Dear Sherif Ahmed Fouad Yehya,

If you applied temperature in hollow cylinders, then you won't see any temperature gradient in your simulation, because that temperature will be constant throughout the analysis and doesn't create temperature gradient. The best option as you already thought is performing a single wall heat exchanger simulation in fluent. This way you can simulate it properly.

Hope this answer helps.

Dear

You could use Ansys fluent with transeint option.

Ansys fluent with unsteady option