NETM porous solid zone: adjacent solid walls default to adiabatic

[Choi Jae Won]

I am running a conjugate heat transfer simulation in Fluent 2025 R2 in which a CPU heat sink fin array is represented as a porous medium with the Non-Equilibrium Thermal Model (NETM). I cannot transfer heat from the heat sink base into the auto-generated porous solid zone, and would appreciate guidance.

Model summary

• Fin block as a porous zone: porosity 0.48, viscous resistance 2.958e6 1/m^2, interfacial area density 320 1/m, h_sf via expression
• Working fluid: mineral oil, single-phase immersion cooling
• Conduction chain: solid_cpu (200 W volumetric source) → solid_tim → solid_heatsink → fin block
• Mesh generated in ICEM CFD with shared topology, i.e. the heat sink / porous block interface is conformal

Problem

When NETM is enabled, Fluent creates the solid cell zone fluid_porous_fin:013. The wall zones adjacent to this zone are one-sided, and their Thermal panel offers only Heat Flux / Temperature / Convection / Radiation / Mixed / via System Coupling. The "Coupled" option is not available, and "via Mapped Interface" is greyed out.

Relevant zones:

 

13   fluid_porous_fin:013         solid (copper)       cell
146  fluid_porous_fin             fluid (mineral oil)  cell
77   solid_heatsink               solid (copper)       cell
158  wall_porous_fin              wall -> solid_heatsink side
163  wall_porous_fin-shadow       wall -> fluid_porous_fin side (coupled pair with 158)
16   wall_porous_fin-shadow:016   wall -> fluid_porous_fin:013, one-sided, adiabatic

A flux report confirms the total heat transfer rate into all zones adjacent to fluid_porous_fin:013 is 0 W, while the user energy source is 200 W.

Because every wall bounding fluid_porous_fin:013 defaults to adiabatic, no heat can enter the porous solid to be redistributed over the fin surfaces. The only remaining conduction path from the base into the oil is the narrow base strip directly wetted by the fluid, and that path removes only about 51 W. The remaining 149 W has no available path: even after the residuals have flattened, this 149 W deficit (75 % of the input) persists and the case cannot close its energy balance. This starved heat-removal path drives the CPU temperature to approximately 645 K and the volume-averaged porous fluid temperature to 447.6 K.

What I have tried

1. Setting the "Coupled" condition on the solid-side wall — the option does not appear in the Thermal panel.
2. Using System Coupling — not applicable, as no external solver is involved.
3. Creating a mesh interface between solid_heatsink and the porous solid zone, following Ansys document 2056744 ("three coincident interface zones"). Manual Create initially did not list zones 158 and 163 because they form a conformal coupled pair.
4. Splitting the conformal face with /mesh/modify-zones/slit-face-zone on zone 158. This succeeded and produced wall_porous_fin and wall_porous_fin-slit as separate zones.
5. Creating a mesh interface with wall_porous_fin in Boundary Zone List 1 and both wall_porous_fin-slit and wall_porous_fin-shadow:016 in Boundary Zone List 2. Fluent accepted this but assigned the entire intersection to the fluid-side zone, producing wall_porous_fin-shadow:016-non-overlapping with an area of exactly 0 m^2. No heat is transferred to zone 13.
6. Creating a one-to-one interface between wall_porous_fin and wall_porous_fin-shadow:016 only. The interface is created, but no Coupled wall appears on zone 13 and no heat is transferred.

Questions

1. Is it possible in Fluent 2025 R2 to transfer heat from an adjacent solid cell zone into the NETM auto-generated porous solid zone? If so, what is the correct procedure?
2. Ansys document 2056744 states that heat transfer between the porous solid and adjacent zones is not included in NETM, and also instructs the user to reassign the wall zones of the generated solid zone to interface boundary conditions. These two statements appear to be in tension. Could you clarify which applies to my configuration?
3. If the auto-generated solid zone cannot participate in mesh interfaces by design, is the recommended workaround to create a real duplicate cell zone (e.g. via /mesh/modify-zones/copy-move-cell-zone) and implement the interfacial coupling h_sf * a_sf * (T_s − T_f) manually through DEFINE_SOURCE UDFs?
4. Does the conformal mesh at the heat sink / porous block interface (shared topology from ICEM CFD) prevent this configuration from working? Would a non-conformal mesh at that location resolve the issue?

I can provide the case and data files if that would help.

[Rob]

Staff are unable to review files, and are limited to what you share and public knowledge. 

The NETM creates a coexistant solid zone to handle the hard bits of the porous zone. That solid is bounded by walls. For the wet sides that's not a problem, as the bulk heat transfer is convection. But, it's harder for wall:fluid and wall:porous_solid boundaries. For an external boundary it's "just" a case of deciding how much heat enters the fluid-wall and solid-wall.

For your case you've got the heat source into a chip? And then need to mimic the heat transfer into both fluid (likely very low) and solid-porous (likely most/all of it)?