CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

how to measure pressure drop through a valve

Register Blogs Community New Posts Updated Threads Search

Like Tree2Likes
  • 1 Post By ghorrocks
  • 1 Post By oj.bulmer

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   April 6, 2013, 12:58
Question how to measure pressure drop through a valve
  #1
Senior Member
 
hmasenger's Avatar
 
hamed
Join Date: Apr 2009
Posts: 148
Rep Power: 17
hmasenger is on a distinguished road
Hi all
I am modeling flow in a butterfly valve which is installed in a pipe .I need to know the pressure drop accruing in the valve by variation in flow velocity.
I don’t know what value I should set for the pressure outlet BC that makes it possible to measure the pressure drop.
Is it all right if I just set the domain's reference pressure to[ 1 atm ] and set pressure for outlet BC to [ 0 atm ] which is about 20 times of pipe diameter downstream of valve and then measure the average pressure difference between locations in 5times of pipe diameter upstream and 10 times of diameter downstream of valve?


Best regards

Last edited by hmasenger; April 7, 2013 at 04:31.
hmasenger is offline   Reply With Quote

Old   April 7, 2013, 07:24
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,841
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
There is no "correct" answer for this, just many different approaches. What you have done makes sense and is easily comparable to published results. But you have also included a fair amount of pipe before and after the vlave, and this may introduce its own pressure loss. So you measured pressure loss is a combination of the valve and pipe pressure loss. And then there is the entrance condition - if you have assumed plug flow then you will have high pressure losses in the first bit as the boundary layers develop.

But these are all details and most engineers do nt worry about them or have simple ways of correcting for them. It is up to you as to whether this is important or not.

Is the simulation incompressible?
ghorrocks is offline   Reply With Quote

Old   April 7, 2013, 13:07
Default
  #3
Senior Member
 
hmasenger's Avatar
 
hamed
Join Date: Apr 2009
Posts: 148
Rep Power: 17
hmasenger is on a distinguished road
your answer was very helpful .thanks for that.
yes the flow is incompressible(water@20) and fully developed so I am considering 10% for turbulent intensity(i am not sure that's enough)
Actually the valve is a 1200mm diameter butterfly valve so I thing pipe friction head loos in that distance is negligible and the reason for taking 10D and 5D distances for pressure drop in literature is for including all eddy dissipation in measurements . I am studding cavitation and pressure drops in various water speeds.although effect of valve opening in cavitation and maybe a new disk shape for decreasing the cavitation for my msc degree.
for inlet BC i am going to use developed velocity profile published in fluid mechanic references
hmasenger is offline   Reply With Quote

Old   April 7, 2013, 19:02
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,841
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
That sounds good, your approach sounds sensible then. Make sure you do sensitivity analysis on all tunable variables to make sure things are under control.
hmasenger likes this.
ghorrocks is offline   Reply With Quote

Old   April 8, 2013, 12:13
Default
  #5
Senior Member
 
OJ
Join Date: Apr 2012
Location: United Kindom
Posts: 473
Rep Power: 20
oj.bulmer will become famous soon enough
One of the major reasons of measuring the pressure drop at those distances (5D, 10D) is to have a measure of "lost energy" in overcoming hydraulic resistance of the valve. And energy is indicative of total of static pressure (p) and dynamic pressure (1/2* density * v^2).

If you plot a graph of pressure in streamwise direction, you'll see that static pressure starts falling before the valve, it goes to minimum in wall region and then recovers. At the same time, dynamic pressure changes as well. If we take only (static) pressure drops close to the valve, these distances chosen may involve the drop and recovery of the pressure, where some pressure is still being converted into velocity or vice versa. Hence it won't be a complete representation but only a half one.

To mitigate this, you can take total energy (static or dynamic pressure) and thus have an adequate indication of energy loss across the valve. But this is a bit tedious. Or simply, let the static pressure recover, which it should typically do over 10D distance downstream. While before the valve, the pressure starts falling before a shorter length prior to valve, hence 5D is sufficient.

OJ
hmasenger likes this.
oj.bulmer is offline   Reply With Quote

Old   April 8, 2013, 15:30
Default
  #6
Senior Member
 
hmasenger's Avatar
 
hamed
Join Date: Apr 2009
Posts: 148
Rep Power: 17
hmasenger is on a distinguished road
tanks OJ .It was really helpfull .
hmasenger is offline   Reply With Quote

Reply


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
help in pressure drop calculation in fluent sinaupdate FLUENT 3 July 10, 2016 06:56
Pressure drop? Munni FLUENT 4 March 25, 2010 15:21
Constant pressure drop condition Jonny6001 FLUENT 0 December 19, 2009 08:36
Pressure Drop in outlet Vent Abdul FLUENT 2 October 28, 2008 13:13
Hydrostatic pressure in 2-phase flow modeling (CFX4.2) HB &DS CFX 0 January 9, 2000 14:19


All times are GMT -4. The time now is 14:25.