Posted Thu, 20 Aug 2015 08:16:43 GMT by Lance Kim
Is it possible to set a temperature-dependent radiation heat-flux boundary condition? I defined an expression for the "Heat-flux BC" to all surface nodes of a simple flat plate using the expression editor as follows with temperatures in Kelvin

Current=0.8*5.67E-8 (T@(Time)^4 - 276^4)

The resultant top surface temperature is nearly identical to the constant temperature boundary condition of the lower surface. Inspecting the boundary condition shows a heat flux of around 0.006W/m2 when it should be more like 500W/m2 at a surface temperature of approximately 90degC.
Posted Fri, 21 Aug 2015 07:46:54 GMT by Björn Kaiser
The Parameter [b]Time[/b] represent the simulation time. In the fem-file [b]Time[/b] is always 0. Accordingly, if you assign an expression containing [b]Time[/b] you assign 0. This is attributed to the technical fact that you cannot assign time-dependent values to boundary conditions. Instead, you can either assign a constant value or you can assign a time-series-ID pointing to an existing time-series in the Time Series Editor. Due to this technical consideration, I suggest to prepare a time-series with a unique ID. Then assign this time-series-(ID) to the boundary condition.
Posted Fri, 21 Aug 2015 08:10:28 GMT by Lance Kim
Thank you for the tip! But can a time series be used to specify a heat-flux BC that is dependent on surface temperature? I would like to model the effects of a buried point heat source on ground surface temperature in the presence of radiation cooling.
Posted Fri, 21 Aug 2015 08:53:21 GMT by Björn Kaiser
I suggest a heat-transfer BC (Cauchy). A heat transfer boundary condition applies a pre-defined reference temperature (e.g. surface temperature) combined with a transfer-rate.

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