Thermal contact in heat transfer

[huda111]

Hi Abaqus Users

I am modeling a solid wire mesh through which the heat is being transfered through its thickness. I need to show air between voids and enclose it in the air environment for real base heat transfer simulation. I am confused about the thermal contact of air with my material. I have tried gap conductance and TIE command but no results. The heat is not transfered through my material where the contacts are involved. Can anybody help me. please........
thanks in advance
regards
huda

 

[corus]

Air isn't normally modelled as heat transfer across it is usually by convection and radiation. For narrow gaps, however, convection would be negligible and gap conduction and radiation would be used without using a solid region for the air. If you're wanting to only model conduction then a better approach is to create a single part and partition it for the different materials. This will ensure conduction and equal temperatures at the interface. Your tied constraints should have produced similar results but if the mesh differed significantly then the distance from one node to another may have been too great to tie the surfaces together.

 

[huda111]

Dear Corus

Thank you very much for your reply. This was just example I am trying how it looks like. In my real model its very complicated. I have to make a cylindrical wire or cylinder which contains different additives (material and latent heat). So what I did is, I made different parts and asseble them and then translate them. then I created thermal contacts between them for heat transfer. I selected for both parts "the surface to surface contact general". But for air I am confused what to do. So I made a solid part, gave air material properties and then assemble them and translate in the same manner (which i think is not correct way). But all my contacts are not working even in my additive contact surface to surface is not working.
So as you suggested that I should make a part and then partition it, may not be possible.
Can you suggest me what to do. I will really appreciate

Huda

 

[corus]

If all the parts are assembled then you can simply merge all parts into a single part. There is an option to retain the surfaces between the parts. Then it would be a simple matter to assign different materials to all the separate regions within the single part. If there are no gaps anywhere then you'll naturally have thermal contact.

 

[huda111]

Thank you Corus.
I have assembled the part and merge them without air. I got the analysis. But I am still confused about air. One option I am thinking is to make a solid shell insulating box around my whole part and then assign air properties to the voids. My questions are
Is that make any sense?
Should I apply the same technique of merge for air ?
And what properties I should assign for air for transient heat transfer analysis.the same like density, specific heat and conductivity?
your help will be really appreciated.
In my analysis the heat transfer through convection is really not important but still I have to include in analysis. How I can tell abaqus or make assumption about air to be negligible?

Regards
Huda

 

[corus]

If the effects of air are negligible then having no boundary conditions at all will assume no heat loss from the solid surfaces. This would be applicable if the effects of radiation between surfaces were also negligible, and convection to the air was also negligible. In general convection can be ignored in small gaps. In those cases you would use gap conduction and gap radiation between close surfaces, though the effects of conduction across an air gap is negligible.

If you wish to include air as a solid medium then you would need to assign thermal conduction, and also density and specific heat in a thermal transient analysis.

 

[huda111]

Dear corus

I am still confused about the air around the cylinder. I have put my cylindrical wire between two plates having boundary conditions. The cylinder does not have any boundary conditions (means it is insulated from outside). All the additives within the cylinder have natural thermal contact as there is no gap between them. the analysis is running OK.

But this is just a prototype, I might have to create no of cylindrical wires crossing each other (as wire mesh) and there will be gap between them. The same plates will be used as boundary conditions. But how the gaps will be defined.
So thats why I was thinking to make a box around my whole model showing air and give that box air property. Basically the following assumptions i have to made in my model

1) heat is only transferred by conduction because the air between gaps and over surface is Still.
2) Air in mesh will be considered as fluid matrix

Can you help me please. I will really appreciate.

 

[corus]

Consider the value of the thermal conductivity of air to the thermal conductivity of the solid parts. In general the conductivity of air is so small that it can be neglected. Heat loss through air is dominated by radiation and convection for which only surface conditions of the solid need to be defined. If in doubt then run a test case to see the effect of including the mass of air in the model compared to a case where it is excluded.