Intuitive explanation of Fanno Flow

[Earhart]

TL;DR

Can anyone explain how is it that friction can both accelerate and cool an adiabatic subsonic gas flow, as it does in Fanno Flow?

OK, I have read all the Fanno flow equations, I understand mass and enthalpy conservation for adiabatic flows give the result that friction causes Mach number go to 1... But I cannot think of a physical explanation for the counter-intuitive fact that friction both accelerates and cools the subsonic flow of a compressible gas. Can someone give an INTUITIVE explanation of how this works? Taking also into account that, for an incompressible adiabatic pipe flow (which should be the limit case for very low Mach numbers), viscous friction work (viscous dissipation) is converted to heat, which raises the fluid temperature, according to the energy equation.
Thanks a lot to anyone who can help me understand this.

 

[boneh3ad]

It's a surprisingly straightforward answer, but the key here is boundary layers. In a real flow, a boundary layer forms, leading to a displacement thickness that grows in the downstream direction. That results in a virtual narrowing of the duct. For a subsonic flow, that means an increase in velocity, which in turn means a decrease in pressure, temperature and density.

Since the Mach-area relationship is reversed for supersonic flow, the opposite happens as a result of the same virtual constriction.

 

[Earhart]

This is the answer I was looking for, which I could not find in textbooks. As you say, it is surprisingly straightforward. Thank you very much!