I can apply a Wall Heat Transfer Coefficient from a CFX results file to a model in Simulation, based on the CFX solution variable "Wall Heat Transfer Coefficient". However I want the Simulation boundary condition to be based on a different variable from my CFX results; this could be my own custom variable for the wall heat transfer coefficient or the transient average of the wall heat transfer coefficient calculated using transient statistics.

In both cases you are starting from a CFX results file that contains a different variable that represents the Wall Heat Transfer Coefficient, so the procedure is the same:

1. In CFX-Post on the Variables tab edit "Wall Heat Transfer Coefficient". Here you have the option to overwrite this variable with an expression. Create an expression that either simply points to another variable in the results file, or is some function of existing variables, then select to overwrite "Wall Heat Transfer Coefficient" with this expression. Note that data is not lost from the results file - you can remove this expression to recover the original results.

2. Apply a CFX Convection load in Simulation using the usual approach, with the Ambient Temp Type set to Constant. Simulation will interpolate "Wall Heat Transfer Coefficient" from the CFX results file, but you've overwritten this with an expression that points to the real variable of interest. The Constant Ambient Temp Type is required so that Near Wall Temperature data is not used from the CFX results file, which is usually not desirable (see below).

If the variable you are transfering to Simulation is based on the CFX variable "Wall Heat Transfer Coefficient" then it's important the "tbulk for htc" expert parameter has been used in CFX. When CFX calculates a Wall Heat Transfer Coefficient it needs to use a reference temperature. By default the near wall temperature is used as the reference temperature ("Wall Adjacent Temperature" is the corresponding variable name). See the CFX Help under "ANSYS CFX-Solver Modeling Guide >> Boundary Condition Modeling >> Wall >> Wall Heat Transfer >> Heat Transfer Coefficient" for details on how this is calculated.
The important point is to make sure Simulation is using the same reference temperature when it calculates the Film Coefficient as was used by CFX when it calculated its Wall Heat Transfer Coefficient. If you have made your own wall heat transfer variable then it probably doesn't use "Wall Adjacent Temperature" as the reference temperature. Therefore you should set the Expert Parameter "tbulk for htc" in CFX-Pre (Main Menu: Insert >> Solver >> Expert Parameter >> Discretisation tab >> tbulk for htc). This value is in "Solution Units", which are Kelvin by default (Solution Units are not the same as the units you use to set boundary conditions etc). Set this value to your reference temperature for heat transfer, which is usually the freestream fluid temperature. CFX will then use this value to calculate the Wall Heat Transfer Coefficient instead of using the near wall temperature. When you import the results into Simulation you can then specify this value as your "Constant Ambient Temperature".

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