Learning Forum

[jvardaman]

I have been in contact with someone previously about identifying temperature mismatch between measured and model results in ANSYS Mechanical. I have discovered something else within the Mechanical model that is not providing accurate results. As you can see in the image attached, I ran a simple experiment where I heated some high density polyethylene (HDPE) in an oven and then cooled it down in ambient air. I included my current model results as well. The measurements show there is a region of heat transfer change in the HDPE material around 120-130C. The model does not account for this. I had to change the convection film coefficient to get the model to reach the same temperature as the measured. Is there someone with more intimate knowledge of the Mechanical program that could help with this issue? I would like to continue using this program, but it is consistently producing inaccurate results. Thank you.

[peteroznewman]

ANSYS staff are not permitted to open attachments. Here are snapshots of the attached pdf.

[Chandra Sekaran]

That knee like behavior can occur due to latent heat where temperature is not changing much but some kind of phase change is happening. In ANSYS for the transient thermal you can use specific heat & density or use enthalpy. In either case the latent  heat should be accounted for. I wonder if this is included in this model. 

https://www.mdpi.com/2076-3417/7/7/722/pdf  seems to show a phase change in HDPE type material at 130 C. So may be this is the issue?

[jvardaman]

Hi, CS

The phase change does indeed happen around those temperatures, and I also believe the 'knee' is due to phase change. I have manually inputted what I am told are the correct specific heat and density values. I have never seen the knee replicated in the simulation results, so I cannot verify that the model is taking phase change into account. Obtaining the correct correct film coefficient is another matter that throws values way off. 

[Sandeep Medikonda]

Hi, Just to add.

Please see the following verification examples 

https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v195/ans_the/Hlp_G_THE3_12.html

especially this particular one:

https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v195/ans_vm/Hlp_V_VM104.html

Also, please see this link below for enthalpy vs temperature data of HDPE

https://thermalmaterials.org/pcm/hdpe-natur-nt-d960-6-1