Re: Does surface finish have a relative influence on how air flow behaves?

Entire teams of engineers across the world work on this very problem. There is no simple answer to your question because not only do the inlets need to be considered, but the mixing in the chamber or piston must be taken into account.

First, fuel will not burn on its own. In an environment where there is no oxygen it is impossible to ignite gasoline. So the fuel (gasoline) must be mixed with the oxidizer (oxygen in the air) to ignite. Furthermore maximum efficiency is achieved when the exact amount of oxygen and fuel required for complete burning are fully mixed together so there is no excess oxygen or fuel leftover. Failing to achieve complete mixing will decrease the efficiency of the engine.

Atomizing the fuel into a chamber or piston and mixing it in a turbulent flow are two possible methods for achieving this goal.

In the case of mixing it in a chamber, the efficient movement of the air into the chamber would require nonturbulent laminar flow of the air into the chamber. Once into the chamber the flow would need to be changed to highly turbulent to help atomize and evaporate the fuel and to mix the fuel and air together. For this type of combustion scheme, highly polished parts up to the chamber would help promote laminar air flow while a coarser finish in the chamber would help promote turbulence. In the case of an internal combution engine, the wear of the piston and cylinder and their seals needs to be considered as well when selecting the finish.

If the fuel and oxidizer are mixed together in a flowing stream such as in a gas turbine (jet) engine, some turbulent flow is normally preferred. Turbulence will decrease the engine's efficiency, so considerable engineering goes into controlling the airflow to achieve the desired engine performance. For small ducts and lines, surface finish can be used to help control the amount of turbulence. For large diameter ducts, small changes in surface finish are virtually irrelevent.

As a reference to the various types of airflow in practical systems, you can try Transport Phenomena In Metallurgy by Geiger and Poirier, chapters 1 through 5. There are also other references used for HVAC systems and by Mecahnical Engineers that can give you further insight.