I'll tell you something though, the improvement isn't happening for the
reason you think.
Against all intuition, I assure you that the scoop is actually causing air
to flow exactly in the opposite direction than you expect.
Weird, but true. But the funniest part is that if you're seeing better
efficiency, it's on account of increased airflow through the drum resulting
from a flow path that's contrary to what you were trying to achieve.
When you think about it, though, that makes sense. An exact opposite flow
would have the same effect.
However, this means that your first crack is actually your second, and your
second is actually your first. This makes stopping a roast between first and
second crack impossible unless you spin the drum in reverse. In that case,
be sure to reverse the polarity on your thermocouple connector.
In all seriousness, when my machinist and I built the ring roaster (http://scott.marquardt.googlepages.com/openskyroaster) we implemented 6
substantial scoops on the outside of the drum halves (if you look at the
picture of the entire drum assembly, the two inner donut rings that extend
an inch beyond the drum radius [on either side of the stationary ring in the
middle] were the anchor for the vanes, six on either side, with closed ends
at the outer extremity of the dum halves). We imagined we'd then use the
fill port in the top of the stationary ring as the exhaust. But lo, we
discovered that our "exhaust" was exhibiting a minor suction. Far from
creating the howling turbine we gleefully thought this would be, we
discovered that we didn't know jack about how this stuff works.
We even extended the scoops dramatically around the circumfrence -- no
change. Instead of making air flow inward, they made it flow outward.
This was one of the most bizarre experiences we had, in an otherwise very
successful engineering effort. I think the both of us are still needing
serious therapy for our intuitions, which were left pretty much in tatters.
You could probably improve the efficiency by just making a flat vane
sticking straight up from he drum surface.
Seriously. Don't be incredulous, trust me. This is weird stuff.
On 11/11/06, Douglas H. Boutell wrote: