CFX heat transfer coefficient vs theoretical value

[L_K]

I have calculated a heat transfer coefficient for a flow over flat plate according to Incropera's heat transfer book.
In addition I created a model in CFX and calculated the heat transfer coefficient (I used expert parameter tbulk set to
air inlet temperature 20C=293K). The values are 7.9 and 4.1, respectively. Does anyone have an idea why the values differ?

The boundary conditions are following:

Inlet 0.8 m/s and 20C
Outlet 0.8 m/s
Other faces are modelled as adiabatic walls.
100W heat source at the bottom of plate.
For air, default properties (turbulence model etc.) are used.

http://imgur.com/a/90z4G

 

[ione]

First thing

L K,

First thing.
Do you know for sure the correlation used by CFX is the one reported by Incropera?
Using Churchil & Ozoe correlation for constant heat flux case and with your data I got h = 5,4 W/(m2 K).
Take those correlations for what they are, namely best fit of experimental data and there might be variations of about 30% from real specific cases.

Second thing
Air properties should be evaluated at film temperature, but you do not know the plate temperature
So calculate air properties at T=20 °C (air stream temperature) for a first attempt.

Then calculate the plate temperature with:

Tw – Tair = (q*L/k)/[0.6795Re^(1/2)Pr^(1/3)]

q is the heat flux. The other symbols should be quite clear

Now evaluate again properties at film temperature and use one of the correlations available for your case.

 

[L_K]

Thanks for your input.

I changed the size of the air domain in the cfd model and got a better
agreement with literature htc values.

http://imgur.com/a/kYyHB

I am moving next to a lot more complex geometry. It is a laboratory with fans on both ends
moving the air through the structure (similar to test model). The aim of cfd modelling is to
study whether there are local flow effects which might increase or decrease heat transfer in parts of the
structure.

According to experimental tests, the surface temperatures are in the range of 35-45C.