[CFDJonas]

Hello CFD Online!

I'm currently trying to model a quite simple problem: a double-piped (laminar) flow. For this, I created a 2D axisymmetric rectangular mesh to model the inside of a normal pipe. The pipe or tube is quite long and has a small diameter (very high L/d ratio).

The annular gap of the double pipe is used for heating. One assumption is, that the heating stream is isotherm and the heat transfer coefficient along the tube axis is constant. Thus, for the modeled 2D mesh I chose for the thermal boundary condition of the wall "convective" and entered the constant heat transfer coefficient and the temperature of the heating agent.

Now to my problem regarding this model: By calculating the reynolds number I am definetely in the laminar regime (around 1200). Therefore my idea was to use the laminar viscosity model. However, this leads into divergence quite fast.

On the other hand, when I use the k-omega model (standard or SST) the simulation converges quite easily.

Do you have an explanation for this observation? Is the idea of choosing the laminar model not suitable when I introduce radial temperature gradients? Can I believe the results of the k-omega model or should I use another turbulence model for this kind of situation?

Thanks in advance!

Edit:
Added a sketch of the model situtation in the attachment (the length of the pipe is in reality much longer):

Edit2:
Added a (segment of) contour plot of the temperature... A diverged simulation with the laminar viscosity model is shown. The temperature inhibits these oscillating "regions" as seen in the picture.