Wondering if we could figure out a different way.

Hi all, the more i think about rocket stoves/ rocket mass heaters, the more i get an itch that perhaps theres a way we can change things to work for different peoples applications.

I've read many different posts of people who have various reasons for not wanting an RMH. Some don't like the look of them, some peoples floors cannot handle the weight, some don't want to babysit it for the 1.5-2 hrs of burn time needed, and some don't want to risk voiding their insurance. These are all valid points to have, and i wouldn't fault anybody for any of these reasons.

When i was in school for my mechanics trade and we were learning about engines, we learned the three Ts of good and efficient combustion:

-Time
-Temperature
-Turbulence

In an RMH, it seems that we achieve extremely clean combustion by only utilizing 2 of these 3 Ts. We use turbulence in the burn tunnel and riser, and we use extreme temperature by means of insualtion. In other words, we burn hot and fast. In my opinion we are probably actually pushing the temperature part higher than we have to in order to make up for the lack of the "Time" factor.

With all the great minds here, it would be curious to see if collectively a design could be made to burn efficiently with all three Ts in place. In other words, a very hot, very clean, but SLOW burn. Perhaps I'm wrong, but i feel like if we slow things down, we may not require the extreme temperatures we are currently using. The current design is essentially an oxidizing flame, which anyone who has used a cutting torch knows is the hottest flame, even hotter than the perfect mixed flame. What this hot oxidizing fire allows is for the burn to be clean in spite of the fact that its burning FAST! Remember, we arent burning clean because we are burning fast, in the science of combustion slow is your friend, what we've essentially done is found a way around the fact that we've almost completely eliminated one of the Ts from the equation.

If something like this could be figured out, a way to burn hot and slow, it may allow other people who can't or won't build a convential RMH to have a cleaner/ more efficient option than on the market woodstoves currently out there. If we could make something that burns clean and slow, it may eliminate the need for quite so much mass that is always a big concern for people and their floors.

Anyways, just a couple thoughts that i figured I'd write down to get them out of my head.

A lot of European style stoves are quite advanced, they have to pass quite strict safety and ani pollution levels.
Most use secondary air and are lined with vermiculite.
I made this thread https://permies.com/t/208777/European-box-stove

Rocket mass heater cores do use all three T's. The reason for the tall riser, aside from generating draft, is to give the flames time to burn fully while they are inside the insulated channel. In my RMH and experiments I have done, once the core is inside an enclosure and not just open to the sky, the flames seldom reach the top of the riser but are done burning before that.

I've mentioned using Tesla valves in the system to slow exhaust in another post but the consensus was that may restrict air pressure too much; to my knowledge, nobody has actually tried it yet.

Maybe I'll give it a go, I'll be visiting Wheaton Labs this October.

What if there was a sliding plate so that after you get the heater going the plate drops in then the exhaust gets diverted through the Tesla valve, maybe poke some holes in the plate so that backpressure air can move through the "ignition" exhaust port but the majority goes through the tesla valve. Then warm air is trapped in the mass longer and the ignition airflow problem is reduced.

Kind of a throttle on how air is drawn through the system.

I think the issue with what you are describing is that when a given amount of wood is heated to the proper temperature for combustion, it becomes a gas at a certain rate. Air must be fed to the fire at a certain rate to keep up with the off-gassing of the wood for clean combustion. The only practical way to keep the mixture correct and draw out the time of burn for a given amount of fuel I can think of is to use the smallest core possible.

We are using all three Ts, but the time part is served by the mass. Hot, fast, clean combustion with some means of storing that short-term burst of heat energy in a form that is useful to the space and inhabitants over a longer timeframe.

The core is optimized, any improvements will probably come from messing with the air pulled through the system or the design of the mass by adding a little complexity.

I really think that if inside the thermal mass there was one exhaust pipe that goes directly through the mass and a second through the mass that contains a Tesla valve.

A switch could be added so exhaust can be toggled between the two exhaust pipes.

So when first igniting the RMH there is enough back pressure to get it going hot and fast.

Then after it is going strong you could "dim" the airflow so it burns slower and the exhaust is trapped in the mass longer.

Hypothetically. I'm sure it would take some balancing and trial and error. I know cold starts are already a problem.

Note: a Tesla valve may be overkill, a second pipe of a different diameter could be sufficient?

Actually time is taken into account 2 ways.  The thermal mass acts as a battery giving the heat long term to the home.  Height of the heat riser generating a sufficient burn time.  And it isn't really an oxygen rich flame.  The physical structure acts to limit it to just barely oxygen rich flame.  Look at the NOx data.  On oxygen rich flame generates lots of NOx.  A RMH running properly produces very little.

T Simpson wrote:I've mentioned using Tesla valves in the system to slow exhaust in another post but the consensus was that may restrict air pressure too much; to my knowledge, nobody has actually tried it yet.

Maybe I'll give it a go, I'll be visiting Wheaton Labs this October.

How is a Tesla valve going to add value??  If you slow the gas down you limit pipe length which reduces surface area which reduces the amount of mass that can be heated.

C. Letellier wrote:
How is a Tesla valve going to add value??  If you slow the gas down you limit pipe length which reduces surface area which reduces the amount of mass that can be heated.

Well if you keep the pipe length the same but have a Tesla valve say in the middle wouldn't slowing the gas down mean more time in contact with the mass which means more heat transferred?

I'm thinking a Tesla valve could be made of cob and lined with foil.

C. Letellier wrote:Actually time is taken into account 2 ways.  The thermal mass acts as a battery giving the heat long term to the home.  Height of the heat riser generating a sufficient burn time.  And it isn't really an oxygen rich flame.  The physical structure acts to limit it to just barely oxygen rich flame.  Look at the NOx data.  On oxygen rich flame generates lots of NOx.  A RMH running properly produces very little.

You can't make the thermal mass the T for time. We're talking about combustion only. The thermal mass has nothing to do with the combustion process, only the heat storage process. Granted the heat riser does add time, but it also adds height and takes up space in the home. The thing i was trying to get to was something that could be used by people that can't have an rmh in their home. If you cant have a big thermal mass in your home, than the only other option is slow the burn even more so that it can have more time to radiate into the room before exiting the flue.

By design an rocket uses wide open air, obviously ypu can't choke this off to try and slow it down otherwise we end up with a smoky and dirty burn. What about somehow controlling the fuel supply? Just as a diesel engine has wide open air at all times by design, it controls its fuel quantity instead to throttle things. What if somehow we could expose less fuel to our wide open air in order to have a slower yet still complete burn, by not throttling our air we would always have ample supply to complete combustion, no idea how it would be done, just thinking out loud.

Phil Stevens wrote:We are using all three Ts, but the time part is served by the mass. Hot, fast, clean combustion with some means of storing that short-term burst of heat energy in a form that is useful to the space and inhabitants over a longer timeframe.

Phil you cant count the mass as the t for time as it has no effect on the combustion process.

Jordan Holland wrote:I think the issue with what you are describing is that when a given amount of wood is heated to the proper temperature for combustion, it becomes a gas at a certain rate. Air must be fed to the fire at a certain rate to keep up with the off-gassing of the wood for clean combustion. The only practical way to keep the mixture correct and draw out the time of burn for a given amount of fuel I can think of is to use the smallest core possible.

Jordan this is a good point, as i mentioned in one of my replies above maybe it would be possible to expose less fuel to the hot zone at once in order to lessen the off gassing without sacrificing the amount of air needed for complete combustion

Without the mass it's not an RMH. If we're talking about the parameters of a system, let's consider the entire system. I don't think anyone here is trying to heat spaces with just a rocket combustion core.

Justin Hadden wrote:

C. Letellier wrote:Actually time is taken into account 2 ways.  The thermal mass acts as a battery giving the heat long term to the home.  Height of the heat riser generating a sufficient burn time.  And it isn't really an oxygen rich flame.  The physical structure acts to limit it to just barely oxygen rich flame.  Look at the NOx data.  On oxygen rich flame generates lots of NOx.  A RMH running properly produces very little.

