Heat Flux Load¶

In one sentence: Apply a surface heat flux — direct, convective, or radiative — to selected faces as a Neumann thermal boundary condition.

Overview¶

Workbench: FEM
Menu: FEM → Model → Thermal Loads → Heat Flux Load

Applies a surface heat flux to selected faces. Supports direct heat flux (W/m²), convective heat exchange with a fluid, and radiative heat exchange.

Flux types¶

Type	Description	Equation
Surface heat flux	Direct heat flow	q = q₀ (W/m²)
Convective	Heat exchange with surrounding fluid	q = h(T_∞ − T_surface)
Radiation	Radiative exchange with a black body	q = εσ(T_amb⁴ − T_surface⁴)

Convective heat flux parameters¶

Parameter	Symbol	Description
Film coefficient	h	Convective heat transfer coefficient (W/m²/K)
Ambient temperature	T_∞	Temperature of the surrounding fluid (°C)

Typical convective coefficients¶

Condition	h (W/m²/K)
Natural convection in air	5 – 25
Forced convection (fan) in air	25 – 250
Flowing water	500 – 10 000

Common mistakes and pitfalls¶

Convection coefficient units

The film coefficient h must be in W/m²/K, not W/mm²/K — a factor of 10⁶ difference. Check the unit system in Preferences → FEM → General.

Radiation is nonlinear

Radiative flux (∝ T⁴) is nonlinear and requires a nonlinear solver iteration. Enable nonlinear solving in the CalculiX settings when using radiation BCs.

Python API¶

import ObjectsFem
doc = App.ActiveDocument
analysis = doc.getObject("Analysis")

heatflux = ObjectsFem.makeConstraintHeatflux(doc, "HeatFlux")
heatflux.AmbientTemp = 20.0
heatflux.FilmCoef = 25.0
heatflux.References = [(doc.getObject("Body"), "Face3")]
analysis.addObject(heatflux)
doc.recompute()