model of pipes inside pipe
 

I 'm modeling by GAMBIT_FLUENT a system that consists of: two vertical steel pipes (D=0.08m L=1000m)(iside the pipes happens a flow of hot gas); another pipe (D=0.2m L=1000m) contains the two pipes and is full of water;

I can use a 2D model (vertical section)? In this way I lose some informations? THANK YOU!