Conjugate Heat Transfer (CHT) in Fluent, OpenFOAM & CFX (Guide)
Conjugate Heat Transfer (CHT) in CFD Software
How to effectively handle coupled fluid-solid heat transfer — what CHT really is, how the coupling works, and exactly how ANSYS Fluent, OpenFOAM, CFX and STAR-CCM+ solve it.
A 10 °C error in a chip's junction temperature can mean a recalled product; the same error in a turbine blade can mean catastrophic failure. Yet many thermal models still guess the heat transfer coefficient (HTC) at a wall. Conjugate Heat Transfer (CHT) removes that guesswork: it solves the fluid and the solid together, coupling temperature and heat flux at their shared interface for a true thermal prediction. This guide explains what CHT is, how the coupling works, and — crucially — how to handle it effectively in Fluent, OpenFOAM, CFX and STAR-CCM+.
What This Guide Covers
What Is Conjugate Heat Transfer?
Standard CFD solves the flow and applies a fixed thermal boundary at walls — a set temperature, or an assumed HTC. CHT extends this by adding solid conduction to the solver. The wall becomes a coupled interface: the solver computes the fluid-side heat flux, passes it into the solid to solve Fourier's conduction law, and iterates until the two sides reach thermal equilibrium.
Fluid Region
Navier-Stokes + energy. Computes convective heat flux at the wall.
Solid Region
Fourier conduction. Receives flux, returns wall temperature.
When Should You Use CHT?
CHT is powerful but more expensive than a plain thermal BC. Use it when conduction through the solid genuinely affects the result:
- Electronics cooling — heat spreading through PCBs, heat sinks and components.
- Heat exchangers — wall conduction between hot and cold streams.
- Turbine blade cooling — internal passages + metal conduction.
- Engine & battery thermal management — solid thermal mass matters.
How the Coupling Works: Monolithic vs Partitioned
At the interface, CHT enforces two physical conditions: temperature continuity (both sides share the same wall temperature) and heat-flux continuity (what leaves the fluid enters the solid). There are two ways to solve this:
| Approach | How it works | Trade-off |
|---|---|---|
| Partitioned | Solve each region separately, exchange T & flux at the interface, iterate until they agree | Flexible, memory-efficient; can converge slowly when coupling is strong |
| Monolithic | Assemble the energy equations of all regions into one system and solve together | More robust & faster for strong coupling / many regions; more complex to implement |
Most mainstream codes (Fluent, CFX, STAR-CCM+, standard OpenFOAM) use a partitioned approach; some specialised OpenFOAM solvers (e.g. conjugateHeatFoam in foam-extend) offer a monolithic option that excels for strongly-coupled, multi-region problems.
CHT in ANSYS Fluent
ANSYS Fluent
Fluent makes CHT relatively straightforward. You include both fluid and solid cell zones in one model, and the shared wall becomes a coupled wall automatically. Fluent solves flow + energy in the fluid, conduction in the solid, and at the coupled wall it matches temperature and heat flux, iterating to consistency — no HTC assumed.
Fluent's maturity, GUI and automatic interface handling make it a popular, dependable CHT choice for electronics, turbomachinery and thermal management. Get near-wall meshing right with the y+ calculator and inlet turbulence from the turbulence intensity calculator.
CHT in OpenFOAM
OpenFOAM
OpenFOAM handles CHT with multi-region solvers: chtMultiRegionFoam (transient, PIMPLE) and chtMultiRegionSimpleFoam (steady). You mesh the whole domain as one mesh with cellZones for each solid, then split it into separate regions with splitMeshRegions. Each region gets its own 0/, constant/ and system/, and the regions couple at their interfaces via special temperature boundary conditions that match heat flux.
splitMeshRegions -cellZones → per-region properties & fields → interface BCs on both sides → run chtMultiRegionFoam. The foamSetupCHT template automates much of the setup.$ foamSetupCHT # set up regions from a template $ blockMesh $ splitMeshRegions -cellZones -overwrite # fluid + solid regions $ chtMultiRegionFoam # run the coupled solver
It's more involved to set up than Fluent — each region is its own case — but it's free, unlimited-core and fully customisable. See our learn CFD with OpenFOAM guide, and the best open-source CFD tools overview.
