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A heat flux can be defined for heat transfer analyses using the surface_load element with type set to heatflux element.

<surface_load type="heatflux" surface="surface1">
  <flux lc="1">1.0</flux>
</surface_load>

The heat flux element takes two parameters, namely flux which defines the flux that will be applied. It has an optional lc attribute which defines the load curve for this parameter. If omitted, a constant heat flux will be applied. The other (optional) parameter is the value parameter which can be used to define a different flux value for each surface facet.