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why the value of wall heat flux is negative?

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Old   September 29, 2006, 04:10
Default why the value of wall heat flux is negative?
  #1
prayskyer
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hi:

i do some conjugate heat transfer(CHT) computations, the results turn out that the value of "wall heat flux" is negative on the face between flow and solid field,and the "wall heat transfer coefficient" is not what i want.

could anyone help me about why that negative value could happen?

thanks a lot
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Old   September 29, 2006, 04:53
Default Re: why the value of wall heat flux is negative?
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anna
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Hi,

this is normal because if you check the wall heat flux at for example water-solid side the same value will appear and will be positive and you are checking now the wall heat flux at solid-fluid interface. Am I correct? It depends on which direction the heat flux is transfered.

For the heat transfer coefficient - how do you estimate that this is not correct - how did you calculate it, analytical??? It is difficult to calculate it analyticaly if there are some ribs, roughness, etc. Take the avarage value of the coefficient.

Anna
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Old   September 29, 2006, 05:27
Default Re: why the value of wall heat flux is negative?
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prayskyer
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hi: yes,you are right,I realize when the solid seems like a heater,the value is positive;when the solid seems like a cooler,the value is negative,!~but,can i ask if i can change the "+" and "-"???

That wall heat transfer coeffecient,is that impossible to compute specified value?are all the values average ones? In CFX post,it can output "wall heat transfer coeffecient" as well as " wall heat flux ",how these coefficients come out,and how i can get the right ones?

thanks a lot~

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Old   September 29, 2006, 06:53
Default Re: why the value of wall heat flux is negative?
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anna
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Hi,

no, the values are not avarega. You can ask for the maximum, for minimum , or average. If you plot the heaft transfer coeff. you are going to see that it is different at different parts of your model. The heat trasfer coefficient depends on many things: for example: flow velocity, dimensions of your model, flow material data, etc. In order to compare it with CFX results you can calculate this per hand using different correlations if it is in a forced convection regime, if not such calculations are much more complicated. cheers.Anna
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Old   September 29, 2006, 07:23
Default Re: why the value of wall heat flux is negative?
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prayskyer
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the problem is where the wall heat flux should be upmost,turning out the least value,the should be least,turning out most.I think because of wall heat flux wrong value,turn out the wrong heat transfer coefficient.

by the way,could you plz introduce some correlations to me,could I read some heat transfer book,hehe

thanks in advance
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Old   September 29, 2006, 07:32
Default Re: why the value of wall heat flux is negative?
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anna
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yes, it depends on which kind of flow you are considering:

forced convection:

1.Greg Naterer; Heat Transfer in Single and Multi-phase flow, CRC Press. Florida, 2000; 2. F.Incropera and D. DeWitt, Fundamentals of Heat and Mass Transfer, fifth edition, John Wiley&Sons Inc, 2002 ; 3. F.Kreith, Principles of Heat Transfer, third edition, Intext Educ. Publishers, 1973;

two-phase flow (boiling, condensation, etc)

1. John Collier, Convective boiling and condensation, McGraw - Hill Book Company;

I hope this helps

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