Please excuse the crudity of the picture. I was using a touch pad to draw it.
The idea is the warmer gases would stay in the top pipe moving more heat into the bench. Lower temp gases would exit out of the bottom pipe. Your basic bell principle.
An alternative would have the gases coming in at the top pipe instead of the lower pipe.
Glenn Herbert wrote:I think the main question would be, what is the width of your proposed bench? If it will be only 18" wide, your idea might be the only practical way. If 24" or more, you can make an actual bell easily, probably be more effective, and possibly cheaper depending on what you have for building the bell space.
I'm still working on dimensions for the bench. I'm going to be working with COB for the mass surrounding the vent pipe. The bench will have an insulated cement base with Rockwool or EPS insulation imbeded in the bottom of it. It will have a stone face for the front and a cement "GFRC" bench top that will match the kitchen countertops. The cement countertop seat will cover the cob for heat transfer but will be supported by a steel frame.