Vortex Stove

[tferr]

Thanks Trev its starting to sink in...its making me rethink a couple of things...one being in your stove you have the heat storage bricks acting as a flue to the chimney. This would be helping maintain the back pressure correct? I had planned on that area to act as a bell which would then exit from near the bottom into a bench this is where I wanted to capture the most of the heat. Now I am concerned I may not have enough back pressure to maintain the vortex

Is there a reason the 60mm of vermiculite board used in the top chamber is made up of 3 pieces. I can only purchase either 1 inch or 1.5 inch if I use one of each I get a total of 63.5mm

[patamos]

Nov 30, 2021 14:06:08 GMT -8 Vortex said:

Why is the top chamber surface area to volume ratio important? Over the last 3 seasons I have been mostly experimenting with different size, shapes and configurations of the top chamber, stumbler and exit port. When I built the first experimental model in my garden in summer 2018, I discovered that a restriction made it more stable and less prone to overfueling. Originally this was the gap between the front of the afterburner shelf and the inside of the glass, but was later moved into the top chamber as the glass was getting burnt. A restriction only works so far though, as over a certain level the bottleneck starts to slow down the gas flow too much, which itself makes the stove overfuel from lack of air.

In December last year I noticed that making the top chamber exit squarer, somehow made it work better. It turned out that in the process of changing it around to fit in the square port, I'd unintentionally increased the surface area of the top chamber, and that was what was causing the effect. I experimented with various ways of trying to make use of this discovery, but it wasn't until late last winter that I realised that I could just do away with the restrictions of the stumbler and exit port altogether and use the resistance of the increased surface area instead. Earlier this season I experimented with increasing the SA:V to the maximum possible level to see what that would do, it created a very stable burn, but also slowed the gas stream down too much. By increasing the primary air and gradually reducing the top chamber surface area, I managed to find a perfect balance that holds the vortex nicely in the afterburner and resists overfueling and thermal runaway.

I think this discovery is the crux of the biscuit.  For anyone building a vortex stove you will have a unique set of variables, eg. downstream restrictions and harvesting dynamics, chimney height relative to roof and surrounding vegetation, altitude, etc...  So playing with the SA:V ratios in the top chamber is the way to dial flow rates up or down, and for those of us without testo or condar PM gear... the shape/pattern of the double rams horn is a good indicator of how clean the burn is.


Trev, you are a great researcher and teacher.

[tferr]

Nov 30, 2021 21:23:02 GMT -8 patamos said:

Nov 30, 2021 14:06:08 GMT -8 Vortex said:

Trev, you are a great researcher and teacher.

Agreed and appreciated 

Tom

[Vortex]

Thanks guys.

Math is not my strong point so hopefully the numbers are correct. SA:V seems to be usually written as a fraction, with the surface area over the volume. I think for our purposes an equivalent length of round pipe can be taken as 1 SA:V, (same as we use 1 CSA), and then however much the top chamber of the same length differs from that being the SA:V number, as in 1.43 SA:V in mine.

This adaptation makes it very stable and easy to use. Observing the vortex and making sure it stays in the afterburner is a reliable way of judging how clean the burn is during the flame phase, and the longer you can keep the afterburner lit during the coaling phase the lower the CO. will be then as well.

Nov 30, 2021 16:25:24 GMT -8 tferr said:

Thanks Trev its starting to sink in...its making me rethink a couple of things...one being in your stove you have the heat storage bricks acting as a flue to the chimney. This would be helping maintain the back pressure correct? I had planned on that area to act as a bell which would then exit from near the bottom into a bench this is where I wanted to capture the most of the heat. Now I am concerned I may not have enough back pressure to maintain the vortex

Is there a reason the 60mm of vermiculite board used in the top chamber is made up of 3 pieces. I can only purchase either 1 inch or 1.5 inch if I use one of each I get a total of 63.5mm

That may be the case with your mass but it looks like we have the answer to it now in varying the SA:V and observing the vortex until it's correctly balanced. Would be interesting to see how the SA:V varies on different systems. Stick with the bell, I only have the descending channels like that as I'm very limited for space and had to squeeze as much mass as possible into a very small space.

You don't have to have 3 layers in the top chamber, it's just what I have in mine, the dimensions are the important thing. I just wanted to show exactly what I have, as the limitations of my stove mean I am unable to test out how important or otherwise some of those features are. I'm pretty sure though that the SA:V is the important part and that's down to the dimensions.

