Bernoulli as I described above.

https://de.wikipedia.org/wiki/Druckverlustbeiwert

First I startetd with the "first law of thermodynamics":
(See "first law of thermodynamics.png")

In my opinion I have to respect the pressure loss separately as I described. But I was not sure when I saw the definition of the pressure loss. And what happens when zeta is < 1?

@ FMDenaro: I've found the testbook from White. Wow almost 900 pages! Can you tell me which chapter or which exercise you mean?





Bernoulli requires the flow to be incompressible and the absence of any dissipation of energy due to viscosity and conducibility. This way the total pressure is invariant. Thus, you can compute the difference between the total pressure and the static pressure.


On the other hand, consider the example of the exact viscous solution in a channel (Poiseuille). You will find the pressure difference between two locations along the streamwise direction. This "pressure loss" is due to the friction.


In your case the flow around a valve produces separation due to viscous effect.The flow is not isentropic and the total pressure changes along the streamwise direction.



Search Bernoulli in the White textbook and compare to the section of viscous flows.
FMDenaro is offline   Reply With Quote

Old   November 12, 2018, 05:00
Default
  #5
Senior Member
 
Joern Beilke
Join Date: Mar 2009
Location: Dresden
Posts: 497
Rep Power: 20
JBeilke is on a distinguished road
https://de.wikipedia.org/wiki/Druckverlust


There you find the bernoulli equation with pressure loss.
JBeilke is offline   Reply With Quote

Old   November 12, 2018, 05:44
Default
  #6
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,760
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Yes, the correction in the Bernoulli integral due to the viscous action highlights that the total pressure is no longer invariant in a non-isoentropic flow.
FMDenaro is offline   Reply With Quote

Old   November 14, 2018, 17:20
Default
  #7
New Member
 
Angelina
Join Date: Nov 2018
Posts: 3
Rep Power: 7
Unicorn91 is on a distinguished road
Okay I forgot to say, in my situation I have a turbulent flow (RE = 10'000).

I tried an example from the textbook "White" for viscouse flow: see "Example Viscouse Flow Turbulent".
The resulting velocity (and massflow) is much too high.

So I think the main input of the pressure loss comes from the valve orifice.

@JBeilke: Right, that's how I calculated my second equation (see "first law of thermodynamics.png") So I think thats the right way.

But I was a bit confused about the definition of the flow coefficient Kv (see "Kv-Wert"). The flow coefficient is normaly given, when you buy a valve.

Maybe this is a simplification?
Attached Images
File Type: jpg Example Viscouse Flow Turbulent.jpg (119.3 KB, 5 views)
File Type: jpg Kv-Wert.jpg (110.6 KB, 1 views)
Unicorn91 is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
question regarding LES of pipe flow - pimpleFoam Dan1788 OpenFOAM Running, Solving & CFD 37 December 26, 2017 15:42
Negative Total Pressure Loss Coefficient near the casing in axial compressor mitra22 CFX 1 December 15, 2017 23:34
High Pressure Let Down valve soletto Main CFD Forum 0 December 20, 2016 17:08
CFX Solver stopped with error when requested for backup during solver running Mfaizan CFX 40 May 13, 2016 07:50
Does star cd takes reference pressure? monica Siemens 1 April 19, 2007 12:26


All times are GMT -4. The time now is 06:24.