Tube Bundle
 

I've simulated an in-line tube bundle in CFX, with a total of 12 rows. I need to calculate the average Nusselt number in a particular cylinder, and to do so I will use this equation Nu=(q*D)/(kf(Ts-T∞)). The problem is that in this type of flow the choice of free stream temperature is not obvious. I've reviewed the literature and I couldn't find anything addressing this situation. What should I use as the free stream temperature?

Regards, Ari Guerrero.