Way back when, in the early days of aviation, some of the radial engines had an airfoil shaped cowling ring in front of the engine, but that was there to aid in cooling the cyliders. The mear fact of haveing a ring or cowling around a propeller will not aid in thrust in any meaningful way. On high bypass turbo fan engines, the majority of the thrust is from the Fan discharge and not the core engine. The fan discharge air is often used to cool CSD oil or other types of cooling. Haveing the fan discharge seperate from the core exaust, which is extremely hot,is a way to seperate the air and have some flexibility with the engine. Also haveing the fan discharge air seperate, is a way to " save" the core from FOD since most debris is thrown outward away from the core engine. The amount of damage to the N1 compressor section can be quite substantial, and still have the core engine intact.. .I'm sure there is more to this that I can't recall right now, but I hope this helps.