So you mean to say that the heat-exchange 'barrel' would rather be a cut-in-half air-compressor-tank, with a dome end facing upwards?
(I have to say, the great thing about having a flat surface at the top is the ability to boil a tea-kettle and cook atop it. Seems like it would be trickier to balance a tea-kettle atop a dome... Actually, prehaps you could rig up a little removable grill/stand that could sit atop the dome and provide a flat surface for pots and pans?? Cool!)
As far as I understand, a dome-top should be fine... Perhaps it would even work beautifully. (However, I'm not an expert, and so perhaps someone else could theorize about it better.)
Anyway, I would guess that you may want to focus on the space between the exterior of the heat riser and the interior of the heat-exchange-tank. Maybe you'd want to sort of curve the top edges of the heat-riser so that they run more paralell to the curves of the tank?
You may want to check out the thread that I started recently,
https://permies.com/t/17705/stoves/RMH-CSA-Heat-Exchange-Barrel ... Although, no one has yet responded to my confusion there... So it may just be more confusing. In any case, it seems relevant, and there are some useful quotes from more people.
Another thing to consider is that the traditional flat-top of the heat-exchange-barrel provides for a lot of... heat-exchange. Heat moves by radiation, conduction, and convection (all very different processes). I would imagine that there is a lot going on with convection when it comes to the shape of the heat-exchange barrel: both inside and outside. Of course convection is occuring like a hurricane inside of the heat-exchange-barrel, but it is also occurring across the
outer surfaces: a 'heat-transfer' takes place through the barrel, to the outside air. The air then rises, making room for new cool air to come in and continue the process, and hence an air current of convection is formed: air flowing across the outside of the barrel. So the aerodynamics (the shape) of the barell would seemingly be very important for this convective part of the 'heat-exchange' process. However, I do not know what that means for a dome-topped tank; it would be different.. maybe better maybe worse.. or just plain different.
Another thing: plainly, the heat-exchange-barrel is meant to exchange heat. So I would assume that the material and thickness of the barrel come into play here. Does your dome-topped tank have a uniform thickness on the sides and the dome? Or is the dome thicker? Is it steel? Etc..
I'm hoping that all of this will work out in your favor: right now I'm imagining that the draft will move up through the heat-riser and then very smoothly flow across the domed interior of the tank, on its way down to the exhaust ducts. Seems like the dome provides for a more smooth aerodynamic motion. --- HOWEVER,, on that note, important to the combustion unit (heat-riser + heat-exchange barrel) as a whole are the right angles that it forces the combustion through: these abrupt curves within the combustion unit provide for an intentional turbulence which, by thoroughly mixing the gases, burns the gases really thoroughly. Smooth laminar flow (aerodynamic smoothness) is desired in the exhaust-ducts, but rough turbulence helps inside the combustion unit. (See page 16 of Rocket Mass Heaters by Ianto Evans and Leslie Jackson) ... However, all of this could be negligible (when it comes to the difference between your dome-topped-tank and the traditional barrel): perhaps it's still abrupt enough of a right angle to jostle the gases around and burn them thoroughly.
There's got to be somebody on these forums who has some hands-on experience with dome-topped heat-exchange tanks? Do they burn blacker smoke (from lack of turbulence in the combustion unit)? Are the tanks themselves cooler than the traditional barrel, while the exhaust-ducts and mass get hotter?
Uhhh...
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