What is conjugate heat transfer and why does it matter for electronics?

Conjugate heat transfer means solving conduction in solids and convection/radiation in the surrounding fluid simultaneously, rather than assuming a fixed heat transfer coefficient. It matters because the flow field and temperature field are coupled — the airflow around a hot component depends on its own temperature, and vice versa.

When does radiation matter in an electronics thermal model?

Radiation becomes a significant fraction of total heat transfer at low airflow rates, high-emissivity surfaces, or elevated ambient temperature — conditions common in sealed or naturally-convected enclosures — and omitting it can produce a materially wrong result.

Can I use a fixed heat transfer coefficient instead of solving conjugate heat transfer?

Only if the geometry and flow regime match the correlation the coefficient was derived from. For natural convection, heatsinks with induced flow, or complex enclosure geometry, a fixed coefficient typically introduces meaningful error that a fully coupled conjugate model avoids.

01Conjugate Heat Transfer Analysis

Why does treating conduction and convection separately keep giving me the wrong answer?

Coupled conduction-convection-radiation simulation for assemblies where the physics can't be separated.

02The problem

A thermal model that treats airflow and solid conduction as separate steps — apply a fixed heat transfer coefficient, then solve conduction, or vice versa — consistently misses the real behavior of assemblies where the two are tightly coupled, such as natural convection around a heated enclosure or a heatsink whose fin efficiency depends on the local flow it induces.

The error compounds when radiation is also significant, which is common at low airflow, high emissivity surfaces, or elevated ambient temperatures, and a model without radiation can be off by a meaningful fraction of the total heat balance.

Why it's hard

Heat transfer coefficients assumed from correlations are only valid for the flow regime the correlation was derived under, and real geometry frequently falls outside that regime
Natural convection problems are inherently coupled — the flow field depends on the temperature field and vice versa — so sequential solving introduces error that compounds over iterations
Radiation becomes a significant fraction of total heat transfer at low airflow or high-emissivity surfaces, and is easy to neglect in a convection-focused model
Fin efficiency and heatsink performance depend on the local induced flow, which a fixed heat transfer coefficient assumption cannot represent accurately
Coupled multi-physics problems (boiling, radiation-dominant, rotating machinery) require solvers built for that coupling, not a general electronics-cooling tool extended past its intended scope

03How we solve it

Predict — simulation

Resolve fully coupled conduction, convection and radiation in one model using Flotherm or FLOEFD for electronics-scale assemblies
Escalate to STAR-CCM+ for conjugate problems that also involve two-phase heat transfer, participating-media radiation, or rotating/moving geometry

Measure — hardware

Measure actual device or assembly temperature with Simcenter Micred T3Ster to establish a ground-truth reference for the coupled model

Correlate — close the loop

Compare the coupled simulation's predicted temperature distribution against measured points to confirm the conduction-convection-radiation balance is represented correctly
Adjust boundary conditions based on measured discrepancy rather than tuning arbitrary heat transfer coefficients to force agreement

04Tools applied

Flotherm

Conjugate heat transfer resolved natively for electronics assemblies

FLOEFD

CAD-embedded conjugate heat transfer for early design checks

STAR-CCM+

Conjugate heat transfer coupled with two-phase, radiation or rotating physics

Simcenter Micred T3Ster

Measured temperature reference to validate the coupled model

05Standards in scope