thaks for reply,
Actually the tank is filled with ethanol and water is passing through the helical tube.
The temperature of inlet water is 20° and ethanol temperature is 5°C. i need to check when the outlet temperarure of water will be 7°C.
For this one I think ( I am not sure ) 2-D axisymmetric will be better to analyze. When I checked the picture that I shared before I can see many circles .So I can define first circle as inlet and the last one as otutley. The issue is I dont know to how connect other ones to eact other.