how can you have a turbulent boundary layer with laminar flow in the middle? Plus laminar flow is bad for an engine that atomizes a liquid fuel. since there is no mixing involved at all with laminar flow. for fuel atomization, the opposite is the goal. volume of air and proper atomization/vaporization of the liquid fuel are the two most important things. so creating a more laminar centerflow seems counter productive.

onto my second point, the dimples create a turbulent boundary layer causing turbulent flow.

 

Flow is only going to be by definition "laminar" at a very low velocity, most likely only in the primaries since the secondaries are closed during periods of low velocity. If fuel droplets couldn't remain suspended in such flow then our engines wouldn't idle.

So maybe instead of "laminar flow" I should've just said "less turbulent flow" since at any RPM higher than idle there's going to be turbulence throughout the runner, even in the center. But the dimpled surface creates more turbulence at the wall, which helps keep fuel from filming on the surface. You don't want too much turbulence through the entire cross section though, because turbulence impedes velocity, right? So having the turbulence hug the wall creates more room in the center for less turbulent, higher velocity flow.