I tried to simulate a counterflow heat exchanger, a 2D domain (axisymetric condition), including pipe walls.

The idea is like this: Heat transfer between fluids involving heat conduction through the pipe wall that separate them, and, for the fluids temperature difference, the heat transfer take place. I want to estimate Nusselts number, so, i don't want to stablish any convective coefficient condition, wall temperature or heat flow condition. I don't know if this is possible.

The model consists in having a section where heat transfer between fluids is zero (where the thermal conductivity for pipe wall is 0), letting the velocity profile completely develop, and then, other section where heat transfer will take place.

So, first i simulate just the fluid flow, and I get what i want, counterflow, then, i turned on the energy equation, and every time i click the "calculate" button, i get this error "Divergence detected in AMG solver: temperature Error at host: floating point exception". I don't know if i have to stablish some special contact conditions between the fluid and the pipe wall, or something like that.

I'm attaching a ZIP file with some images: 1) Quick look of the mesh, 2) Velocity vectors of first simulation (Only fluid flow), 3) Schematics of the physical model.

Please, any help will be greatful.