Nozzley Rockety

I've been experimenting with a batch style rocket stove, using a 'nozzle' between the firebox and riser and it's giving pretty good results so far, so thought I'd share the journey.

In short, it's a 150mm system, with a 100mm port, on top of which sits a stainless steel cutlery holder (!), which is surrounded by a square riser that's 400mm high and 200mm wide. Flames get forced through the small holes in the cutlery holder and burn off any remaining fuel. Pretty simple really!

YOUTUBE VIDEO: STAINLESS STEEL NOZZLE IN ACTION







From my testing so far, it works really well. Quick to get going with barely any smoke at the beginning and able to handle refuels well. It also gets really, really hot! Initially the nozzle was 100mm tall (so 100x100x100 internal dimensions) and 4 x 25*100 ports, but I found that making it taller, 130-200mm, seems to be able to handle things better when you throw a lot of fuel in halfway through.

The problem lies in the material of the nozzle. So far, the 1mm thick 304 stainless steel is holding up pretty well, but I can't imagine it having a very long lifespan at these temperatures. It only cost £7 so I'm not too upset. I've been down a crazy habit hole, looking at ceramic nozzles for the oil and gas and cement industries, a porcelain tea strainer and various other items not really designed for the task.

I tried a simple box made from ceramic fibre board initially, with 4 slits on each face - it worked pretty well and would certainly be useable, but the cutlery holder was definitely more effective, glowing bright red and burning off anything that passed through it.

Ceramic Fibre Nozzle:  Ceramic Fibre Nozzle Video

I'm going to try a jewellers casting flask next, these are an ideal shape and much thicker, so should last a while, but it's still standard 304 or 310 stainless. I'm currently speaking to a chap in India about making one out of 310s stainless, which should hold up well (and is used elsewhere for similar things), but there will be minimum order quantities. Group buy?!



Also thinking of trying a larger cutlery holder, lined with ceramic fibre paper inside and out, made stiffer with sodium silicate. I imagine its going to be fiddly punching the holes to match the cutlery holder, but it might work and could potentially swap out the lining every year or so if needs be. It would certainly be cheaper than anything out of 310s stainless.

Let me know if you have any ideas for off the shelf items for the nozzle, or want me to try any specific configuration. If anyone would like to give it a go I'd be happy to help, I think you could try it with quite a lot of firebox and riser/secondary chamber configurations, it seems quite unfussy from what I can tell so far.

Sounds great!
So you have a fire box with a hole in the ceiling and an exit somewhere out of the riser?
Can you post a basic diagram?

Yes that's basically it. Hole in the middle of the ceiling, at the moment its exiting straight out the top of the riser, works a little better if the top is slightly closed so its 100% CSA, but not much difference I don't think.

It might work without a riser, I don't know yet, but could be tried with a full width top box and a cook plate

Does the nozzle have holes in the top?
What size is the fire box, does it have a door?
Have you calculated what all the holes add up to in relation to the system size?

I blocked off the holes in the top, so it just comes out the ones in the sides. I think their total surface area was around 10,000mm2, so a little more than the 50% port. One thing I noticed is that at the beginning of the burn, gasses come out of the lower holes, then when the burn gets more intense, higher up holes are utilised.

I have a feeling you might be able to have more holes/higher surface area, I'll test it next week on a larger cutlery holder with ceramic paper and that it would kind of use the holes as needed - the 50% first port still limits the flow of gas, so as long as it's not being restricted more than this then its all good. But will see.

Once the thing gets red hot it really does a good job of burning the gasses, not many yellow flames in sight, felt much hotter than when I used ceramic fibre board for the nozzle.

It also initially favours 2-3 sides - I think this is because air is coming through the front door. I'm planning to have air injected at equal points through square tube pillars, all facing the same direction to create a bit of a swirl inside the firebox.

The firebox is cylindrical - its a continuation of the sauna stove I've posted about elsewhere. It's currently lined with 50mm superwool, giving an internal diameter of around 400mm and is 300mm high.

Air supply comes in through a hole in the door, hits a plate and rises to the top of the firebox. It's around 10,000mm2, so quite a lot, I think I can probably get away with less but will see. Again, planning to change this.

