Fully developed temperature profile for laminar/turbulent flow in FLUENT
 

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

What are the boundary condition u r applying.

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)

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.

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.

Yes we can solve both energy and momentum eq. simultaneously.

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.

Hi,
entrance length does not depend upon the mesh physically but numerical error certainly does. so for any simulation, grid independence should be checked.

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

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

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?