Neutral Plane



When the hot gas layer is sufficiently deep it will exit a compartment starting at top of openings (doors, windows).

Cool air moves into the compartment through the lower portion of the opening to replace the exiting hot gases.

The demarcation at the opening (where the exiting and entering interface is) is known as the neutral plane.

Hot gases exit above the neutral plane (higher pressure inside) and cool air enters below (lower pressure inside).

This simultaneous outward and inward flow at a single opening is known as bi-directional flow.