This is not a counter-current heat exchanger. It is two phases, Gas and Water, flow in one flow path but in opposite direction.
I have found many source doing analysis on flow pattern of the counter-current flow but usually stops at pressure drop topic. No one goes to Reynolds Number level, let alone the heat transfer coefficient.
I suppose that the key of the problem is to find a proper velocity instead of Vl or Vg, to bring into Reynolds Number. But different flow pattern comes with different velocity.
The problem is two pipes centered each other, gas and water flow in annulus and water flows back through the center pipe. I am trying to simulate the temperature profile in both spaces.