free convection or forced convection 1

[roudan]

Hi

I am modelling heat transfer shown as below figure. The left boundary has a constant T of 250C, and it will transfer from steel A into gas then into another steel B. The top and bottom are insulated BC. The gas is constrained between steel A and B. Now I will not include left steel A and the heat transfer from steel A to steel B and the heat transfer is replaced by air convection using surface film condition on steel B directly. In this case, which kind of convection for gas, free convection or forced convection? Thanks

 

[FEA way]

The difference between these two is simple - free convection is caused by buoyancy effect while forced convection is caused by external means (like fan or pump). So the choice here depends on what initiates the flow.

 

[roudan]

Thank you FEA way, in my case, there is no external means, but the air is constraint inside the annulus inside two closed end pipes, the temperature is set as constant at one pipe, so heat is transfer via gas inside the annulus. for my case, it is not just due to buoyance effect, i am not sure? Thanks for further advice.

 

[FEA way]

It would be best to include those two additional layers (steel and gas) in the model. Otherwise you have to assume some value of film coefficient which is hard to determine with good accuracy.

 

[roudan]

Thanks FEA way, if I include these additional layers, then I don't need to use convection method (surface film), instead, I just use conduction method by input gas conductivity and gas specific heat, is it correct? Thanks

 

[FEA way]

Yes, and you also have to account for heat transfer in each interface (between layers) but this can be done with tie constraint to model heat flowing through the interface without resistance.

 

[roudan]

Thank you FEA way, it is so good to have you here helping us. I appreciate it.

 

[roudan]

Hi FEA way, why we need tie constraint? can I just partition one part into different sections, apply different materials for each section, for example, one section for steel, one section for gas, etc? Thanks

 

[FEA way]

Tie constraint is only necessary when each layer is modeled as a separate part and thus meshes aren’t continuous. If you model layers by partitioning the same part and assigning different sections to each one then you don’t need the tie constraint.

 

[roudan]

Thank you FEA way