Sunny Boy wrote:wsherrick,
Yes, by "bell-mouthing" the entrance of each feed hole, you will reduce the edge affect and increase airflow - without having to increase the hole size. How you'd do that to all those holes from inside that long tube, I'd be interested to see.
With a bit of experiment, it might even be possible to come up with the correct size feed holes for that size fire bed. The result would be, not having to adjust anything. As draft pressure changes, the air flow volume through the feed holes will change because the air can change speed through the feed holes. If I remember correctly, up to a point. Then, I think resistance increases out of proportion to pressure difference and starts to limit flow. Much like motor speeds can often be limited by the size of the carburetor.
As long as the total flow area of feed holes in the large pipe does not exceed the flow area of the large pipe, the feed holes should pretty well do the self regulating. Size of the feed holes then becomes the question.
See, we're back to "size matters".
I see the logic that you can't have the exit holes add up to a larger area than the main pipe. There has to be some form of natural pressure to create enough force to make the air cover the needed area. Without an induced flow (fan) it would take a keen understanding of how all of these elements work together.
On my Glenwood, which has an over fire secondary air supply, the little holes are not drilled straight through, they are slightly angled and are conical in shape, sort of like tiny funnels. When you can see the flames around the secondary air outlets. They come out horizontal for a few inches then go upward. The flames also move around the combustion chamber in a swirling motion, probably due to the holes being at a slight angle. It would be nice if the research behind this wasn't thrown away when somebody decided to clean out some file cabinets years ago.
My thought was that the goal should be to keep the pressure in the delivery pipe as consistent as possible along the entire thing so to even out the distribution of air. That was my thought behind having smaller holes closer to the front and larger ones near the back.
And I am thinking the same thing as you as far as making it self regulating. If the holes are all the correct size for the fire box then it should be so.