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Fully developed temperature profile for laminar/turbulent flow in FLUENT

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Old   April 16, 2012, 07:49
Default Fully developed temperature profile for laminar/turbulent flow in FLUENT
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Hello FLUENT users,

I am new to heat transfer and am trying to simulate a simple case in FLUENT. The flow is between 2 parallel plates (so it is an internal flow). My task is to analyse the HTC (heat transfer coefficient) against the first cell height. So, basically it means that I need to check what mesh is best suited.
I am trying to analyse both laminar and turbulent cases (only with komegaSST model). I use FLUENT 6.4.

In laminar case (and also in turbulent), I have a problem. The velocity Profile is fully developed but thermal profile according to

Le = 0,034*Re*Dh*Pr (from Lienhard and Lienhard (2008))

is never achieved. My geometrie includes a 5m length of the plates and a distance of 0,05m between them. So Dh = 0,1m

1) For the laminar case, I have analytical solutions with me. The results from simulations seem to match the results from analytical solutions with an error of 3% (this happens even though the temperature profile is not developed!!!!). Could some1 explain the reason for this?
P S: However, I use the T_bulk as the reference temperature for all calculations in laminar case. Even if I consider the T_centreline, I get results with 20% error. This I understand is pretty normal.

2) For the turbulent case, I have analytical solutions too. But here, the velocity profile and the thermal profile dont seem to develop at all while infact I should be able to achieve the fully developed profile earlier than the laminar case due to turbulence.
Could any1 clarify if I am performing the simulations correctly?

Thanks in advance.......
Raghu
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Old   April 16, 2012, 14:18
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What are the boundary condition u r applying.
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Old   April 17, 2012, 02:35
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Thanks for the reply.

Initially I simulate with the working fluid as air and then I also do simulation with water as the working fluid.

I apply a velocity inlet of 1 m/s and an inlet temperature of 283K.
outlet is a pressure outlet with 0Pa
wall is a temperature of 10 W/m2-K and 293K (I simulate both the constant wall temp and constant heat flux case.)

I also feel there maybe some differences because of the geometry.
I have a geometry of 3m length, 0.05m height and a small width of 0.001m. I assume this assumption of 0.001m as the z-coordinate serves the purpose of a simulating a 2D flow. Basically I would like to simulate a 2D flow. (This is because I need to migrate to OpenFOAM if I am sure about the results and the setup)

I can attach a sample .cas if necessary or send it to your email directly (the file size may be a little too large to upload in this forum)
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Old   April 17, 2012, 03:46
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Update:

I understand that I have not enough length of the domian to obtain a fully developed profile for the turbulent case.
TO obtain this, should I first develop the velocity profile and then run the energy equation or is it possible to develop them simulataneously in FLUENT?
The entrance length according to my calculation is ~16.6m.
I assume its best to write a UDF.
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Old   April 17, 2012, 08:09
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Geometry is fine if u are trying 2D simulation but make sure that there is only one cell in z-direction and apply symmetry boundary condition to z-direction boundarys.

U can check the range of Z-Velocity after simulation. it should be in range of 10^-10 to be compatiable with 2D result.

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Originally Posted by raghu.tejaswi View Post
Thanks for the reply.

Initially I simulate with the working fluid as air and then I also do simulation with water as the working fluid.

I apply a velocity inlet of 1 m/s and an inlet temperature of 283K.
outlet is a pressure outlet with 0Pa
wall is a temperature of 10 W/m2-K and 293K (I simulate both the constant wall temp and constant heat flux case.)

I also feel there maybe some differences because of the geometry.
I have a geometry of 3m length, 0.05m height and a small width of 0.001m. I assume this assumption of 0.001m as the z-coordinate serves the purpose of a simulating a 2D flow. Basically I would like to simulate a 2D flow. (This is because I need to migrate to OpenFOAM if I am sure about the results and the setup)

I can attach a sample .cas if necessary or send it to your email directly (the file size may be a little too large to upload in this forum)
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Old   April 17, 2012, 08:10
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Yes we can solve both energy and momentum eq. simultaneously.
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Old   April 18, 2012, 08:34
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hi Banty,

Thanks for ur instant reply.
a simple Q, according to calculations in laminar flow, laminar entrance length for velocity as mentioned in the first post.

Is there a mesh-dependency on fully developed velocity profile? Here one has to understand that I am uisng a simultaneously developing flow and not inputting a fully developed velocity profile at the inlet.

The above Q s coz, for a 3m length of the pipe for air as working fluid and v = 0.1m/s, I'm not able to achieve a fully developed velocity profile even at the outlet. the profile is still varying. But for a very fine mesh and a first cell about 0.1mm, I get the velocity profile developed.

has mesh got to do with development of velocity profile. according to my understanding, NO.
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Old   April 18, 2012, 09:02
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Hi,
entrance length does not depend upon the mesh physically but numerical error certainly does. so for any simulation, grid independence should be checked.
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Old   April 25, 2012, 03:45
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Hi banty,

I just happened to find the solution to achieve a full developed temperature profile in FLUENT...
Use Periodic boundary conditions from FLUENT. Since I'm simulating incompressible flow, its easy for me......

Now I have a new Q,
In fully developed turbulent flow, when I use air as the working fluid, everything is perfekt, but when I use water as the working fluid (all other settings remaining same),
1) for constant wall temperature: the solution is diverging
2) for constant heat flux: Im getting unphysical solutions, heat transfer coefficient is too high than analytical solutions.

This problem is due to the case setup , or do I need to have a better boundary layer to simulate higher density fluid?

Raghu
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Old   April 25, 2012, 03:48
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hi banty,

I have done grid independence check using GCI method. But I now need to know for what values of y+ do i get a good result. My case is a simple 2D one and this proves as a basis for further complex geometries..... But understanding this is utmost priority...

Thanks
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Old   November 7, 2012, 10:57
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Hi
I need to reach fully develop flow in microchannel(D=100 micrometer,L=1mm)
q=500,Re=100 to 1000,Tin=300,working fluid is water.
i can not reach thermal fully developed,i don't know why even when i apply Le=.05 Re D Pr
would you please show me a solution?
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