Convective heat transfer coefficient

avb

I have model of a box with air inside it at 300K . Outside box temperature of ambient is 273K. In order to model natural convection inside the box, do I need to define convective heat transfer coefficient(h) inside surface of the box? 

Also how should I define the initial temp of 300K for air in AcuConsole.

acupro

You can use the tutorial ACU-T: 3000 / SL-2100 - Enclosed Hot Cylinder: Natural Convection as a starting point.

You will mesh the air volume inside the box, typically use the Boussinesq density model, and apply the gravity body force.  You can define the initial condition for temperature and other quantities.  Quite possible this is a transient flow rather than steady state, but you can try steady first.