It seems somewhat counter-intuitive that for subsonic flow through a constant area duct in which heat is added, the Mach number increases, even though the added heat increases the speed of sound.
It seems also counter intuitive that friction in a constant area duct would make the flow speed up instead of slow down. (Though Poiseuille flow does not slow down due to the friction either.)
This document explores how the various effects really tie together for the simplest case of low Mach numbers.