Open boundary condition

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Hi i am dealing with a problem having conjugate heat transfer solid domain, which was placed in atmosphere.....so that it gets cooled naturally through free air

So what would be the best suitable boundary conditions..hence it would come under Natural Convection plz suggest the realistic BC's

Secondly my doubt was what would be best suited to define as input heat source -- whether heat flux or heat generation value -

plz kindly suggest

thanks in advance