[hof]

Dec 1, 2021 6:26:26 GMT -8 Vortex said:

What height of chimney of your stove?
Do you use secondary air feed?
May I use chamote 20 mm thick for afterburner floor or better to use vermiculite/CFB/IFB?

[Vortex]

My chimney is 4 meter from the bottom of the stove.

I used to have secondary air around the sides of the door, but have recently found that it works better instead if you just increase the primary air at the bottom of the door to 50% and cover it with a mesh screen so it's about 30% when fully open. The position across the whole bottom of the door means that all the air has to travel up across the fire, and the screen diffuses and mixes the air into the gases. (ATM my air control is just held in place by 2 magnets until I get around to doing it properly)





I also have a small amount of air up through the ash trap in the middle of the V-shaped floor, but only <3% and only after the fire has peaked. This helps a lot with the coaling phase.

You can use dense refractory for the afterburner floor but not the other parts.

[hof]

Dec 2, 2021 3:21:49 GMT -8 Vortex said:

Is it possible to make opening on top of the top chamber? Not on side, as on your stove. Will it make difference on quality of combustion?

[Vortex]

I don't know for sure as I've never done it. I would think that if you maintain the full length of the top chamber before the exit in the roof then it should be OK, as it's the surface area that's important.

[Karl L]

Dec 2, 2021 3:21:49 GMT -8 Vortex said:

I used to have secondary air around the sides of the door, but have recently found that it works better instead if you just increase the primary air at the bottom of the door to 50% and cover it with a mesh screen so it's about 30% when fully open. The position across the whole bottom of the door means that all the air has to travel up across the fire, and the screen diffuses and mixes the air into the gases. (ATM my air control is just held in place by 2 magnets until I get around to doing it properly)

Thanks for that, Trev. Do you still have the 30mm high threshold, in the firebox, just inside door?

I looks like the mesh you are using is around 7mm x 5mm aperture - is that right?

[Vortex]

The threshold is still 30mm high and in the same position. I do plan to try it half an inch farther in from the door some time soon.

The mesh is a type called expanded metal, this bit came from an old air filter. The size of the diamonds seems to be around 9mm x 4.5mm, possibly imperial 3/8" x 3/16". I looked it up online to see if I could buy some more, and the one I found the same size said it was 55% open area.

I had a piece that I used last season over the smaller primary air hole, that was from a gas cooker grill, used to make the glow for the radiate heat. After 1 season it looked like it would survive another year and it was pretty old to start with.

[patamos]

Trev, your discovery regarding air intake low across the front echoes (somewhat) the design Alex Chernov arrived into for his masonry heater fireboxes.  He places a Pisla 4" x 16" grate across the front floor, and a much smaller one 2" x 7" across the very back floor.
I've built a few heaters this way now and find it burns pretty much as clean as an Austrian eco-box - with the added advantage that during coaling phase you can rake the coals forward and back onto the grates for that turbo effect. 

We've been tweaking the air flows on that last vortex I built.  Lots of downstream harvest and restrictions to account for, but coming along.  Will send a video soon

[Vortex]

Interesting about the Pisla grates, is that what Alex uses for the primary and secondary air supplies, and is the air control on the ashbox door?

I like the separate supply to the bottom of the V-shaped floor grate as there's no opening of the door or coal raking involved. I do get a few coals falling down behind the primary air but the mesh keeps them in and the fast primary air moving over them burns them up quite quickly.

[patamos]

Primary and secondary all in one. Air control in the ashbox door or with a damper on external air supply routed through the ashbox. There also tends to be a small volume of air intake low on most Pisla doors, mostly as a 'airwash' for the glass, but it amounts to about 1"sq at best.

As with many masonry heater builds for clients, Alex (and many of us really) try to keep the amount of fire tending to a minimum. It can be a challenge enough getting people to store wood properly and load/light effectively...

