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Porous medium metal foam heat exchanger.

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Old   February 9, 2006, 11:37
Default Porous medium metal foam heat exchanger.
  #1
David Dunavant
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Am wondering if my approach with this is correct. Have modified thermal conductivity k and specific heat cp of my fluid based on volume fractions of metal foam and fluid. Also, seems like pressure drop will not affect thermal performance.

Heat exchange of metal foam is about twice that of conventional baffle-type design. Furthermore pressure drop can be greatly reduced, see above.

Also would appreciate if anyone can provide information about permeability K and form coefficient C of aluminum metal foam since suppliers won't release much data.

Thanks for any assistance. David Dunavant
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Old   February 9, 2006, 14:45
Default Re: Porous medium metal foam heat exchanger.
  #2
Andrew Hayes
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if you have already done pressure drop readings you can calculate the K and C values. You need to take multiple pressure drop readings for various velocities, and then do a pressure drop reading across the open channel. Subtract the two to find what the pressure drop was due only to the porous media. Then, fit your data to a second order equation.

Then, knowing the constants for the variables you can solve for K and C. C is the inertia coefficient.

dP=[(mu*L)/K]*Vd + [(rho*C*L)/K^.5]*Vd^2

where mu - kinematic viscosity L - length of porous section K - permeability Vd - darcian velocity rho - density C - inertia coefficient

Jiang (and other authors) have equations for K and C pertaining to packed beds, but I found them to be different for foam. Burmeister gives an equation for K for porous media;

K = (porosity*Ap)/(8*pi)

where Ap = area of pore

you can estimate that if you know the ligiment diamter and the pores-per-inch (PPI). Or, if you know the pore diameter.

K = (porosity*Dp)/(32*pi)

where

Dp = pore diamter

I am modeling this now, and I did not change my Cp value. I did modify the thermal conductivity of my fluid based on the temperature. Pressure drop changes all kinds of things; F, C, Re, Ux, and Vy, among other things - all of which play a part in the heat transfer
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Old   February 10, 2006, 08:24
Default Re: Porous medium metal foam heat exchanger.
  #3
Harry Fulmer
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Interesting app. What about the issues of (dirt) fouling and subsequent increased blocking of the foam?
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Old   February 10, 2006, 10:30
Default Re: Porous medium metal foam heat exchanger.
  #4
diaw (Des Aubery)
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Thats a very good practical question. Suggest looking into a pre-filter placed before the unit. Allow for the pressure-drop in your circuit hydraulics.

Des Aubery...
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Old   February 13, 2006, 15:11
Default Re: Porous medium metal foam heat exchanger.
  #5
Harry Fulmer
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a pre-filter would increase the amount of air bypass for an open type system or decrease fan flow rate or both. Either way it drops the air flow through the foam. An interesting trade off to investigate!
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Old   February 13, 2006, 20:51
Default Re: Porous medium metal foam heat exchanger.
  #6
diaw (Des Aubery)
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Without a pre-filter, the airflow will definitely drop off... as the foam becomes blocked. Then the problem becomes how to unblock the foam, which is not always too easy.
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Old   February 16, 2006, 10:13
Default Re: Porous medium metal foam heat exchanger.
  #7
Andrew Hayes
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I have also found that the thermal non equilibrium model is the model to use when modeling a porous media heat exchanger. Except at very low Re where the residence time of the fluid in the porous media is high and, therefore, can reach the same temperature of the porous material.
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