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My transient thermal analysis doesn't fit to the reality

Question asked by Louis Collienne on Apr 6, 2020
Latest reply on Apr 10, 2020 by Jay Seaglar

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Hello, as part of an internship projet, I've made a transient thermal analysis but I need help because it doesn't fit to reality.
The purpose is to heat an aluminium cell with 6 cartridges heater inserted inside the cell.
I've designed the cell, the heat insulator and the cartridges. I set the initial temperature for all the parts to 300K, the heat power to 40W to each cartridge.
(given by the cartridges datasheets) and the convection to all exposed faces to 10W/(m².K)(I've used the table for natural air convection and I've seen that the value is between 5-25).
I've also chosen the right material for each part.

To reach 200°C, in the software I need 80 second and, in reality, I need 10 minutes ...
What am I doing wrong?
Thanks

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Josh Brady Apr 6, 2020 9:02 AM
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If this is an internship, you should have a mentor, right? Do they have any opinion?

How are your 40W heaters controlled? I'm guessing you have the simulated heaters set to blast out straight up 40W into the aluminum. Unless you've done a superb job of matching things, your heaters probably can't run at 100% power during the entire warmup without overheating. Usually in the real world you have a temperature controller somewhere, that controls relays that allow current into the heater. Depending on the location of the temperature sensing device that's providing feedback to the controller, it may be limiting the heat output. Usually there's a light on the controller or on the relay that will tell you when the controller is allowing current to flow to the heaters. Are those lights on 100% of the time during heatup?
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- Louis Collienne @ Josh Brady on Apr 7, 2020 4:21 AM
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  Yes I have a mentor but he never uses the thermal analysis on SW. He just says that the difference between the simulation and reality is anormaly high.
  So I try to find by myself how I can may be fix this.
  The heaters are controlled by an ON/OFF controller. I manually set the temperature on the controller.
  When I put 200°C I have the full power until 202 degrees (I have a 2 degrees hysteresis).
  For this reason, I thought I should have similar result between SW and the reality during the heating part (from the ambient temperature to 200°C) but this is not the case.
  In response to your comment, I checked the cartridge temperature and they set up to 400°C (after 80 seconds) and if I made a 1 hour transient analyses, they reach 900°C (whitout thermostat !). I don't know if in reality they can reach such a high temperature.
  The problem is that there are no detailed datasheets for the cartridges (it's a 5 euro cartridge for the prototype).
  To do the cartidges on solidworks, I just design a stainless steel cylinder and I put the 40W on surfaces.
  I know that I must have some differences bewteen SW and reality but in my case I think it's a huge difference.
  If I cannot do a better simulation then I need to justify why I have such a difference if I want to put my results in my thesis.
  
  Thank you for your answer !
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Jay Seaglar Apr 10, 2020 2:34 PM
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The following SOLIDWORKS Knowledge Base (KB) solution might help - it summarizes several of the most common reasons that a transient thermal study could give unexpected results:

S-040493: "My transient thermal analysis results seem unreasonable. I've checked my setup including initial temperatures, thermal loads and convection settings. What could be causing this?"

These include:
1. Specific Heat material property is missing or inaccurate for one or more materials (for details, see S-021183: "What are the required material properties to run a thermal analysis?")
2. Initial Temperatures are applied to outer faces instead of entire solid bodies (S-019007: "Defining an initial temperature for a transient thermal study")
3. Time step/increment does not satisfy the recommended range based on the thermal balance equation (S-059741: "Maximum recommended time step for transient thermal analysis")
4. Heat Power applied to multiple entities is actually using 'Total' instead of 'Per Item' or vice versa (http://help.solidworks.com/2020/english/solidworks/cworks/idc_help_load_heatpower.htm)
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