Chimney - rectangular or round, does it matter?

[Ken.thai]

Hi,

Simple and maybe stupid question, does the chimney have to be a cylinder shape, or is a rectangular one okay?

I've finished my dome and the opening for the chimney is roughly 12x19cm, so I'm thinking to make a rectangular chimney with those inner measurements, and then wrap an outer layer 2cm thicker and put ceramic fibre in between. Attached is a rough sketch

[UtahBeehiver]

I read an article once where round is more flow efficient than square or rectangular in terms of square area so if your oven chimney requires an eight in ID vent which is (pi x r2) = 50.24" then a eight round vent is used if a square or rectangular vent you may need to use dimensions that are greater the 50.24" how much I don't know. Either shape work just be aware of flow efficiency.

[Ken.thai]

I read an article once where round is more flow efficient than square or rectangular in terms of square area so if your oven chimney requires an eight in ID vent which is (pi x r2) = 50.24" then a eight round vent is used if a square or rectangular vent you may need to use dimensions that are greater the 50.24" how much I don't know. Either shape work just be aware of flow efficiency.

Thanks for the reply, It makes sense that a round chimney would have a better flow. Aesthetically (?) it looks nicer with a round chimney as well.
I managed to find a metal workshop just down the road here in town (Thailand). A couple of guys came around and took measurements, and I provided a few illustrations how I wanted it, and some ceramic fiber for isolation (double barrel). I received the custom-made stainless steel chimney a few days ago and I'm really happy with the result! I reckon I got a good deal too, paid 4000 Baht for the lot ( 120 USD)!

Some pics: