The heat transfer coefficient model varies in the time since it is dependent on the "a", "b", and "c" coefficients of the Colburn equation, as described in the documentation page below:
If you want to change the value of the "Fooling factor" or the "a, b, c" coefficients during the simulation, it is a current limitation in Simscape. The Simscape network has a different architecture than Simulink. More specifically, it is acausal, resolving the set of equations with respect to the used components. In this regard, you can change parameters at the beginning of the simulation but not during it. To find more information on how Simscape simulation works, please consult the following documentation page:
A workaround might be to use Simscape Fluids fundamental components to create a simplified heat exchanger that might be relevant to your workflow. An example of a simplified custom heat exchanger could be something like the screenshot below:
By using the "Controlled Heat Flow Rate Source" block and "Simulink-PS Converter" you can control the heat transfer rate at every timestep from Simulink. I understand, though, that creating a custom condenser evaporator might not be trivial.
