thanks Ayurzana & Amir.

This is a classical problem and there are many papers published on the topic because the problem also used as a benchmark problem. As mentioned above, if you are using non-dimensional problem, then set the temperature to 1 on the heated wall and zero on the cold wall, other connecting walls should be adiabatic, dT/dn=0. 

Definitely, the answer depends on the software that you use., ANSYS Fluent or your own code? Besides the physical shape that you want to simulate.!

Hi Nimesh 

I think the natural convection can be added but not as a boundary condition. Firstly, you have a wall and you should add the boundary conditions for the wall like constant heat or constant temperature. However, the natural convection work when you change the properties of the density of the fluid . So , you should choose "Boussinesq approximation"  for the density. 

Hi Nimesh,

I am not sure that it is possible to answer your question without knowing what problem you are trying to solve. Do you have a description of it?

Hi Nimash . It depand to your physical problem ,for wall  you can use boudary condition like constant heat flux or temperature.

Here i attach a problem definition.....There is one rectangular computational domain in which one heated object is placed.....Which boundary condition should apply for computational domain and object...

Unfortunately, I can see u put a forced air velocity which means not natural convection. Also, you have to mention the third dimension of the air duct. 

However, for this problem without the natural convection you should set up the source of heat as wall and heat flux or heat generation. and you should set up the inlet wall of the  fluid as inlet velocity then set up the velocity number and the temperature. 

Regards 

Hasan Alasadi 

Leeds university , the UK

Here i attach a problem definition in which i want to do natural convection. so, which boundary condition is suitable. Also i am working in 2-D problem. 

Hi Nimesh,

I am afraid that what you have sketched is still ambiguous. How is the natural convection caused? Is the central object held at a uniform temperature? Or does it have some form of internal heating? Or is it heated by an external radiation source? Is the outer boundary which you have sketched  a physical boundary or a computational boundary? Is it cold, insulated, held at a constant heat flux? What application do you have in mind? Is there a fluid between the two rectangles where the inner one represents a solid, or is the fluid contained within the inner one and the space between the two rectangles a solid? There is nothing in your sketch to tell us!

It is essential to be absolutely clear in your own mind what it is that you are trying to do, and only then to communicate to others if you need any assistance or advice. At the moment I still have no idea!

See this video and pay attention for minute 11 about changing the properties of the density.

https://www.youtube.com/watch?v=ThJ3VYhTwMI