Also, have you built this yet? You could look at rocket stoves, which are also made of cob but have a well designed combustion chamber, unlike what you describe.
Agh! You stole my thunder!
When I first started playing with rocket stoves one of the things you immediately realize is that anytime you see smoke - you've got really bad combustion.
Rocket stoves, Rocket mass heaters, masonry stoves - all burn the exhaust gasses 2, 3, 4 times and get SO HOT (with almost no wood) that there is -0- smoke and almost -0- emission.
The folks these day who preach the whole "Rocket Stove"- "Rocket mass heaters" make it sound like it's some new-fangled development that a bunch of concerned eco-60s hippies came up with, when in fact the Russians (Euros) developed the idea about 400 years ago.
Google "Russian Stove" and you will find images of whole house heater/stove/oven units --- THAT HAD NO FLUE!
Fact is it burned so hot and so clean it would heat the entire home (slowly), cook and bake, and the "exhaust" (as it were) dumped right back into the house.
If there is still time, might want to consider the whole masonry/Russian/Rocket Mass Heater concept.
These types of "stoves" basically have a long running (sideways, up and down, back and forth) flue. The convection out the ass end sucks the air through-FAST, gets up to 1200+ degrees (try THAT with you Franklin Stove!) - and by the time it finds open air it has burned up all the gasses, no smoke, with a fraction of the wood burned. It really is very cool, and unfortunately something very few people know about.
I have built several, including a rocket stove that I use for our a campsite. I bought a fireplace thermometer and with a good handful of ripped 1x4 I pegged the thermometer at over 1400 degrees. Even if heat production is not your goal, the exhaust concept still applies and is something you should at least be aware of