Flow Simulation. How to relate inlet and outlet temperatures? (virtual closed loop)

I want to design a passively cooled radiator. I want to know how much heat can be dissipated with water staying within a certain temperature range.

There is a water volume flow inlet at 35C, and a 1500W heating surface after it. And there is a water volume flow outlet at the same flow rate. The inlet and outlet are just boundary conditions placed on the lids; they are not actually physically connected. I got everything set up, except for one thing: How to make the inlet water temperature change to be equal to the outlet temperature? I need this to simulate a closed loop effect for the water flow.

If it's not possible and I need to physically connect the inlet and outlet outside the radiator (creating an actual closed loop), how do you enable a specified volume flow rate for the water?

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