Finding heat transfer coefficient "h" at tube bundle surface -help!-
 

Hello guys!

I am trying to find the "h" coefficient of a tube bundle in a duct. You can see the geometry as attached.

Problem conditions are:
Tubes are at 90C constant.
Fluid is water at 10C with 0,5 m/s inlet.

The steps I followed on FLUENT are:

1- Models: energy equation, k-omega standart & enhanced wall treatment
2- Materials: conditions of water at 10C
3- Cell zone conditions: changed to water as described above
4- Boundary conditions: Inlet 0,5 m/s at 283 K with 1% turbulent intensity and hyraulic dia. of the duct
Outlet is outflow
Tubes are walls at 363 K constant
Duct walls are walls and insulated, thus no heat flux
5- Reference Values: Compute from "inlet"
6- Monitors: Area-weighted average & surface heat transf. coeff. of tubes
7- Solution Initialization: standard initialization compute from "inlet"

After 1000 iterations, I adapt boundary and region twice and run another 1000 iterations, and I get these values:

h = ~7300 w/m2K
Nu = 11242
dT= 2,75K

But if I do a manual calculation according to Zhukauskas' equation for staggered heat exchangers this is what I get: (please see Empirical Calculation.pdf)

h = 11388 w/m2K
Nu= 290,3
dT= 4,5 K

Why I get that different solutions? I am new at CFD and I tried a lot of different ways in FLUENT such as different meshing, different turbulent intensity or models but i still don't get what I should be getting.

Can you please help me where I am doing wrong?

Thank you in advance,
bluejojoba

anyone guys? I am really desperate.