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Nusselt number calculation in comsol 4.2

Posted 26 apr 2013, 04:56 GMT-4 Version 4.2 4 Replies

Sin Kim
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Dear All,

I simulated a simple laminar flow of a liquid metal inside a 1 inch tube of 0.5m length with a constant heat flux acting at its wall in comsol 4.2. I used both the mesh options i.e. the automatic mesh option based on the physics controlled mesh featured in v4.2 and also the manual mesh option, however in both cases the temperature at the wall comes out to be less than the bulk liquid temperature at that axial location which is giving a Nusselt number value that does not matches with the theory at all. Can some one identify what is wrong? (Note that in a short tube of 0.5m length the flow always remains developing and therefore the Nusselt number must remain higher than the value of 4.36 (Fully developed laminar flow) at each axial location.


Kindest regards.
4 Replies Last Post 22 apr 2016, 03:59 GMT-4
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 26 apr 2013, 05:32 GMT-4
Hi

which Nusselt number ?

if iths the one predefined in COMSOL then take care that the "Nusselt (or Reynolds) Cell number " is defined in COMSOL with respect to the average mesh size "h" and not with respect to the "model geometrical characteristic Length", that COMSOL cannot anyhow guess what is

So these number should be considered w.r.t. the mesh size for mesh density analysis, not for the model analysis.

define your estimate of L and plug in the formula in a variable or a derived value and you will see it gets closer the the "book" values

--
Good luck
Ivar
Hi which Nusselt number ? if iths the one predefined in COMSOL then take care that the "Nusselt (or Reynolds) Cell number " is defined in COMSOL with respect to the average mesh size "h" and not with respect to the "model geometrical characteristic Length", that COMSOL cannot anyhow guess what is So these number should be considered w.r.t. the mesh size for mesh density analysis, not for the model analysis. define your estimate of L and plug in the formula in a variable or a derived value and you will see it gets closer the the "book" values -- Good luck Ivar
Sin Kim
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Posted: 1 decade ago 26 apr 2013, 11:11 GMT-4
Thanks Ivar for your reply. Actually I am interested in finding values of the local Nusselt number at the tube wall and it is defined as Nu= (q*D)/(k*(Tw-Tb)) where Tb is calculated by the formula [Int(T*w*r*rho dr)/Int(w*r*rho dr)]. This calculation is standard and I have already calculated the correct Nu with these formulas using FLUENT and STARCD. I cannot understand why COMSOL is giving wrong results.

q = wall heat flux (uniform)
Tw = temperature at the wall
Tb= bulk liquid temperature
D= diameter of the tube
k= thermal conductivity
w= stream wise velocity component
r= radial coordinate
T=temperature of the fluid
rho=density of the liquid.

Kind regards





Thanks Ivar for your reply. Actually I am interested in finding values of the local Nusselt number at the tube wall and it is defined as Nu= (q*D)/(k*(Tw-Tb)) where Tb is calculated by the formula [Int(T*w*r*rho dr)/Int(w*r*rho dr)]. This calculation is standard and I have already calculated the correct Nu with these formulas using FLUENT and STARCD. I cannot understand why COMSOL is giving wrong results. q = wall heat flux (uniform) Tw = temperature at the wall Tb= bulk liquid temperature D= diameter of the tube k= thermal conductivity w= stream wise velocity component r= radial coordinate T=temperature of the fluid rho=density of the liquid. Kind regards
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 26 apr 2013, 14:04 GMT-4
Hi

I agree they should give similar results ;)

could it be that you are in 2D-Axi and that your surface section integration is set to volumetric (implicit 2*pi*r multiplication) such that the "r" in your integration formula is dedoubled ?

or that the wall heat flux value units are different and that another 2*pi*r might be mising ?

So far I have not really catched COMSOl from being that different, so I rather trust my results, but on the other hand I have no longer other software to compare with

--
Good luck
Ivar
Hi I agree they should give similar results ;) could it be that you are in 2D-Axi and that your surface section integration is set to volumetric (implicit 2*pi*r multiplication) such that the "r" in your integration formula is dedoubled ? or that the wall heat flux value units are different and that another 2*pi*r might be mising ? So far I have not really catched COMSOl from being that different, so I rather trust my results, but on the other hand I have no longer other software to compare with -- Good luck Ivar
Farah Souayfane
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Posted: 8 years ago 22 apr 2016, 03:59 GMT-4
Hi

I have a question about Nusselt Number.
actually I am confused.
does the local Nusselt number increase with the vertical height (H) or decrease??


Thank you.
Hi I have a question about Nusselt Number. actually I am confused. does the local Nusselt number increase with the vertical height (H) or decrease?? Thank you.

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