Surface Flow or Integrate Variables
 

Hello.
All my results (the values of temperature flow in mesh nodes) are obtained from OOFEM.
Now I need to calculate heat flow through the all surface of my 3d model. For this purpose I use filters of ParaView, such as Extract Surface->Surface Flow->Plot Selection Over Time. Should I plot Surface Flow or IST_TemperatureFlow(Magnitude)? What is the difference between these two calculated parameters? Maybe I wrong, and filter Integrate Variables is more appropriate instead of filter Surface Flow.
My second question is how to calculate heat flow through the separate surface (for example, top or bottom side of my model).
I’m looking forward to your help and advice. Thank you in advance for your response.