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WALL HEAT TRANSFER COEF...AGAIN

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Old   July 5, 2005, 07:23
Default WALL HEAT TRANSFER COEF...AGAIN
  #1
Carl
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Hi all, I have read some discussions on the way CFX calculates the wall heat transfer coefficient (April 21, 2005, Glenn, MNHK), but any other idea is certainly welcomed: I ran a rotating radially channeled disc subjected to 373K and using PBC. I got some plots of the 'wall heat transfer coefficient', which are higher than the some experimental readings I was given. Then, in order to 'confirm' these values (given by CFX) I plotted the 'near wall adjacent temperature' and the 'wall heat flux' and I selected a region and made some calculations using:

q_w=h_c*(T_w - T_nw),

where T_w is my boundary condition and q_w, T_nw are the ones given by CFXpost and h_c is the "unkown". I got results 'higher' than those plotted in 'wall heat transfer coefficient'!!..?? So, I went to books and used the classical relation:

h_c=k_air*(T_w-T_nw)/((dn)(T_w-T_infinity))

I used the a guessed (dorfmann, 1963) boundary layer (dn) thickness for a rotating disk, and I got a lower h_c but that is closer to the experimental values.... so, in this equation the key value is 'dn'...

So I am looking for the formulation used in CFX to obtain this value... I can not find it. All I found is that is follows a paper from Kader B A, 'Temperature and concentration profiles in fully turbulent boundary layers'...Intl J Heat Mass Transf, 1981. I am going to read it... but in the mean time, any further thought is welcomed...

Carl
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Old   July 5, 2005, 18:51
Default Re: WALL HEAT TRANSFER COEF...AGAIN
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Glenn Horrocks
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Hi,

If you are asking how CFX calculates heat transfer at walls, for laminar flows it is by simple conduction, and for turbulent flows it is by turbulent wall boundary conditions (wall functions or integration to the wall). The turbulent wall boundary conditions are discussed in the documentation.

CFX calculates h from the calculated wall heat transfer. This parameter is based on an "ambient" temperature, and the choice of an ambient temperature is not always obvious. Hence CFX uses a near wall temperature. However most analysis on heat transfer coefficients use a far field or supply temperature, so to calculate h based on these temperatures and the calculated wall heat flux.

Also note CFX10 will allow you to define an arbitrary ambient temperature to calculate wall heat transfer coefficients against. This should simplfy things a bit.

Glenn Horrocks
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Old   July 8, 2005, 01:35
Default Re: WALL HEAT TRANSFER COEF...AGAIN
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Sandra
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Sometimes the parameter is not based on an "ambient temperature". For internal flows, a bulk temperature must be found.
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