Alex came up with this firebox design (see: www.stovemaster.com/html_en/drawing_for_self.html) in response to recent pressure to build eco-boxes (with air vents all up and down the side and back walls).
As with all kinds of 'group think', building officials in Europe saw that the Austrian ecobox was getting slightly better numbers than bottom air feed systems, so began mandating eco-boxes. Trouble is they are relatively difficult to build and also quite fragile. Further, every seasoned heater builder knows that how we load and light in a big firebox (top down with drywood etc...) is a much bigger factor in combustion efficiency.
Also, with a solid flat floor the eco-box fares poorly in the coaling phase, so they advise closing off the air supply to starve that phase of air, and use the coals in the next burn. Works well in theory, but once your door gaskets are a little worn... or if operator forgets to close air supply at that stage... well, that is when the air from a v-floor works way better.
So, all that to say, when various real-world factors are weighed in... including the likelihood that many operators are just going to 'load, light and leave'... the real-world improvements with an eco-box are a bit of a mixed bag.

So, Alex, being a no-nonsense kind of guy, and being a very experienced builder who has seen and tried all kinds of designs over the years, came up with this design and had it tested with Norbert Senf's Condar PM analyser, to discover it is pretty much as clean as an eco-box thru the flame/burn phase - but waaay more rugged and easier to build.

Alex is also the fellow who has worked harder than anyone to promulgate the Russian Bells in masonry heaters (see: www.stovemaster.com/html_en/designsystem.html); and has led a number of studies on the relative harvesting efficiency of bells vs flue runs.

btw. he is also the distributor for Pisla hardware in Canada

Can you tell I have a lot of respect for the guy?

[Vortex]

I helped a guy rebuild his ecobox stove a few years ago. All the internal castings were cracked and falling apart. My guess was that it was the result of the numerous secondary air holes all over the castings - heating one side of a refractory while cooling the other seems bound to stress them a lot more. Within a few months of the rebuild the same was already starting to happen again.

I'm also a fan of Russian bells, but unfortunately down at the small scale of my stove mass they don't really work well, as the gas turbulence overcomes the stratification of the gases. I think the reason I can get away with having 13 90 degree bends in just 16' of 1 CSA channels, is that it's all in a tiny 0.3 cubic meter / 10.8 cubic feet of mass that weighs 1 ton and has 3 meters of ISA. All those bends break up the laminar flows improving the transfer of heat into the masonry and the channels maximize the contact of the gases with the available mass. It's amazing how much heat it can suck up for it's size, 900C at the afterburner down to 150C at the chimney. It also wouldn't work well on a larger scale though - horses for courses as they say.

[tferr]

Nov 30, 2021 21:23:02 GMT -8 patamos said:

Nov 30, 2021 14:06:08 GMT -8 Vortex said:

Why is the top chamber surface area to volume ratio important? Over the last 3 seasons I have been mostly experimenting with different size, shapes and configurations of the top chamber, stumbler and exit port. When I built the first experimental model in my garden in summer 2018, I discovered that a restriction made it more stable and less prone to overfueling. Originally this was the gap between the front of the afterburner shelf and the inside of the glass, but was later moved into the top chamber as the glass was getting burnt. A restriction only works so far though, as over a certain level the bottleneck starts to slow down the gas flow too much, which itself makes the stove overfuel from lack of air.

In December last year I noticed that making the top chamber exit squarer, somehow made it work better. It turned out that in the process of changing it around to fit in the square port, I'd unintentionally increased the surface area of the top chamber, and that was what was causing the effect. I experimented with various ways of trying to make use of this discovery, but it wasn't until late last winter that I realised that I could just do away with the restrictions of the stumbler and exit port altogether and use the resistance of the increased surface area instead. Earlier this season I experimented with increasing the SA:V to the maximum possible level to see what that would do, it created a very stable burn, but also slowed the gas stream down too much. By increasing the primary air and gradually reducing the top chamber surface area, I managed to find a perfect balance that holds the vortex nicely in the afterburner and resists overfueling and thermal runaway.

I think this discovery is the crux of the biscuit.  For anyone building a vortex stove you will have a unique set of variables, eg. downstream restrictions and harvesting dynamics, chimney height relative to roof and surrounding vegetation, altitude, etc...  So playing with the SA:V ratios in the top chamber is the way to dial flow rates up or down, and for those of us without testo or condar PM gear... the shape/pattern of the double rams horn is a good indicator of how clean the burn is.


Trev, you are a great researcher and teacher.

Do youthink there would be any advantage to making the ISA adjustable from the exterior of the stove.
Either by a verticle louver type systemor by a cam lobe lift system .....crazytalk?