Learning Forum

[ricardo]

Hi all, 

So I have this enclosed box (without an inlet or outlet). It has a heat source inside (like a coil used in an oven). Then I put a small fan inside to distribute the heat from the heat source.

I was wondering why the calculation won't converge. I followed many tutorials on YouTube regarding the use of fans and heat distribution. But all of them are using inlets and outlets, not in an enclosed space.

Here is my geometry:

It has 4 solid:

1. Fan exterior - just a casing for the fan
2. Volume - the fluid volume after I boolean all the geometry inside
3. Fan blades - the rotating fan, but since we only need fluid, I suppressed this object 
4. Fan fluid - the rotating fluid volume after I boolean it with the fan blades

As for the meshing, I don't think there is an issue. I put body sizing around 25mm for the exterior wall (it is around 1mx1m box). Then around 2mm mesh size for the fan.

Then I named the selection as follows:

In the fluent setup, I put these as setting:

Energy on, viscous realizable- k epsilon

In the cell zone, I rotate the fan fluid volume to simulate the rotating fan with this setting in cell zone fluid:

Then there are so many objects in the boundary condition which kind of confusing for me.

What I am doing is that I modify the boundary condition of the wall- fan fluid. I did similar stuff but with cell zone condition. According to a tutorial on YouTube, I did the same thing with the boundary condition of the fan fluid with this setting:

Another setting are:

1. Solution methods changed to second order upwind
2. General setup use transient (but I also tried steady state, both are not converged)

After I do the initialization and run the calculation, it won't converge

Can somebody suggest what steps I should try or steps that I missed?

Really appreciate it

 

Thank you

 

Regards,

Ricardo

[SRP]

Hi,

Since it is a closed system, did you monitor any other parameter to check for the convergence?

Thank you

[ricardo]

Hi,

Thank you for your reply!

No I did not check other parameters for convergence.

Since I am pretty new to CFD, I was just following tutorials where they only looked at the residual graph.

If I tried the steady state simulation, the residual looks like this:

Still way above the threshold of 0.001

If I do the transient, it looks like this:

Interestingly, all the tutorials that I watched use inlet and outlet, not a closed system like this. So I am not sure if I am doing it correctly.

Do you think a closed system will have issues in reaching convergence? I read somewhere that in transient simulation (since it is more difficult to reach convergence compared to steady state) we need to play with the inlet value so it reaches convergence. The issue is that I do not have an inlet :(

Thank you

 

Regards,

Ricardo