I've watched a lot of videos and tried to read through the forum, but I don't understand yet why the barrel around the heat riser is exposed. Wouldn't this be a prime candidate for adding mass? I would think that you would want to basically use the barrel as a mold and encase it in cob, especially at the top, where the greatest heat occurs.
Understanding the thermal flows within the barrel/aka bell are critical to understanding the way the rmh actually works and answers your question.
It's easy to understand how the flame wants to go up the riser, but the reason the flow then wants to turn down relates to the critical part of your question. The heat from the flame starts to dissipate through the barrel's radiation, cooling the exhaust and this cooler air wants to fall to the bottom of the barrel and creates the push on the exhaust out of the barrel and through the bench.
If the metal barrel is insulated with cob or whatever, the gases will not flow as efficiently and as the heat builds up in the bell/barrel they will eventually stall altogether.
The masonry bells of the newer batch burners have sufficient mass and interior surface area to absorb heat for a long time supplying the cooling to the exhaust, just as the heat radiating metal barrel used to cool the gasses for the original j tube rockets. But even so they can reach a heat saturation point (depending on specifics of the design) and after a few batches of wood reach internal temps that would slow or stop airflow through the bell,--- again leading to smoke back in the room.
There are lots of design factors that can work together to make or break the operation of any individual RMH. The push on the gasses out of the barrel can make up for deficiencies in other part of the system, or if it is sufficiently weak can be the straw that breaks the camels back.
So there is no one answer as to how much of the barrel can be covered, but I would start with a naked barrel when firing a new system, and once it was dry and working well, think about adding mass to the barrel, but be on the lookout for smoke back on lengthy burns, then shorten your burn times or take away some cob.
no real maximum space, and minimum space has to do partially with the cross section of the riser, but also has to do with the manifold or exit from the barrel
The gases need to be able to flow freely out of the barrel so some manifolds or plenums could require up to three times the area of the riser depending on those dynamics, sorry I don't have the link offhand, but there is a specific and rather complicated formula-- or like I did years ago, I looked at my manifold design as if I was watching water flowing out of the barrel space and tried to shape it into the smoothest transition possible, like a funnel narrowing down to the specific exhaust pipe size with no obstructions to the flow.
If the space between riser and barrel is too narrow, it could interfere with that smooth transition and require a larger "funnel to make up for the obstructions.
I have pretty much the same surface exposed, as a barrel. Tho, the gas movement is far freer. What i found, is that i feel there is too much "radiator" area. Tho, now that it is starting it's third winter. and that i live above. I like it more. Tho, if i had to redo one of this type. I think, i would take it the other way. A big mass bell above the heat riser. So there is a huge temperature differential between the mass and the hot gases. So they heat more. And if i needed quick heat, i would put a "radiator" metal bell, before the chimney.
What i see in this arrangement:
If it's too hot, and you need heating the mass à bit more. You could bypass the metal bell.
Gases entering this bell are cooler. So the place doesn't overheat as easily. This, allowing you to heat more the mass.
You're less at risk of burning yourself badly with a cooler "radiator"
I agree, the barrel/radiator is a pusher, not a sucker
Any sucking being done is at the base of the riser , and the hot gasses flowing up the riser pull the flame from the firebox. In the j tube it's easily seen as the fire burns sideways through the burn tunnel, but if you don't first start the heat rising in the riser, or at least down the burn tunnel, starting a cold rocket stove can be a smoky mess.
I think a lot of that misunderstanding has to do with more precise definition of terms and understanding the individual functions of each part of the system
I got this wonderful piece of soapstone that covers the top of the barrel, and that moderates the heat up there, although after a long burn even that can get quite hot. But the circulating water in the coil I placed around the barrel extracts a tremendous amount of heat keeping a very constant cooler temperature air in the barrel, hence I have a very strong push of cooler air sinking to the bottom of the barrel pushing exhaust out of the system.
And for the sake of argument, I will simply remind everyone of the Permies motto about heating water--BOOM SQUISH. Heating water without enough information and safety measures can be a deadly dangerous thing.
So, if I understand this correctly, it would be beneficial to have some kind of mass on top of the barrel, e.g. a stack of bricks or a slab of soap stone? A mass of cob would be rigid and would crack as the barrel expands and contracts, so it needs to be a material that can move.
Has anyone tried building a wall of bricks or some other mass around the barrel, without actually being in contact with it? (Some kind of reflector?) I'm imagining a half circle, going around the back of the barrel, with the front exposed. I'm thinking of a system for my shop, and I don't really have a place to encase a run of ductwork, so I am trying to maximize the mass right around the unit, itself.
Remember all you've learned about rockets and go to the next level.
But more directly to answer your question, I put the soapstone on top just as an indestructible (more or less) top. I knew the top of the barrel would probably burn out eventually (three years and counting) and wanted to be ready if/when it did.
Plus as I explained, I was getting plenty of cooling effect on the sides so didn't need to worry about the insulative effect of the stone on top.
chances are good that using the curved brick wall a couple inches away from the barrel will not interfere much with it's radiation, but if you have limited floor space building a more massive bell out of masonry straight up is an adequate substitute for a bench.
The site above has directions for building a batch burner which is quite similar to a rocket stove, the heat riser works on the same principles but the options of design are expanded beyond the barrel and bench. It also has a fair amount of theory and definitions of terms as well as tables for construction all in one place.
If you want to keep the smaller burn box of the rmh that is possible to use with the alternate bell
Location: Southern alps, on the French side of the french /italian border 5000ft elevation