Another thing I noticed is that it seems to be quite sensitive to having the door open, in that it really starts roaring, before quickly dialling back when the door is closed. Other designs I've tried haven't seemed quite this twitchy, but seems that the air intake size has quite a big effect on this one.

Found a better video in slow motion when it was at full blast: Slow Motion Rocket Nozzle

I'd be interested to know whether the little jets slamming into the walls of the riser is aiding combustion or not, could potentially try removing the riser altogether and letting them travel further to the edge of the stove body

Great Video Dave!
It looks like laser's blasting out!
You will be wanting a testo unit soon to see how efficient your design is!

Dave,

I really like how it works - a set of mini ports creating a lot of small turbulences. It would be nice to test it with Testo like Thomas said. If it turns out to be clean it would be worth to determine the lifespan of 304 and then 310 nozzle.
I wonder if ceramic nozzle only seems to work worse but is equally clean burning. If that was a case the nozzle could be carved from high-alumina 3" thick firebrick straight.

A testo would be nice, but I gather they're not cheap! If anyone with a Testo wants to give this a go, I'd be happy to share specs and buy them a cutlery holder 😄

The ceramic holder may have performed worse due to the reduced distance between the port and the wall - the total area was still around 100% CSA, but it might be that a bit more space is better, I'm not sure. Also instead of lots of little holes, one of them had slits and the other larger holes. I'm sure you could make it work with ceramic though. At one point I was looking at casting crucibles and drilling lots of holes in them, but was put off by how difficult the drilling would have been. I think the advantage of metal is it's easier to get lots of holes close together while maintaining structural integrity.

By the way, I was inspired by this stove

It uses a stainless steel nozzle rated to 1100c, would be interesting to see whether its guaranteed for a certain amount of time (edit: seems to be guaranteed with the rest of the stove for 5 years and isn't offered as a standard replacement part)

I'm planning to keep a tally of the various nozzles over time and see how they perform.

It might work with a square nozzle or perhaps for ease of build, the latest trend of a triangular shape!
In my case, I would try using vermiculite board rated at 1200c and proven to last for years in the vortex stove but it is 25mm thick.
Perhaps a kiln shelf but not so easy to join the corners.
However I would agree from what you have said that thin metal might well be the best option.
Cast iron tube might work a bit longer than stainless but hard work drilling the holes!

I use a piece of 1” steel mesh in my J tube vortex stove and regard that as a disposable component, it last about 20-30 burns but is very cheap and easy to replace.
On that basis and certainly for your own use, a disposable part might be just fine?

Yeah I wouldn’t mind replacing every few months or so, but a year or more would be ideal.

I think you could probably use vermiculite or firebrick, you’d just need to configure the space around it. I have an idea for a corbel structure for the firebox, with a firebrick nozzle and a pizza oven above the nozzle, but that’s for another day!

You could leave gaps between stacks of bricks like this…

The casting flask turned up today so gave it a quick try. It’s pretty solid and weighs a lot more than the cutlery holder. Unfortunately, it didn’t perform well. Slow to get going and although for much of the burn it was mostly clean, it definitely wasn’t as clean as the cutlery holder. On refuel it overfueled - too much fuel was coming through which led to around 3 minutes of dark smoke.

I totalled up the surface area of the holes, there’s a big difference between the two. The cutlery holder had 120 small holes equalling 2356mm2 and the casting flask 84 holes totalling 6636mm2. I was surprised at how the cutlery holder has such a small total holes surface area (14% CSA!) yet seemed to burn so well.

There’s casting flask was also taller at 200mm, cutlery holder 130mm, both 100mm diameter.

There’s a chance I’ve messed up some calculations somewhere. Tomorrow I’ll block the top off the casting flask and try and make the holes have similar area. That should tell me whether more small holes or less big holes is better.

EDIT: I should add that the two tests I randomly first using a square ceramic fibre nozzle, both had slits or holes totalling 50% CSA and didn’t overfuel.

Take this with a pinch of salt as it came from Chat GTP, but according to our artificial overlord, given the same surface area, 100 small holes will create more draft than one large hole, due to higher air velocity/better pressure differential and better turbulence and mixing. Frictional losses will be less significant as while smaller holes have less resistance individually, the overall resistance from many small holes is not necessarily higher than that of a large hole.

Who knows if true, but it's worth investigating!