While conducting a nonlinear transient h.t. analysis, my results blow up and I cannot wrap my head around it. All of the loads that I have applied to this assembly are as follows:
-a linear temp ramp up from 100 to 2650°F over a 10 hour period
-an initial condition temperature of 90°F for ambient air
-and several different convection loads on my walls, roof and floor due to having separate parts of the same material to make up the walls
This same assembly worked in a nonlinear steady state analysis and got temperatures relative to what my company actually sees with this particular kiln. However, due to this analysis having proprietary material data information I cannot directly share anything. Is there anyone out there who could guide me toward an article or two that can explain the steps in trying to find out why my analysis blows up?
Solved! Go to Solution.
Solved by John_Holtz. Go to Solution.
Hi @dstuebgen
Since the steady state analysis gives expected results, I suggest that you check the items that are different in the transient analysis. The transient analysis assigns Time and/or Temperature Dependence tables to the loads. Perhaps one of the tables is incorrect.
John
Hi,
I agree with your use of entering 1 for the thermal conductivity and entering the actual thermal conductivity in the table. The end result (the thermal conductivity) is the same whether the analysis is using the table as the thermal conductivity or if the analysis is multiplying the table by 1.
I suspect your issue is a table that is used for the loads. For example, you have a temperature load that changes over time. How was that entered? (The load type, load magnitude, and table input.) If the convection loads change over time or temperature, how is that entered?
It might be a good idea to try to duplicate the issue with a small test model that includes the loads (assuming they do not contain proprietary information) but make everything else as simple as possible (simple geometry, constant thermal conductivity, and so on). If you can reproduce the problem and provide the model, someone has a better chance of understanding what is happening in the analysis.
John
Hi John,
-The temperature load was entered with a magnitude of 1° F with a table starting with (0 seconds, 100°F) and goes up to (36000 seconds, 2650°F) to simulate a ramp up in temperature for a simple kiln.
-The convection loads are entered as an assumed vertical, horizonal hot face up, or hot face down (depending on geometry of course) with a constant convection coefficient (accounting for radiation without directly applying a radiation load) entered at a specific film temperature.
*The convection coefficients should technically change with temperature load, but for simplicity's sake we kept the coefficient constant.
It all sounds okay. You may need to reproduce the setup and problem in a model that you can share. Or if the model can be shared through technical support (instead of through this public forum), you can create a support case through www.autodesk.com/gethelp.
John
To summarize the outcome of this thread for other readers, the time step size was too large. Using the Adaptive time stepping kept the temperatures following the prescribed temperature loads.
To provide a little more detail,
John
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