Thanks for your reply. However, I'm afraid I don't agree with your physics. It's been over 40 years since I last studied fluid mechanics but I'm reasonably sure that your explanation isn't right. I'm less sure that mine is
There are very good reasons for building conventional external flues straight-sided. One is that it's easier that way and there is no good reason to build it any other way, given that combustion is not happening in the chimney. Two is that it makes it easier to put a rain cap on. Three is that you genuinely need speed and momentum to punch the air past any contrary wind gusts and through any low-lying inversion layer. Historically, number four was that getting the smoke and pollution as high as possible dispersed the crap over as wide an area as possible to avoid complaints from neighbours. Even so hyperboloid cooling towers, which are chimneys, do bell out.
Many modern industrial furnace systems deliberately slow down the combustion gases in order to get more complete combustion, often in a hot cyclone. Of course, they are using forced-draft.
Hmmm If the condensation soaks into the cob it must be evaporated out again, or the cob would eventually go soggy. The latent heat required for this will simply suck back out of the room air all the heat you have gained from the condensation in the first place! What you have is a humidifier in effect, not a heat recovery condenser. And you will also re-evaporate any condensible pollutants, unless these bind to the clay in the cob. After a few years, the binding sites inn the clay will become saturated, however, I guess it wouldn't be hard to rebuild the bench when it starts to pong! Although a humidifier is a good idea in many cold-climate winters, It wouldn't do here, or in Seattle, for that matter. Average relative humidities in winter in Seattle and year around here are over 80% and at night over 90. we need all the de-humidifying that we can get!