You can't make the thermal mass the T for time. We're talking about combustion only. The thermal mass has nothing to do with the combustion process, only the heat storage process. Granted the heat riser does add time, but it also adds height and takes up space in the home. The thing i was trying to get to was something that could be used by people that can't have an rmh in their home. If you cant have a big thermal mass in your home, than the only other option is slow the burn even more so that it can have more time to radiate into the room before exiting the flue.

By design an rocket uses wide open air, obviously you can't choke this off to try and slow it down otherwise we end up with a smoky and dirty burn. What about somehow controlling the fuel supply? Just as a diesel engine has wide open air at all times by design, it controls its fuel quantity instead to throttle things. What if somehow we could expose less fuel to our wide open air in order to have a slower yet still complete burn, by not throttling our air we would always have ample supply to complete combustion, no idea how it would be done, just thinking out loud.

First off we need to talk terminology.  RMH is rocket MASS heater.  You are talking rocket stove heater which is a completely different design.   Suggest researching Rocket stoves.  There is a bunch of good info here on permies and a lot of really good info on the donkey boards.  Donkey Boards  as well as many other places.  Look at the right type of design for what you want to accomplish

Second in a pure form you are right that only the flame front counts in the time and thus the combustion.  But the comparison to the diesel engine is unfair because you are looking for different output goals.  The engine you don't care about the heat gain efficiency.  But the stove if you pull more air thru without as much burn you are actually quenching your heat and reducing your delta Temp which reduces efficiency.  The amount of heat you can extract is dependent on that delta Temp.  More heated air up the chimney is heat loss because you paid in fuel to heat that air.  Just guessing but will bet for your goals a lot of feet of stove pipe radiating into the house will be more effective than a tesla valve.  The valve's goal is better flow one way than the other.   You don't need that much complication.  You want to prolong the heats time in the pipe.  That is longer pipe, heat bell etc.  The valve running backwards gets you some of that at the cost of a lot of drag.  Running forward it would just be a long pipe.  The key point you miss is that a true rocket mass heater is totally dependent for its efficiency on that delta Temp. and its ability to max that out is dependent on the heat stored in the mass so in a less pure form of thought that time does count in the equation.  It still doesn't reach what a good condensing gas stove does but it comes close for that delta Temp.

Then an aside.  My crazy dream rocket mass heater does include a tesla valve but it is on the water part of the set up to aid in being sure the water flow only ever goes one way for a convection loop.  I want to build the barrel with a large water tube spiral around it 3 or 4 times as part of the wall of the barrel.(not just wrapped around the barrel but actually part of the wall of the barrel.)  Probably 1 1/2 inch or 2 inch pipe to have enough water to avoid squish boom with it going into a non pressurized tank.  Bring the colder side of the water in thru a forward tesla valve and then into the spiral pipe up the barrel with a fair long riser pipe going up.  If it produces steam bubbles in the spiral route them to the middle of the riser pipe to act as an air lift pump.  If it ever gets hot enough to really percolate the valve would be to keep it from trying to ever back flow.  Between the valve and head pressure the flow should only ever be one way.  Put this on a huge tank of water heavily insulated with a fair amount of vertical height for stratification.  In winter it would help heat household water while heating the house.(most of the heat from solar)  In summer activate a mass bypass valve in the chimney so the mass isn't heated so the stove can be cooked on.  Wrap the barrel in a series of high temp insulating panels in summer so it doesn't  heat the house either with the hope that the water will cool enough to maintain draw with the drag of the mass part of the chimney not in the circuit.(efficiency will suck but it should still draw just like any stove and since the only goal is cooking efficiency this time of year is of less concern)  Yes we all have our crazy RMH thinking.  This is one of two of mine.

I've long pondered over a primitive way to throttle the aperature for secondary air.  Recently, while working on a 1965 Chevrolet Corvair engine, I took notice of how the cylinder heads make use of a bimetallic spring connected to an actuator rod that travels through a metal sleeve.  As the spring warms, it begins to open and this action pushes the rod, which in turn pushes a small metal valve open.  It could work much the same way in a RMH as the secondary air isn't desired until the stove is warm.  The hotter it gets, the more air would be allowed into the system, and as it cools, the secondary air is restricted until it closes completely.  All automated, with zero electronics.