I have been reading the book and reading threads in this forum. What I think I understand is that in a RMH the pressure to drive the exhaust through the thermal mass side of the system and out the chimney is provided by two “pumping” dynamics. First there is the combustion in the heat riser, and then the cooling of the post combustion gasses in the drum bell.
The book indicates that the space between the drum wall and the insulated riser should be about 1.5 in. with an interior heat riser diameter of 6 in.
Do I have my facts right?
How sacred are these ratios?
I can deal with numbers when I have to, but in less technical terms to start, just how fixed are these ratios? For example, can all of the volume in the drum be shifted to one side by having the heat riser tangent to the side of the drum? [That is off center and moved all the way to one side of the drum.]
I have several related questions as to the physics of pushing exhaust, especially cooled exhaust, out of the system as it heats the mass. I will likely initiate another thread to those ends.
By "The Book", I assume you mean "Rocket Mass Heaters: Superefficient Woodstoves YOU Can Build" by Ianto Evans. On page 16, yes it shows the "push and pull" effect. And on page 24, he says you can have an eccentric gap for more efficient direction of radiant heat. The diameter of the inside of the riser is dependent on the diameter of the burn tunnel and exhaust pipe, which should have the same cross section area through-out.
I understand what you are saying, and I assume that the that the physics involved hear is based on the volume of the gases being moved. However, I have also read comments by Erica/Ernie that the second pump in the system to drive the gases through the heat storage side is the cooling of of the gases by the radiant heating by the drum. I was also asking if that cooling is affected by the geometry of the volume of the gases as they are pass through the drum.
The volume of the space in the drum [excluding the heat riser and its surrounding insulation] is much larger than the volume of the heat riser itself. I assuming therefore that the heated gases as they emerge from the riser have an expanded volume that contracts as it cools and flows down to the exhaust at the bottom of the barrel. These dynamics are a bit of a mystery to me, but I am sure there are explanations in the physics of the heat exchanges I really do not need to know.
For the moment I am more interested in the geometry of the volume, and not its heat loss. The key factor seems to be the constancy of the cross section surface of the input and the output pathways, and not the relative volume with in the drum. I am curious about how the geometric shape of the combustion unit will affect this flow even if the cross sectional area remain equal.
For example, if I went to a rectangular design for my feed tube and heat riser, and I replaced the drum with rectangular space made of fire brick, how does that affect my flow through the system even if I keep the cross sectional 1:1 ratio of the in and out for the system? [I understand that the lack of gas cooling by the convection from the barrel has an effect, but that is a question for another time.]