Assigning Network Properties
A network is a two-dimensional modeling boundary condition that can be used to approximate the thermal behavior complex components or systems. They consist of nodes linked together using either thermal resistance (R link) or unidirectional mass flow heat transfer (MC link) connections or links. The network schematic editor allows you to make fairly complicated, arbitrarily connected thermal network topology for modeling various components, like an IC package, a heat pipe, a cold plate, or a desktop server. Each network can consist of a set of external nodes (faces) that are connected to internal network nodes, which, in turn, can be connected to other internal nodes or boundary nodes. External face nodes can be connected to each other as well.
Steady-state or transient thermal analysis can be performed by adding thermal mass properties (Mass, Specific Heat) to the network internal nodes.
The Icepak network boundary condition is designed to be used inside a CFD model, that is, it cannot be used in a "standalone" mode. Within a network, the governing flow and heat transfer equations are reduced to simple algebraic equations. The node temperatures are related to the conduction heat transfers through the R-links and to mass flow heat transfers through the C-links for given mass flow rates. Each defined network node must be connected. Network object communicates with the rest of the Icepak model through the two-dimensional face nodes of a three-dimensional solid block. Since there is no need to solve CFD equations within network object, there is no need to mesh inside its embedding three-dimensional solid block. All volumetric CFD mesh must only exist in the rest of the Icepak model outside the network object. To do this, deselect Solve Inside in Properties Window. So, volumetric mesh inside is disabled as it is considered a hollow block.
A simple way to create a network object in Icepak is to create a solid block with network volume dimensions. Then, in face-selection mode, you can select one or more faces of the network volume and assign a network boundary condition (Icepak > Thermal > Network).Then, edit it in the Network Thermal Model dialog box. Based on the number of selected faces, the schematic is initially populated with their R links to a single internal node. This basic network topology can be easily revised to adapt network object to more complex model.
Since there is no need to solve CFD equations within a network, there is no mesh inside. All volumetric CFD mesh must only exist in the rest of the Icepak model outside the network object. This is easily achieved by creating a 3D geometry that has:
- higher object priority than other intersecting and contacting objects.
- its Properties > "Solve Inside" is disabled to ensure no volumetric mesh inside
You can refer to this 3D geometry as the hollow block.
In face-selection mode, you can select one or more faces of the network volume and create a network boundary condition using the schematic in the Network Thermal Model dialog box. Based on the number of faces you select, the schematic is initially populated with nodes and links.
Network Nodes and Links
Networks consist of a set of external nodes, boundary nodes, and internal nodes. External nodes represent the geometry of the network face, so that you can visualize the geometric shape to which the internal nodes are connected.
Boundary nodes represent boundary conditions for the network node. They are just like regular boundary conditions that you would apply at a wall, such as fixed heat flux or fixed temperature.
Internal nodes represent objects that you want to lump together to have a single temperature. For example, if you are modeling an IC package or a chip, the junction would be an internal node. The assumptions here are that the internal details of the chip are not important and that the most important characteristic that would be of interest in modeling is the junction node. The rest of the chip is modeled through resistance connections to surfaces.
External nodes and internal nodes can be connected by resistive links (R links) or thermal mass flow links (MC links).
For IC packages, R-links can represent sophisticated thermal networks, for instance, modeling a package with multiple heat sources. The internal nodes will be dedicated for observing the various junction temperatures of these heat sources. Their connections to the external nodes, positioned on the faces of the hollow block, or to other passive internal nodes, are composing the interlinking thermal heat flow paths of the package.
For cold plate equipment, the created network model is combining flow and thermal heat transfers. Unidirectional MC links allow to specify mass flow rate as well as the flow direction. The mass flow rate entering and leaving any internal node must sum to zero to account for conservation of mass. Face nodes/surfaces must be located on domain boundaries (for example, a cabinet boundary or surfaces of objects with Solve Inside disabled ) since they represent a connection to an external domain that you do not want to model in detail.
For complementary information about the building of a complex thermal network for IC package, the JEDEC standard JESD15-4 could be useful: JEDEC-JESD15-4 DELPHI Compact Thermal Model Guideline.