RE: DLE 111 Engine
pat is right, the purpose for the horizontal separator is to eliminate hot air reversion from the exhaust side to the upper side.
Two things to remember when visualizing air flow inside a cowl are these: 1) We cannot direct the air so much as "exclude" it from a certain area, and 2) we cannot "push" the air from one location to another, but we CAN "suck" it from point to point.
Effectively we have a high-pressure area immediately aft of the prop, at the cowl entrance. The path the air takes from that point depends upon the pressure differential. If we have a lower pressure inside the cowl, the high-pressure air WILL flow that direction. If we can further create a suction at the cowl exit, then the air within the cowl WILL flow out. If we have a pressure buildup at any point within the cowl this flow will be interrupted. The total pressure differential may be less than 1/2 psi, but it is enough to get the job done. It helps to place a lip on the forward edge of the cowl exit, and then work forward from there.
When it comes to baffles one thing to keep in mind is any time we ACCELERATE the airflow, the local air pressure DROPS due to Bernoulli's principle.
The best baffle will exclude any airflow that does not travel directly through the cooling fins.