Hope it's not too late ...
It looks like the Furnance block is set up such that it produces (1) a burning temperature in the H port, based on the air-fuel ratio and (2) amount of heating energy available in the C_exh signal port. This is of course a simplied way of describing the behavior of a furnance, and as a result, changing the fuel flow won't directly affect the burning temperature but only the C_exh signal. This is because it's assumed that the amount air is proportional to the fuel mass, as set by the "percent excess air", and when you have less fuel, less air comes in, and the burning temperature remains unchanged, just with less available heating energy. This explains why the temperature at the H port doesn't seem to change no matter what you vary.
One way to change this is to make that "percent excess air" an input instead of a parameter, by making your own custom block. It should be a very simple change based on the original code. You may then probably couple the fuel flow and excess air, with some function or look-up table, so that when less fuel comes in, there is more excess air, and the temperature and available heat will both drop. Here is more info on using custom Simscape blocks: https://www.mathworks.com/help/physmod/simscape/lang/creating-custom-components.html
