Isolated thermal regions and power balance

A steady temperature requires the heat added to a region to be removed from that region. Before starting a simulation, Vanellus checks prescribed heating and cooling separately in each connected thermal region. The same check applies to dry runs.

What forms a thermal region

A thermal region contains fluid and conducting-solid cells connected by heat-transfer paths in the mesh. Fluid–conducting and conducting–conducting interfaces join their neighboring cells. Insulating components interrupt these paths, and an adiabatic wall separates the thermal regions on its two sides.

Compact thermal models connect the regions touching their active resistor-network faces. Linked fans connect the regions at their intake and exhaust patches through transported heat.

Connectivity is checked on the generated mesh. A gap lost during meshing or a conducting contact removed by CAD voxelization can therefore change which inputs belong to the same region. Inspect the mesh when the reported grouping differs from the intended model.

When prescribed power must balance

A region with a fixed-temperature boundary, a temperature-bearing inlet or inlet-outlet boundary, or a radiative boundary with nonzero emissivity can exchange heat according to its solved state. Its prescribed heating and cooling do not need to sum to zero before the solve: the remaining transfer is determined during the simulation.

A region without any of these boundaries is thermally isolated for this check. Its prescribed heating and cooling must balance. This includes a sealed enclosure whose exterior walls are all adiabatic or have prescribed heat fluxes.

Vanellus includes these contributions, with positive power adding heat and negative power removing it:

• conducting-component power, integrated over the component cells in the region
• boundary heat_flux, multiplied by the meshed boundary area in the region
• absorbed incident radiation, absorptivity × incident_irradiation, multiplied by that area
• CTM power, counted once for its connected region
• linked-fan thermal_power, counted once for its connected region.

A radiative boundary with zero emissivity supplies no temperature-dependent emission. Any absorbed irradiation still counts as prescribed heating.

For each isolated region, Vanellus compares total heating with total cooling. Heating in one disconnected region cannot be balanced by cooling in another.

Example: a sealed heated enclosure

A 10 W component in an otherwise adiabatic sealed enclosure leaves 10 W of excess heating. With reference_temperature supplied, the request reaches the power check and is rejected because that region cannot reach a steady temperature with the specified inputs.

If a selected wall patch in the same region has a meshed area of 0.01 m², a heat_flux of −1000 W/m² removes 10 W and balances the prescribed heating. Alternatively, model a physical heat-transfer path, such as a fixed-temperature wall or radiation to the environment. Choose the correction that represents the intended system.

Whenever any region is isolated, supply the top-level reference_temperature, even if other regions have temperature-setting boundaries. It sets the total thermal energy in each isolated region.

Example: a radiation-cooled body

A radiative surface with nonzero emissivity can remove heat as its temperature rises. A heated region connected to that surface therefore does not need an equal prescribed cooling load for the request to pass this check. The radiative heat transfer is calculated during the solve.

If the model has no qualifying boundary temperature for automatic reference selection, supply reference_temperature explicitly. The radiative environment’s ambient_temperature is a separate input and does not supply the automatic reference.

See also

• Conducting and insulating domains explains which cells participate in heat transfer.
• Thermal boundary conditions explains heat-flux signs and radiation inputs.
• Boussinesq buoyancy and reference temperature explains explicit and automatic reference selection.
• Diagnose a rejected or failed simulation explains how to inspect API feedback.
• Monitors and derived quantities explains conservation checks on computed results.