coupled wall or ...
 

i designed shell and tube heat exchanger for cooling high pressure natural gas.
there are hot gas and cold fluid in this system.
natural gas goes in steel tube and there is a cooling outside of the tube.i define interface between outer and inner side of the tube.i define coupled wall in interfaces.the problem is heat transfer occurs during the process but i didnt define any heat transfer coeffcient.there is an option to define heat transfer coefficient for tube body. what are the differences between them?

other question is can i define different heat transfer coefficient inner and outer side of the tube?

I have a hard time following what is going on here... but the answer is straightforward.

If you solve the fluid flow problem, the heat transfer coefficient is a flow property and will be obtained from the flow. Hence, you do not specify a heat transfer coefficient there. On these boundaries there should be some sort of fluid|solid interface and the boundary condition should be coupled. Note that you can still select a convection boundary condition even on an interface even though in reality it doesn't really make sense. Funny things will happen, it is an expert option.

You only need to specify the heat transfer coefficient as boundary conditions on boundaries where you are not solving the fluid flow problem. On these boundaries, there will not be any interface and you will not be able to select the coupled condition at all because it's not coupled to anything.