side by side heat exchanger
 

Hello
I have to model a side by side heat exchanger. The inner fluid is geothermal water (inlet temperature= 70°C and outlet temperature=40°C). the outer fluid is cold air circulating inside a circular collector. The heat exchanger is constructed
of side by side pipes (diameter = 20 cm) placed at the surface of the soil. I have constructed a 2D geometry but the problem is how to mention to Fluent that the water enters every pipe and quits the other side after increasing the temperature of the air. What type of inlet boundary I have to chose to These pipes? Did I have to calculate the flow rate for enery pipe and put it as boundary? Did I have to write a program please help me?