Thermal Resistance Types
When assigning contact, the required properties you need to define depend on the Thermal Resistance Type that you choose in the Contact dialog box. There are five available options, as follows:
- Thermal Conductance: Total conductance for the combined area of all selected faces (in units of power per Δtemperature). Do not use this option when the actual contact area differs from the area of the assignment faces (see Note below).
- Thermal Conductance per Area: The distributed conductance per unit area (in units of power per area per Δtemperature). This option is suitable for all contact cases.
- Thermal Resistance: Total resistance for the combined area of all selected faces (in units of Δtemperature per power). Do not use this option when the actual contact area differs from the area of the assignment faces (see Note below).
- Thermal Impedance: An area-based resistance value (in units of Δtemperature times area per power) for which the resultant total resistance is inversely proportional to the contact area. That is, the greater the contact area, the lesser the total resistance. The term "Resistance per Area" is not used because that term would imply a proportional relationship between resistance and area, rather than an inversely proportional relationship. This option is suitable for all contact cases.
- Thickness: This option simulates the resistive effect of an intermediate object (not included in the model geometry) sandwiched between the two contacting objects. For this resistance type, you define the thickness and the material. The effective conductivity per unit area is determined by the solver from the thermal conductivity of the material and the specified thickness.
An example of when you might use this option is to simulate the effect of a thin epoxy layer bonding two parts together. You could also interpose an electrical insulator between a semiconductor and a heat sink when the insulator is not included in the geometry. However, this option would only account for the thermal resistance of the insulator and not for the contact resistance between the insulator and the object on each side of it. To account for both the insulator material and the two contact resistances, choose one of the other four options and define the appropriate value to account for all three effects (RContact + RMaterial + RContact).
When the total area of the selected model faces differs from the actual contact area, you should not use the Thermal Conductance or Thermal Resistance option to specify contact. For either of the following two cases, the selected face area and the actual contact area differ, and you should only choose the Conductance per Area, Impedance, or Thickness resistance type:
- When a large face is selected to cover contact with multiple smaller adjacent objects, and less than 100% of the face is in contact with other objects (see the first case under Contact Examples in the Contact Guidelines and Examples topic.)
- When any model face in the selection set is only partially in contact with an adjacent object (that is, when there is only partial face overlap between the selected face and target parts)