Annulus!! Another great term! This pizza oven is getting more respectable all the time!
Sonotubes are terrific, and that sounds like a slick application. In applications like this (using them as interior block outs) caution is advised when hydraulic form pressures get large. I have a great friend, Dustin Ferch ...(phenomenal concrete man, who helped me place and finish my forge) who learned this the hard way. Casting a chimney for a "Russian?" stove...24 inch OD, 12 inch ID, good rebar section etc... pumping the mud into the form, when the lift got to about 8 feet high the inner tube collapsed. Shipwreck!
The moral of the story is that hydraulic form pressures mount at approximately 150 pounds per square foot for every vertical foot of formed height and sonotube does NOT resist crushing as well as it resists tension. Probably at least partially because the outside of the tube is not water resistant.
The other moral of the story is that Blacksmithing is way more fun that concrete.