Solar steam generator
 

Hello All
I'm trying to design a solar steam generator, that is a solar collector directing the radiation to a 2cm diameter, long copper tube having water inlet and a proposed steam from the outlet. I'm only simulating a tube having a solar radiation flux incident on its outer surface.
My questions are:
1- is this a steady or unsteady state simulation
2- any advice of what models should be chosen in water like two phases water and vapor etc.

Thanks in advance