Vortex Stove

[Jura]

Nov 1, 2020 10:06:49 GMT -8 Vortex said:

Today I removed all the ceramic fiber board and blanket from the afterburner and rebuilt it with a double layer of vermiculite board. Also put a small piece of ceramic glass in the hottest part of the afterburner..

Hi

Have you happened to check How the change influenced the emissions, efficiency ?

[Vortex]

Hey Rafał,

There was no noticeable difference to the testo results when I changed over to vermiculite board, (which is still perfect showing no signs of degrading) but I have made an interesting discovery since, the squarer the top chamber exit port is the cleaner it seems to run.

I was making an adjustable exit port to see the effects different sizes have in real-time, rather than one fire at a time, where there are too many other variables. I made the adjustable exit port using a sliding piece of vermiculite board on it's side, connected by a length of rigid wire through a hole in the front of the stove. The port had to be higher and narrower to accommodate the design. To my surprise even with it set to the same 78% CSA as before, it was much harder to overfuel, the double vortex seemed to want to stay in the rear of the afterburner, and even when it did overfuel didn't produce as much carbon monoxide as before.

I thought it might be the slightly larger volume of the top chamber, as I had to make it a bit bigger to fit the squarer port in, but I tried increasing the volume more and it didn't seem to help, made it worse if anything. I then took a grinder to the exit port and managed to get it slightly squarer again: 131mm x 106mm, and it was even better. Unfortunately that's as square as I can go without major redesign work. I cant understand why it works though, so if anyone has any theories I'd like to hear them.







Adjustable exit port fully open:



Testo results:

[cork]

Maybe when stove is cool draught is fairly even across sectional area bar laminar resistance near surfaces. When stove is hot draught is strong near the top of sectional area and with square port there is less top section and more resistance if that makes sense?

[fishalive12345]

Something to do with vibration. I have zero knowledge about this and it may be obvious anyway. Satamax? someone? posted a very cool flow visualisation film (discussion of the Coanda effect) from around the fifties in which you can see how the double vortex is formed by pulsations coming through alterating edges of the long thin port. This affects the gas flow downstream of the aperture but maybe the shape of the aperture can also affect gas flow upstream.

Trevor, I was wondering if changing the depth of the top box has had a noticeable effect on the temperature of the cooktop. Looks like changing the shape of the exit port to make it more square will be limited by the placement of the port anywhere except on the top.

[Vortex]

Time for an update.

I managed to make the top chamber exit port square without too much redesign work. Had to slope the edge down to it, but it's good enough for experimenting with. The effects I noticed with the 131mm x 105mm exit port seem more pronounced with a square port (117mm x 118mm).

Thanks for the above suggestions guys. James, I like your idea, but the effect is noticeable right from the start, which I wouldn't expect if it was temperature related - I think you might on the right track though. Brian, I haven't noticed any change in the temperature of the cooktop. It's definitely a resonant system, so it could be that. It could be that the total length of the edges of a square port are shorter than a rectangle of the same area, so will have a little less friction, but if that was the case a slightly larger port should have the same effect.

The job of the top chamber exit port is to convert the force of the expanding gases (from the firebox being injected into the afterburner) into spin and not forward motion through the system. Also it limits the gas speed which stops the system from runaway overfuelling. Normally an exit port small enough to perfectly confine the vortex to the rear of the afterburner will have the downside of slowing the whole system too much, which means less force to spin the gases up. Conversely a larger exit port doesn't have enough resistance to the forward motion of the expanding gases, so doesn't convert it into spin well, and is prone to runaway overfuelling. My favorite theory is that the square port makes a longer more convoluted path for the gasses, that somehow creates more resistance without actually reducing the gas flow. I need to do some experiments to see if it holds up.

[Karl L]

One thing that comes to my mind is that with the longer/flatter exit port there is a shorter path length for the gases on the left compared to on the right. The square port equalises this a bit. I'm not sure why this would give the effect you describe, though.

But maybe in this part of the stove there is a shape-dependent and very non-linear relationship between pressure and flow rate and the new layout changes this relationship?

Perhaps it's possible to purposely design a shape that lets low velocity gases pass very easily, but becomes much more resistant as gas velocity builds up? -- a bit like a Tesla valve?

[Vortex]

Thanks Karl, That's interesting, I hadn't heard of the Tesla valve before. The path of the gases around the new square exit port and the chimney, are similar to the 'S' shaped path of the reverse flow in the Tesla valve picture on Wikipedia.

The shape of the top chamber with that step down in front of the exit port now looks like it would induce a spin into the gases before they go through the port, wonder if that's part of it.

Since I made this discovery the average %O2 has been higher on every burn. Used to be around 10.5-11%, since the change it's been 11.8-12.3%. You'd expect to see that with a larger exit port (as the faster the gas stream the more air is drawn over the fire), but I have the same area port size with a more arduous path.

[Jura]

Any new experiments, observations, thoughts to share ?

