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

Correlation of loss coeffient

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

Reply
 
LinkBack Thread Tools Display Modes
Old   June 16, 2003, 23:28
Default Correlation of loss coeffient
  #1
Suping
Guest
 
Posts: n/a
Hello friends,

I have a fluid mechanics question:

A pressure drop vs flow rate is measured for air flow through a porous medium. This gives a relation DP=1/2*density(air)*C(air)*v^2, where C(air) is the loss coefficient.

If the fluid is changed to water through the same porous medium, I am wondering if the above formulation for air can be used in some way for predicting the pressure drop (water) or the loss coefficient C(water).

Please help. Thanks!

Suping
  Reply With Quote

Old   June 17, 2003, 04:46
Default Re: Correlation of loss coeffient
  #2
Nicola
Guest
 
Posts: n/a
As far as the mach number of the flow is very low so that the fluid can be regarded as incompressible, the loss coefficient depends on the Reynolds number only, as simple dimensional analysis shows. This means that the loss coefficients obtained for water and for air are the same if Reynolds numbers are the equal.

Hope it helps

  Reply With Quote

Old   June 17, 2003, 14:38
Default Re: Correlation of loss coeffient
  #3
Jonas Larsson
Guest
 
Posts: n/a
Note that this formula is only valid for very thin porous media, for thicker media you need to use Darcy's Law which has a linear dependence on the velocity (instead of the quadratic). The type of formula you give is commnly used for perforated plates and screens (used for example to even out and calm the flow in wind-tunnels etc.) - if you need correlations for C let me know - I have a couple of good references which I recently digged up for modeling a screen in a CFD simulation of a jet-engine test-cell.
  Reply With Quote

Old   June 17, 2003, 21:59
Default Re: Correlation of loss coeffient
  #4
Suping
Guest
 
Posts: n/a
Jonas,

Thanks for the helpful message. I am very interested in reading the references you mentioned.

Our model based on Darcy's law is developed for general porous media including thick media and thin perforated screen. According to Darcy's law, the pressure drop increases linearly with velocity in thick porous media. For thin perforated screens, however, we intent to use DP=1/2*density*C*v^2.

Since some experimental measurements are available for air flow, it will be helpful to know the correlation for loss coefficient in order for predicting other fluids through the same screens.

Thanks again.

Suping
  Reply With Quote

Old   June 18, 2003, 01:10
Default Re: Correlation of loss coeffient
  #5
Lars Ola Liavåg
Guest
 
Posts: n/a
Exactly how thick is a thick porous medium? Is for instance a water-air heat exchanger featuring bundles of tubes with/without fins placed in the air stream thick enough to follow a linear dependence on velocity?
  Reply With Quote

Old   June 18, 2003, 03:16
Default Re: Correlation of loss coeffient
  #6
Jonas Larsson
Guest
 
Posts: n/a
The best reference I found was:

Cornell, W. G., "Losses in Flow Normal to Plane Screens", Transactions if the ASME, May 1958, pp 791-799 (since it is a bit old it can be a bit tought to get - let me know if you want me to fax it to you).

Another reference is:

Salmirs, S & Aljabari, S., "Prediction of the Pressure Loss Coefficient of Wind Tunnel Turbulence Reducing Sreens", AIAA Paper 92-0568, 30th Aerospace Sciences Meeting & Exhibit, January 1992, Reno

You can also find a quite long list of other references on:

http://vonkarman.stanford.edu/tsd/pb...el/rscreen.txt

and

http://vonkarman.stanford.edu/tsd/pb...el/screen.html

I hope that helps.
  Reply With Quote

Old   June 18, 2003, 04:07
Default Re: Correlation of loss coeffient
  #7
Harry Fulmer
Guest
 
Posts: n/a
Generally a laminar type loss has a linear relationship between velocity and dP, a turbulent loss has a quadratic one. Transitional flows obey a combination of the two of the form:

f = A/Re + B/Re^n

values for A, B and n can be found in the literature, Try Idelchik.

Good luck!
  Reply With Quote

Old   June 18, 2003, 05:31
Default Re: Correlation of loss coeffient
  #8
Rami
Guest
 
Posts: n/a
Suping,

We apply the Darcy-Forschheimer model

-dp/dx = A*mu*u + B*rho*u^2

where A and B are nondimensional correlation coefficients, mu is the dynamic viscosity and rho is the density.

We have used this form for various types of porous media (e.g., ceramic foam, tube banks and honeycomb) at various Re ranges. The A and B are found based on experimental data (e.g., Zukauskas for tube banks) for the appropriate porous medium micro-structures, and are generally related to the micro-structure parameters (e.g., the tube arrangement, diameter and pitches in tube banks).

Having the correlation coefficients for the needed Re range, the model may be applied to any fluid.
  Reply With Quote

Old   June 19, 2003, 15:17
Default Re: Correlation of loss coeffient
  #9
Suping
Guest
 
Posts: n/a
I am not quite sure for your case.

In general, for high Reynolds number flows where Re=rho*U*k^0.5/mu >=1, Darcy law is inadequate, where rho is density, k permeability, and mu viscosity. A remedy to this problem is to employ an inertia term, containing a loss coefficient, to the standard Darcy equation.
  Reply With Quote

Old   June 19, 2003, 15:23
Default Re: Correlation of loss coeffient
  #10
Suping
Guest
 
Posts: n/a
Dear Friends,

From your responses, I really found valuable information. Many Thanks to all of you.

Best Wishes.

Suping
  Reply With Quote

Reply

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Pressure Loss dependet on Flow Direction MichaelN CFX 2 June 5, 2011 11:22
loss calculation in simulation of turbomachines playmaster Main CFD Forum 0 August 21, 2010 05:59
Mass loss! So what? jinwon park Main CFD Forum 13 May 22, 2008 09:29
How to deifmne head loss co effcint ? Angelina FLUENT 1 February 8, 2006 10:41
How to deifmne head loss co effcint in cfx? Angelina CFX 4 February 7, 2006 14:14


All times are GMT -4. The time now is 15:18.