## Fluids

#### How could I know that the Convergence in a conjugate heat transfer problem is reached?

• Mohamed Abdulazim
Subscriber

Hello everybody,

I am trying to solve a heat transfer problem(conjugate heat transfer )by the fluent, which is a heat transfer enhancement in a rectangular channel with baffles. I left the computer solving the problem for about 24 hours and it performed about 4774 iterations. I need to know whether I have reached to the convergence and the results are reliable or not. The residuals for the equations reached to the values as they appear in the captured photos of the screen and the value of the net mass transfer rate is 4.0530945e-06 and the net heat transfer rate is -0.0058049306Watt. Also, I used monitors for variables such as a mass-weighted average at the outlet temperature, Area -weighted average for lower plate surface heat transfer coefficient and the Area -weighted average of all heat fluxes from all entities, but I am confused about the monitor for the surface heat transfer coefficient as the curve fluctuated up and down as shown in the figure. Is this enough for convergence knowing that the under relaxation factors(URFs) are the default values?

Some detailed information about my model: The problem is a rectangular channel with a 1 m length and there is heating by constant heat flux from the above and lower plates in the channel and there are solid baffles in a staggered arrangement on the lower and upper plates. I used SIMPLE discretisation scheme with second-order upwind and used the default under relaxation factors. The turbulence model is k-epsilon(RNG) equations with enhanced wall treatment and the air properties are constant. Thanks in advance.

• peteroznewman
Subscriber

• Mohamed Abdulazim
Subscriber

Hello,peteroznewman

Thank you very much for your kind response.

• Mohamed Abdulazim
Subscriber

• Mohamed Abdulazim
Subscriber

• Rob
Ansys Employee

First off update to 2020R1, R17 is about 4 years old now and the technology has moved on.

Secondly, have a look at the flow field, and carefully explain what's going on (with pictures).  Focus on things like shedding and recirculation zones.

• Mohamed Abdulazim
Subscriber
Thank you very much for your insightful response, rwoolhou.I really need to upgrade Ansys program, but unfortunately this requires the installation of Windows 10 as I have Windows 8.1,I am going to work on that. You have mentioned that I should have a look at the flow field with focusing on circulation and shedding zones,do you need contours of flow field to understand the nature of the flow or advice me to refine mesh in these zones.
• Rob
Ansys Employee

I think I know what's happening, but you'll need to provide evidence: contours and vectors will be needed. The mesh is probably OK, and mesh refinement studies can be done to confirm this.

• Mohamed Abdulazim
Subscriber

• Rob
Ansys Employee

Yes, now you've done that what's happening in the flow? Why would the steady solver struggle with convergence?