fan modelling in star-CCM+
 

How to create the fan interface, for a simple outlet fan in electronic cooling case.
I tried to model using momentum source, but it asked for the upstream and downstream boundaries. How to create these?
Secondly, I created fan interface from a boundary and tried to mesh, but it said those two interfaces are already connected

Sounds like you have not read the documentation on fan modelling via momentum source - there are two options - a 'fan interface' which is a fan on an interface between two regions, or a fan as a separate region which is interfaced at the in and out faces of that region to other model region(s). Maybe you need to learn about multiple regions and interfacing them first.

hwo to get exact cfm for a fan
 

Yeah, I have learnt about the interfaces but now, I am stuck in judging the exact cfm the fan should work. How to give this in momentum source or fan interface. It may be possible from fan curve, but that is for a range of values not for a single value. I want to see the difference the fan makes for different values of mass flow rates with different simulations for each.

I imagine you can input a simplistic fan curve in - eg a straight line polynomial with mass flow constant for a wide range of pressures - very easy if this works, but I have not tested it.
But there is a much simpler way now you have explained yourself - add the interface, then set the type to Fully Developed, then have the option to specify the mass flow or pressure jump across that interface.