Issue:
A thermal analysis is performed. The minimum calculated temperature is colder than the minimum applied temperature, or the maximum calculated temperature is hotter than the maximum applied temperature.
Causes:
The abnormal temperatures occur because:
- The heat flow through the elements is calculated based on the applied loads and the element shape.
- The temperature of the nodes is then calculated based on the calculated heat flow through the element and the element shape.
- Sometimes the temperature gradient necessary to support the calculated heat flow through the element can result in one of the nodes being colder (or hotter) than it should be. Such conditions usually occur at a corner or edge of the model that has a load applied.
The abnormal temperature is less accurate (colder than the coldest applied load, or hotter than the hottest applied load) when any of the following occur:
- the element is large or distorted.
- the applied load has a high magnitude (such as a large convection coefficient).
- the thermal conductivity has a large change from one part to another.
- the time step is large in a transient analysis, so the heat flow during the time step is large.
Solution:
To improve the calculated temperature range, make changes to the model to improve the path of the heat flow through the model.
Mesh Size
Use a finer mesh, especially at corners or edges where the abnormal temperatures occur and where the thermal loads are applied.
Contact Setup
If the temperature excursion occurs at a contact face, change the contact and mesh as follows:
- Edit the contact pair where the temperature extreme occurs.
- Change the "Penetration Type" to "Unsymmetric Contact".
- Note which faces are on the Secondary Entity.
- Click OK to save the changes.
- Add a mesh refinement ("Mesh > Mesh Control") to the faces on the secondary entity. Make the mesh at least 10% smaller than the mesh size on the primary faces.
- Regenerate the mesh.
- Run the analysis.
Time Step Size
In a transient heat transfer analysis, the stability of the temperature calculation is also dependent on the size of the time step. Use a smaller time step to improve the accuracy of the calculation.
Load Curve
An incorrect load curve type in a transient heat transfer analysis can cause thermal results to go outside of expected ranges.
- Make sure any time dependent loads are set to Load Scale Factor vs. Time
Thermal Capacitance
When using "mm" CAD units, the thermal capacitance (specific heat) is in units of mili-Joules (mJ), not mega-Joules (MJ). Specific heat is only used in a transient heat transfer analysis.
Material Properties
Using capacitance or thermal conductivity values that are orders of magnitude different can cause issues with the stiffness matrix. If some materials have a very low conductivity and other materials have a very high conductivity, the heat flow will be determined primarily by the material with the low conductivity. Reduce the high conductivity so that the range of (maximum conductivity)/(minimum conductivity) is 1000 or smaller.
Products:
Fusion; Simulation Mechanical; Nastran; Inventor Nastran;
