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-   -   Determining alpha and beta for porous baffle (http://www.cfd-online.com/Forums/cd-adapco/52471-determining-alpha-beta-porous-baffle.html)

Liaqat Khan October 25, 2000 18:28

Determining alpha and beta for porous baffle
 
I am a new user of Star-CD, and not sure how to determine alpha and beta values of porous baffle.

The porous baffle in my case represent a screen. When I run the model without the baffle, the solution converges in less than 100 iterations. However, when the baffle is included, and if I use arbitrary values of alpha and beta the solution does not converge in 1000 iterations, and the convergence parameters never fall below 100. I have tried various combinations of alpha and beta, changing their values by order of magnitude, with the same result.

From field and physical model studies, the head loss across the screen is approximately given by HL = 0.125 V^2, where velocity is in ft/s and HL is in feet of water, and the porosity is 0.755.

I would like to use the following equation (equation(8-6) in Methodology)

delP = -rho*(alpha*V+beta)*V

to determine alpha and beta.

From equation for head loss: For V = 1 ft/s (0.3048 m/s), HL = 0.125 ft = 0.054 psi = 373.463 Pa. For V = 2 ft/s (0.6096 m/s), HL = 0.500 ft = 0.217 psi = 1493.85 Pa

Using these estimates of head losses, equations for delP can be written as (rho = 998.2 kg/m^3)

373.463 = -998.2*(alpha*0.3048+beta)*0.3048 1493.87 = -998.2*(alpha*0.6096+beta)*0.6096

If I solve these equations for alpha and beta, I get negative value of alpha and beta is almost zero. Obviously, something is not right.

I will appreciate if some one can give me suggestions how to approach this issue.

Thanks.

Liaqat A. Khan


Brian October 27, 2000 04:44

Re: Determining alpha and beta for porous baffle
 
Alpha is coming out negative because your pressure drop should be negative. The way you have formulated it, the baffle causes a pressure INCREASE!

But whatever the values, remember to test them against the allowable limits for alpha and beta. You can find these limits in the manual.


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