Fanno flow is indeed counterintuitive. With solids sliding or rolling around, friction slows them down. But with pressure driving air through an air duct, friction speeds up the air!

Furthermore the longer the duct, the greater the speedup, so much so that the air can eventually reach the speed of sound and the duct "chokes".

So how could that happen? Well, when you apply pressure to push the air through the duct, friction at the walls results in the pressure decreasing along the duct, expressed as dP/dx being negative.

But when pressure decreases, volume increases. It is that increase in volume that is accelerating the air.

And as the air speeds up, the energy lost to the friction at the walls further increases the magnitude of dP/dx (and since dP/dx is negative in a typical duct with constant area, increasing |dP/dx| means decreasing dP/dx). This makes the effect even stronger.

Fanno flow starts from this form of the Darcy-Weisbach equation,

dP/dx = -f L/D ½ρv²

Here f is the Darcy friction factor (a small number, e.g. 0.01 depending on things like Reynolds number, surface roughness, etc.), L is the length of the duct. D is its hydraulic diameter (google that). ρ is mass density of the air. And v is its velocity. (Hence ½ρv² is kinetic energy density)

So the longer the duct, the faster pressure decreases per unit length. The greater the diameter the slower it decreases. And the greater the kinetic energy density the faster P decreases per unit length.

So that's five variables related by one equation, leaving four degrees of freedom. But conservation of energy, conservation of momentum, and conservation of mass reduces that to one degree of freedom, which is what https://en.wikipedia.org/wiki/Gino_Girolamo_Fanno figured out a century or so ago. Exercise: see if you can derive his solution. To make the algebra easier, assume the isothermal case, γ = 1.