Dave Rose

Fox James wrote:Good luck with the new build, exciting for you and hopefully a Success!
PS good point with the ceramic fibre riser as that would be extremely unhealthy option!
If you even touch the stuff once super heated, it just produces  so many air born particles!

Thanks and yeah, keen to not use the stuff again!

Ran a few more tests and pretty happy with the casting vessel, so going to go ahead and build it and hook up to a chimney, It should be easy to swap out nozzles later if needs be. Please let me know if you have any tips on the build!

Here's the plan for the construction, it's all fitting inside a 10mm thick, 500mm diameter, 880mm tall steel tube, which acts as the stove body and bell. On top of this will be sauna rocks. Heat goes up the riser, hits a removable lid, falls back down to the level of the firebox ceiling and exits horizontally. I think you could quite easily adapt this to fit inside a square or rectangular stove, or potentially fit it all inside a standard oil drum.


An air frame swirls air around in 1 direction, to mix as much air and fuel as possible before it reaches the nozzle. This one is fed from outside air via a pipe that connects to the rear bottom of the frame.


It also supports the ceiling and holds vermiculite panels in place.


Insulating firebricks or superwool go between the air frame and stove body. The nozzle sits in a hole in 50mm of vermiculite board, which is supported underneath for the most part by a 10mm steel plate, or potentially cast iron if I can source.


Next is a riser made of insulating firebrick. You could use a superwool 5 minute riser, but if I'm going to be reaching in and swapping out nozzles regularly I figured something sturdier would be better. Its wider at the bottom - around 200mm, could be 250 I think, then there's a step to make it 150mm. I found that sometimes swirls of unburnt material rose up the edges and the step help interrupt them and burn them off - I've only tested one version of this so not 100% sure.



Here's how it fits inside the barrel...

Cristobal Cristo wrote:If the lifespan of the cheap 304 holder will be less than a month then it would make sens to try to fabricate it from RA330 high nickel alloy. They sell 4" pipes  that could be drilled and topped with ceramic material. The topping could be also welded from RA330 sheet, but welding this material is quite demanding. However the pipes that they sell are thick walled, 4" would have the walls 6 mm thick so maybe it would affect the burn negatively.

Interesting, I’ve not come across that alloy I’ll look into it. The one I’ve been looking at is 310s which sounds like it would do the job, I can get it locally but would have to drill it also, which would be a pretty big task for 50-100 holes, or have it lasered which would be costly. I’m hoping the company in India who make the casting vessels will do one in 310s for a reasonable price but will see. I have a feeling the 304 one I have at the moment, which has 3mm thick walls will last a while, but we’ll see.

Nancy Reading wrote:Thanks for the update. It is logical that turbulence aids combustion, but you'd have thought more constriction wouldn't help - it's a bit of a contradiction isn't it?
Maybe a zircon paint might help give the metal some longevity?

It is, I haven’t quite got my head around it yet! All I can think is the extra velocity of lots of very small holes counteracts the smaller area.

Re zircon paint - I’m not sure it can be used on metal? I have lined one of the nozzles with ceramic paper and covered it in sodium silicate so will see if that extends the life.

A few more nozzles tried with some behaving quite well, others less so.

A couple of videos…

15% CSA port, casting vessel: https://youtube.com/shorts/DHPXV2oDulw?si=hieiGdGpNi2APmfe
Venturi rocket nozzle (turn up the volume!): https://youtube.com/shorts/jl94vJ4NPB0?si=NXXqUlUjIWdr1_RB

Main takeaways so far:

When using a perforated chamber as a port, total size of the perforations can be as low as 15-20% CSA. This is not true of a round Venturi port. Smaller holes mean less overall area required (I think).

A 250mm riser seems to work better than a 200mm one - the little jets that shoot out seem to like being given some space before they collide into the wall. I’ll try 300mm at some point. Circular riser works better than square.


Casting vessel (10mm holes)


I ran two tests on, blocking off some of the holes. It overfueld last time, with total port size 34%, so tried 20% (44 holes) and 15% (33 holes)

15% was slow to get going, but got there eventually and could handle everything I threw in without smoking.

20% was much quicker to start, overall a good clean burn bar the odd little bit of smoke escaping on refuel. It wasn’t quite as quick to get going as thinner metal (this is 3mm thick) - I think it robs some heat right at the beginning for a couple of minutes until it’s up to temp.

Larger cutlery holder lined with ceramic paper (7mm holes) 16% CSA [EDIT: incorrect, more like 24%!]



This performed well, was on the edge of being too frisky, but for the most part it was super clean bar the odd little puff of smoke on a big refuel. With some tweaks to the air and riser size I think it could work well.

Tapered tubes

I wanted to try something similar to a Venturi port so sourced a stainless concentric reducer tube, to try and build a true rocket!
50-100mm
Choked the firebox, restriction too narrow. However for short periods, it really did blast like a true rocket.
75-100mm
A bit slow and lazy, a few rocket blasts but also some slow idle periods.
Neither were very clean so I’m giving up on this one. I’ve a feeling it could work with a small port similar to the first and a very long riser or strong chimney to force it through, but was too unpredictable in my setup. Sounded great,  though.

Next experiments will be with the air in the firebox and the riser size, I don’t think the central port is working well with front air, so planning to inject equally from 4 sides.

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!

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.

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…

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.

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