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Old   August 13, 2003, 06:28
Default Need advice for intercooler problem
  #1
joon
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hi~! I'm working on an intercooler problem and I need some help here!- -;; Well, I know the total heat transfer rate q[Watt] so I calculated heat flux[W/m^2] and input it in wall boundary condition for fins[long slim tubes where heat transfer actually occurs]. I don't know the porosity of the fin so I can't solve it as porous media and since I don't know the heat transfer coeifficient so I can not use "fixed" option in wall boundary condition panel. The problem is that after I've solved it, there is minus temperature regions in the fin which has too ridiculously low temperature The inlet temp. is about 443K(170 celsius) and outlet temp. 333K(60 celsius) So this is about 100K temperature drop!(and this fits the test result!) But some end regions of the fin(especially at the surface of the fin) the temp. is about 23K(-250 celsius __!!) I don't know why this happened and I need your help! Please advice me about using heat flux condition in wall boundaries! I think I did somethin wrong there and also the grid(which I didn't checked out the y+ values yet~!!)

Thank you for advance.
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Old   August 13, 2003, 10:47
Default Re: Need advice for intercooler problem
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cjtune
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You have too many unknowns...

Are you simulating the performance of the intercooler itself, or the intercooler when in some environment (ie. under your car hood)?

If you're focusing on the heat exchanger itself, then you have to mesh every single fin, tube, etc. but usually you can shortcut by just simulating one repeating 'chunk' and then assume that the heat transfer correlation (Nusselt number) and head loss you've got for that chunk applies to the entire exchanger too. This will obviously be a conjugate HT simulation.

If you're trying to find out how it performs in its environment, you still have to know the HT and head loss correlation, either through measurements, a prior CFD sim done on the intercooler (as mentioned above), or come up with correlations from some textbook if the geometry's simple enough. Here, the intercooler is just represented by a two blocks of porous media cells. One block for each of the two fluid streams (inside and outside of intercooler). You'll need to couple them (for the exchange of enthalpy) with user subroutines. See http://www.adapco-online.com/user_ar...con/index.html.

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Old   August 13, 2003, 20:21
Default Re: Need advice for intercooler problem
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joon
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Thank you for your details. Yes, I realize now that I have too many unknowns *^^*

I am solving intercooler itself only(not in some environment)becouse I am only interested in pressure drop and temperature drop of the charged air(from turbo charger) after it passes through an intercooler.

I've meshed every single tubes but neglect the fins inside and between the tubes. So I not simulating the tube's flow resistence. (not using the porosity factor!) And I am negelecting the ambient air velocity

I want to simplified the flow as an internal flow which passes through an intercooler geometry in still ambient air! So it will be air(ambient) to air(charged air) heat transfer simulation only.

The one way I've tried is to assume h(Heat Transfer Coeifficient) and put it as resistence factor in the wall boundary condition with ambient temperature. I tried repeatly assuming different h, until I get the desired temperature drop.

But since I know the heat transfer rate q[Watt] of the intercooler I tried using "flux" condition. And result turned out to be somewhat different than I've expected.(the problems I've metioned before)

Now, is it wrong to simulate this way? or is how should I use "flux" condition in wall boudary condition in Star-CD. Please remember it is simpified the probelm due to lack of test datas!
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Old   August 13, 2003, 20:55
Default Re: Need advice for intercooler problem
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cjtune
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Problem is, h will depend on flow conditions (natural convection in your case) and that'll strongly depend on the fins but since you neglected them... h is not necessary constant throughout the intercooler and neither is flux.
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Old   August 13, 2003, 21:18
Default Re: Need advice for intercooler problem
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joon
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hmm...what if I would only consider the heat transfer through the tubes....? Since I am only considering inner flows... There are 27 tubes in my case, I gave the conditions as I written before on the surface of those tubes. (These tubes are thin long pipe things that is in between inlet and outlet and originally contains louvred fins inside and between each tubes) As you mentioned fins are main source of the heat transfer, but let's guess all heat transfer occurs between tube surface and ambient air.

Thank you
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Old   September 14, 2003, 00:06
Default Re: Need advice for intercooler problem
  #6
azmir
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If measuring the pressure drop between the inlet and outlet of the main tubes after cooled down by the 26 fins, why are you concerned so much with simulating the heat transfer? I would just model inlet BC with right temperature at inlet, an outlet BC with right temperature at outlet (your expected test data), and just check for pressure drop in post data.
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