Manifold Question

Need some understanding. in the excellent book “Rocket Mass Heater Overview and Terms” under the paragraph

Bell/Barrel

Size: The practical bottom line:

“In the barrel and manifold areas, flow areas from twice to four times the system CSA seem to work fine.”

Can someone explain this?

CSA is in2, so for a 6” diameter riser the CSA is 28.3in2.
The CSA would be the same as the rest of the bell/barrel.
Unless they meant inches cubed with CSA and gap between top of riser and barrel.
Which I understand the gap is from 1.5 - 2 inches.

The cross sectional areas in the barrel around the heat riser, and in the manifold where flow transitions from the barrel to the horizontal ducting, can be two to four times the general system cross sectional area. Indeed, these regions need to be larger, because there is more surface area in the barrel for drag to act, and sharp turns in airflow in the manifold which cause damaging turbulence and drag.

http://donkey32.proboards.com/thread/1406/calculating-ring-circumference-projection-gap

https://en.m.wikipedia.org/wiki/Boundary_layer

Gentlemen,

Thank you for your response, after reading, I realized that I was suffering from severe Brain Flatulence (also known as brain fart). In my confused state, I wrongfully thought the book was referencing a CSA at the top of riser and barrel gap not the flue.
Your posts helped cure the aforementioned medical condition, so until the next flare up, I will keep studying.

Scott Robuck
Srobuck56@gmail.com