Newbie, temperature not changing in CFD simulation

[Anonymous]

I’m trying to find out what will be the temperature of an aluminum box when it placed in a room without air flow and heat is applied into it.

I set the box parts as aluminum, set the air box, added heat flux to one of the box internal surface.

Changed the solver to both flow and heat transfer, but still the temperature will not change in the model.

What am I’m doing wrong?

cfz file here: http://wikisend.com/download/413534/CFD4_support.cfz

[marwan_azzam]

Hello,

Thanks for sharing the model.

1- Applying a heat flux at internal surface is not good practice.  You can move it to an outside surface or use part-based internal heat generation.

2- There are no flow boundary conditions.  As such, Auto Forced convection should not be checked.  This box is to be checked when fluid flow effects the temperature but not vise versa.

3- If you want the flow to be driven by the temperature gradient (buoyancy), you need to set the Air material environment to Variable not Fixed and set the direction of gravity and definitely turn off Auto Forced Convection.

4- I would start without radiation to make sure all runs well. 

Marwan

[Anonymous]

Thanks Marwan.

I’m still missing something 😞

I've started a new project.

Added a part for Volumetric Boundary with Total Heat Generation of 5.5W

Set the air properties to variable.

Added heat transfer to the solver and gravity to earth & 1,0,0 (no auto forced convection or radiation)

Run to solver and the temperature is stuck on 0!!

What have I missed?

cfz file: http://wikisend.com/download/826548/CFD5_support.cfz

 

[marwan_azzam]

Hello,

You have the analysis set to Steady State. In that case no need to run an analysis.  The temperature will be uniform at a value of infinity.  The software just didn't want to tell you that.

If you set it to Transient, you will see a temperature gradient.

Marwan

[modess.seyednezhad]

So basically, to see the temperature difference we need to set it in transient mode. It is weird!! 

Because I have sort of the same problem. I cannot see the temperature variation even though I have applied all BCs. 

[marwan_azzam]

"So basically, to see the temperature difference we need to set it in transient mode"

No.

Think of it this way:

Heat Transfer calculations will not be performed if the applied boundary conditions will result in a uniform temperature throughout the model.  That would be a trivial solution and hence will not be done.

Such boundary conditions, however, will result in a non-uniform temperature distribution if the analysis is run as Transient.

If analysis type is Steady State, you need to define a set of boundary conditions that will result in a non-uniform temperature distribution (i.e. temperature gradient) in the model.

I hope this clarifies what I said earlier.

Cheers,

Marwan

[modess.seyednezhad]

Thank you, Marwan!

I have defined the temperature and volumetric flow rate boundary conditions to the inlets but unfortunately there is no temperature difference along the channels. Please see the attached file. 

Thank you 

[marwan_azzam]

Hello,

Not sure why you say there is no temperature gradient.

I ran the model as-is and got a temperature gradient as seen in image below.

[modess.seyednezhad]

Marwan , 

I believe on my computer the software has some technical issues!! 

It is so weird to me. 

[modess.seyednezhad]

I have another question. I increased the thermal conductivity and I expect to have a higher temperature drop but it was vice versa. I am not sure what is not working and I do not know how to debug it. 

[marwan_azzam]

Hi,

Are we still talking about the same model?

There are three materials in the model. Which one did you change (higher conductivity)?

Marwan

[modess.seyednezhad]

Yes, it is the same model. I changed the solid material. The fluid is the same. 

[marwan_azzam]

I doubled the thermal conductivity of the solid in all three directions and the outlet temperature did increase.

Not sure why the model behaves differently on your end.

Marwan

[modess.seyednezhad]

I want to change the thermal conductivity and increase it. We expect that the hot outlet decreases because of higher thermal conductivity.