[Vortex]

Hey Rafał,
Not much new. I'm still doing daily test runs. Been having some problems with the signal from the thermometer on the testo probe dropping out randomly, same as Peter's was. As far as I can tell it's caused by a bad design in the connection of the thermometer into the probe. It has 5 prongs that just insert into 5 metal tubes soldered onto a small PCB and form a tenuous connection. It's also temperature sensitive due to the expansion and contraction from the heat. Probably works fine when new but the more often it's removed for cleaning the more wear there is.

Still trying to figure out why a square port works better. Unfortunately it's very tight in my top chamber and I'm quite limited in what I can change without reducing the CSA somewhere. If I had more room there I could create more of the Tesla valve type path, so nothing conclusive yet. The ribs in the surface of the vermiculite board may be adding resistance, so I'm considering removing the firebricks that form the back wall of the top chamber and putting in some thinner ones, so I can have a layer of vermiculite board over the surface with ribs perpendicular to the flow.

Also been experimenting with defuser screens on the inside of the primary air to spread out the air flow a bit, so it's not all blowing on one spot which can lead to overfuelling if there are gaps between the firewood there for it to blow up through. Playing with some ideas of a larger primary air or maybe an air-frame, but would have to push the core back an inch or two to make room for that though...

[Jura]

Jan 9, 2021 16:11:37 GMT -8 Vortex said:

Still trying to figure out why a square port works better. Unfortunately it's very tight in my top chamber and I'm quite limited in what I can change without reducing the CSA somewhere.

Unfortunately there is no response from my professor. And it would be unbecoming to mail him again. (he is rather oldshool mannered)
I tend to agree that in your case it may be just a bit longer way for the fumes. but...
I'm still trying to understand fluid mechanics and kinetics but... it's kinda... hard as mathematical description of non steady state processes are presented in form of numerical methods. of which understanding (without a graphical presentation ) are a bit out of my remit.

There is opensource freeware software OpenFoam for allowing graphical representation of the solutions. with some nice tutorials on YT.
But somehow I didn't have time to learn it. (I still can not understand that ppl get border or do not know what to do with free time...)

[Vortex]

Another possibility is that the changes I made to the shape of the top chamber (to fit the square port in) have altered the gas flow and that is what is causing the effect.
Previously the top chamber was the same height throughout and the exit was along the left hand side, between the back of the stumbler/afterburner exit and the rear wall. Now the top chamber is a lot deeper at the rear and the square port is at the far rear left side. The stumbler/afterburner exit is only slightly bigger than the firebox port, and so has a similar effect on the gases passing through it. Previously that would have been a linear motion propelling the gases through the system, whereas now because of the change in the shape of the chamber all that energy is being converted into spin. That could be why the double vortex is a lot more inclined to stay in the rear of the afterburner, as there is less pull on it from the top chamber.

I removed the piece of vermiculite board in the bottom right of the picture below, it seems to run best like that even though there is less insulation on the afterburner ceiling:



Previous layout of the top chamber:

[martyn]

The vermiculite still looks like new, it is obviously holding up!
I have read many post about how vermiculite won’t last for any length of time in the fire box or riser but perhaps that is not always the case as you are proving.
Potty on his you tube channel mentioned that his fire box vermiculite pieces (not the floor) would last two whole seasons before falling apart .
I think Peter use vermiculite on the risers of his 4’’ J tubes?

[Vortex]

It's getting 900C-920C daily. The largest piece that forms the roof of the afterburner has some very fine hairline cracks in the bottom surface now, but it still has a good ring sound to it, so I'd be surprised if it didn't last a season. It's about a quarter the price of CF board of the same size, so wouldn't be too expensive to replace once a year. Some of the small pieces at the front sides behind the glass have been in there for two and a half years. Seems to be fine so long as there isn't too much differential expansion across it, so smaller the better. Still want to try out a kiln shelf.

[martyn]

I have started work on my new Vortex stove!
I have taken out the old stove so there is no turning back now, i want to keep the rooms steam punk theme and i have a few ideas but at the moment i am in a quandary about the afterburner shelf.
I have available to me 25mm ceramic board, 20 mm vermiculite board or ceramic glass.
I cant find any kiln shelves for sale that will deliver to my address .
I also have some 5mm ceramic paper that is interesting stuff, it is like flexible cardboard and does not seem to produce any dust. I have considered using this underneath some ceramic glass.
I have 32mm fire brick or 28mm cast refractory.
I am thinking the 25mm ceramic board coated in water glass would be the long lasting answer but I would prefer something as thin as possible, i could cut the board in half ?
I have a couple of days to make my mind up!

[Vortex]

Why do you want the shelf so thin? I would go for the ceramic glass with the vermiculite board on top. I have some of that 5mm ceramic card, it falls to bits quite quickly if it gets to afterburner temps. If you make the cooktop so it can be lifted off, then it's very easy to lift out the shelf and replace it with something else if needs be.

I've been experimenting with different stumblers to see if that triangular one of mine can just be replaced with a rectangle. It seems the internal surface area of the aperture has an effect on gas flow, so a 2" wide rectangle of the same size makes the system run slower as there is more resistance. It's possible a slightly larger aperture would compensate for this but I haven't tried that out yet.