CHT in CFX & STAR-CCM+
ANSYS CFX
CFX handles CHT natively with automatic solid-fluid domain interfaces. You define fluid and solid domains, and CFX creates a conservative interface that couples temperature and flux. Its robust coupled solver and strong turbomachinery heritage make it a favourite for turbine blade cooling and rotating-machinery thermal analysis.
Simcenter STAR-CCM+
STAR-CCM+ excels at large, complex multi-region CHT with its polyhedral meshing and automated interface creation. It's widely used for full-vehicle underhood thermal, battery packs and electronics, where many solid and fluid regions must couple in one model.
Both, like Fluent, use a partitioned coupling with automatic interface handling — the difference is largely workflow, meshing style and ecosystem.
Software Comparison
| Code | Cost | CHT approach | Best for |
|---|---|---|---|
| ANSYS Fluent | Commercial | Coupled wall, partitioned | General CHT, electronics, ease of use |
| OpenFOAM | Free | Multi-region (chtMultiRegionFoam); monolithic in foam-extend | Custom/large studies, no licence cost |
| ANSYS CFX | Commercial | Automatic domain interface | Turbomachinery, turbine cooling |
| STAR-CCM+ | Commercial | Automatic multi-region interface | Large multi-region (underhood, packs) |
Best Practices for Effective CHT
- Refine the mesh at the interface. The steepest temperature gradients live there — under-resolving them ruins accuracy.
- Resolve the near-wall turbulence properly. Match y+ to your wall treatment — check with the y+ calculator.
- Use accurate, consistent properties for both fluid and solid (k, ρ, cp) — wrong solid conductivity is a common error.
- Apply realistic BCs and heat sources — inlet conditions, power dissipation, external cooling.
- Monitor the energy balance and a solid temperature, not just residuals — the solid lags.
- Do a mesh-independence study with a grid-independence test, and check the flow regime with the Reynolds number calculator.
Frequently Asked Questions
What is conjugate heat transfer (CHT)?
A CFD method that solves fluid and solid heat transfer simultaneously, coupling temperature and heat flux at their shared interface — removing the need to assume a heat transfer coefficient.
When should I use conjugate heat transfer?
When solid conduction affects the result — electronics cooling, heat exchangers, turbine cooling, engine and battery thermal management. Skip it for thin walls where a simple BC suffices.
How does ANSYS Fluent handle CHT?
It includes fluid and solid cell zones and treats their shared wall as a coupled wall, matching temperature and flux and iterating to equilibrium — no assumed HTC.
How does OpenFOAM handle CHT?
With multi-region solvers chtMultiRegionFoam (transient) and chtMultiRegionSimpleFoam (steady). One mesh with cellZones is split into regions via splitMeshRegions, coupled at interfaces.
What is the difference between monolithic and partitioned CHT coupling?
Partitioned solves each region and exchanges at the interface (flexible, can be slow when strongly coupled); monolithic solves all regions together (more robust/faster for strong coupling, more complex).
What are the best practices for an accurate CHT simulation?
Refine the interface mesh, resolve near-wall turbulence, use accurate consistent properties for both regions, apply realistic BCs, and monitor energy balance and a solid temperature (which converges slowly).
Conclusion
Conjugate heat transfer turns "what HTC should I assume?" into "let the simulation compute it." By solving the fluid and solid together and coupling temperature and heat flux at the interface, CHT delivers true thermal predictions for the applications where a 10 °C error is unacceptable. Fluent makes it easy with automatic coupled walls; OpenFOAM offers a free, customisable multi-region route via chtMultiRegionFoam; CFX and STAR-CCM+ shine on turbomachinery and large multi-region models. Whichever you choose, the fundamentals are the same: model both regions, refine the interface, use correct properties, couple temperature and flux, and watch the energy balance. Do that, and your thermal predictions will be as reliable as the physics allows.
For more CFD, heat transfer and simulation tutorials plus free engineering calculators, explore Free CFD Tutorial. If this guide helped you, please share it with your fellow CFD and thermal engineers.
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