I've been working on "Natural Convection Between two horizontal parallel plates" for some time. but i've several problems on my project. is there anyone who can help me and who worked on such a project before. i've fixed temp. boundary condition on plates and Pressure outlet boundary condition for the side boundaries. are they true or am i wrong on this ?

Two fixed temp plates and pressure outlets sounds right. You probebly need to be more specific about the problems you refer to.

This project was actually done experimentally before. Now, I want to re-calculate the Nusselt number that was calculated experimentally and make a comparison. (In the experiment bottom plate was heated uniformly with a resistance and the top plate was insulated from above. After the experiment both the bottom wall temperature, top plate temperature and surrounding temperature was measured as they became stable. So I want to use these temperature values as my boundary conditions and calculate Nusselt number. But I'm not sure if I'm right or wrong to select these boundary conditions? I think I must get the same result of the experimental one. ) - From the experiment, Known data values listed below.

1) Distance between parallel plates: for example 6 mm

2) Bottom Plate: I use wall boundary condition with constant temperature. (Do I have to use shell conduction? I 've confusion on this point) Tbot = 326,5 K (Experimental data)

Top Plate : I use a wall boundary condition with constant temperature. (Do I have to use shell conduction? I 've confusion on this point) Ttop = 315,3 K (Experimental data)

Surrounding temperature : T∞ = 289,8 K (Experimental data)

QUESTIONS: Question : I use Boussinesq approximation and I don't know if I'm right or wrong ?

Question : I use Tfilm temperature for the fluid ( air ) properties ; where Tfilm=(Tbot + T&infin / 2 is it correct?

Question : I enter "Tref =Surrounding Temperature" is it correct ?

Question : I use 20*30*30 (NY*NX*NZ) grids for the numerical analysis is it enough ?

Question : Are my boundary conditions correct for this experimental apparatus?

PS: if anyone wants to help me i can send my mesh and case files to his/her private mail.i'll be glad if this could happen.

There are several things you need to consider.

1. Are the experimental measurements such that you know the plates are isothermal? If there is a large gradient on the plates the results will be affected.

2. Boussinesq should be OK, as your temperature range is pretty small.

3. The film temp should be OK.

4. Make sure you also set the side boundaries to the surrounding temperature.

5. There is no way to know if your mesh is good enough ahead of time, and it depends on other factors like if you are using turbulence and, if so, which wall approach you use.

6. If the plates are not actually closed to make a closed cavity, you may need to model some portion of the surrounding air.