So what I have is a counterflow heat exchanger, with water being continuously cooled down with an ethanol flow (where the ethanol is at constant temperature -2). I would like to simulate transiently, so in the end I can monitor specific positions in the water channel with respect to time. So for example, the channel is 1m, and at x=0.9 m, what is the temperature every lets say 16s. So for this, I would need the water to recirculate from the outlet (after it has been cooled by the ethanol channel) to flow back into the inlet, so the inlet temperature would be constantly decreasing. nI have looked into UDFs, but I have no idea where to start with recirculation boundary condition since I don't have a specific delta T to apply to apply from the outlet to the inlet or a constant heat flux to applynnn