Experimental batch rocket(ish) sauna heater

To us Wannabes, learning what doesn’t work is also important .. as well as the thought process.

Absolutely, although lately I’ve been finding the more excited I am about a new theory, the more likely it is to be wrong!

With that in mind, I’ve been revisiting Peter’s Shorty and realised that the best design I have so far is also the closest to Shorty dimensions (no surprises), with the following differences:

- Riser is centred above the firebox rather than behind it
- as a result gasses take a longer path to get from firebox to riser (could be good or bad?)
- my riser was equal to system CSA and 700mm long, exiting straight out the top, Shorty’s riser is wider after the initial restriction, a little shorter at 600mm and gasses exit the side rather than the top

So I tried the following:

- 200mm secondary chamber with horseshoe (same as I had before)
- 150mm diameter initial riser within the secondary chamber, 200mm tall


- 216mm diameter for the rest of the riser, 400mm tall (600mm total riser length, same as shorty)


- restriction on top of riser to force exit out of sides (different shape to Shorty but same principle)

In theory, this would just about fit in my 880mm barrel - I could potentially have the riser stop just short of the top of the bell with some ceramic glass to look into/light up the sauna rocks.

The test was mostly successful! Nice quick start with clean burn 7 minutes after lighting. From time to time I’d see little puffs of smoke after this, as though it was on the brink, but it was mostly clean.

Threw in a large log after 24 minutes - lots of dark smoke initially, clear again after 5 minutes, bar the odd little puff of smoke.

So all in all a very good test but I need to calm things down a little.

I was thinking of restricting the second port in the horseshoe a little at the top to try and put the brakes on a little?

Gasses are also taking a longer path to get to the riser and going through two restrictions. I’m not sure if this is good or bad, but it would be possible to have them take a more direct path, which would also free up some space in the bell.

Ideas welcome 🙏

Squeezed another test in and added a purposefully large restriction to the horseshoe port of 80mm, giving it a CSA of  just under 33%, whilst the first port remained around 50%.

Big difference, much slower in the firebox. It burned clean after 8 minutes, but it was still hot and I suspect it would have taken longer if not. Once clean, it was very clean, not a whiff of smoke.

I added a big log after 18 minutes - some thick smoke initially, but it was gone after 1 minute and burned clean from then. Much better, but not perfect.

Promising - I think getting the speed just fast enough is key to this and I reckon it can be done through the right relationship between the first and second port.

I do wonder what it was about the test with 2 200mm chambers and no riser, that enabled it to not smoke one little bit on refuel. Perhaps just ample time and space for everything to combust - but in this case at the expense of a very slow start.

Next I’ll try reducing the first port a little and increasing the second, so they’re the same size but a little smaller overall, maybe 40%.

Two more tests and I managed to solve the smoke on refuelling, but at the expense of a slower start. Not that slow though, so think I'm getting closer to a better balance.

Test 1
Port 1 (firebox to secondary chamber) at 40% CSA and Port 2 (the horseshoe between secondary chamber and riser) the same. It took around 15 minutes or so for the smoke to disappear completely, but it burned very clean after this, with no smoke at all on refuel.

Had a peak down the riser - flames were just periodically licking the bottom of the riser, suggesting gas had ample time and space to combust. Looking in the firebox, it was much slower than previous attempts with bigger ports, flames gently licking the ceiling and not rushing at speed towards the back, suggesting more combustion is allowed to happen in here.

Test 2
Port 1: 40%, Port 2 100%. I just wanted to see what would happen here and how these two ports interacted. Things got off to a quicker start, but it was already quite warm from the previous burn.Secondary combustion started after around 8 minutes, then burned clean and again no smoke on refuel. Things looked a little faster inside the firebox, but perhaps also a little uneven. I didn't hear any traction engines, but something faintly resembling them. My guess is that increasing the second port has increased the draft a little but also potentially put it a bit out of balance, so I'll keep these the same size for now going forwards.

As with the previous burn, I had a look down the riser and flames were only really just licking the bottom, ample time and space to burn off all the gasses. I'd hazard a guess that a shorter or thinner riser could probably work with this smaller port.

I'm seeing how critical the port size is to everything else. Things get going much more quickly with a bigger port, but too big and it can overfuel if it doesn't have enough time/space in secondary combustion. Smaller port means slower to get going, but also potentially enables a smaller secondary combustion zone and can handle refuels without a hitch.

I wonder if anyone has implemented a damper on the port itself, rather than the air, in order to facilitate a rapid start, but slow things down once going?

I'm planning to gradually increase both port sizes from 40%, until I see smoke on refuel, so I get an idea of where the limit is for this particular size/shape secondary combustion zone. The first test with Shorty style riser smoked on refuel and had a 66% port, so in theory it should be somewhere between these.  

Following this, I might try something different for the secondary combustion zone, then attempt to gauge the size of the port required for this, kind of the opposite way I'm doing things at the moment. I wondered if I could get away with a much smaller path to the riser, which would free up more space in the bell for heat extraction.




Essentially, I'd build a little vortex box on top of the main fire chamber, so gasses exit out the rear port, then get funnelled towards the central riser. James, if you're reading this far, I think this is similar to your vortex J tube, but you have your riser behind your main fire chamber and here its on top.

Tested with two 50% ports - seems this is about the limit. I got a little smoke on re fuel, but it was clear after 1 minute. So for this design, optimum port size seems to be around 45-50%. Start was ok, around 12 minutes to clean burn, but not as fast as I’d like. The general pace in the firebox felt quite nice, lively enough but not roaring towards the back.

What I’m interested in now is exactly how effective the secondary chamber, which directs air to the riser, is. In Peter’s Shorty, port size is larger and it seemingly handles refuels well, but the gasses are also taking a shorter path.

So it’s possible the design of my secondary chamber isn’t optimal and I might be better off trying to get the gasses to take a more direct path from firebox to riser? Let’s see.

Well, finally I think I might be on to something!

Video: https://youtube.com/shorts/HOAaPBFrz44?si=DQ_qaAhY6lMWPlHV

Cut a long story short, I ran a few more tests with a DSR3 like afterburner and a rear exit Shorty style afterburner. Both were pretty good, the DSR3 one started very quickly but easily over-fueled, the Shorty style one was a little slower to get going but handled refuel better, but still with some smoke.

Then I decided to take a gamble on a wacky design, but it seems to have paid off. Let’s call it the Small Vortex Collider … basically, it plays on the DSR3 design, but has 4 slots instead of one, with 100x100mm square tubes all firing the gasses towards each other. They collide in the middle, then go up a short riser, system CSA.






The results were unexpectedly good - not perfect, but by far the best effort yet. A quick start, although a little back and forth between light smoke and clean, but was fully clean after 8 minutes. Once clean, it stayed clean, no matter how much fuel I threw at it.

Things were burning at a nice lively pace in the firebox and it was fun watching the gasses smash into each other - my guess is this really helps raise the temperature at the bottom of the riser.

Still a long way to go to test using final proper materials - there’s a lot of metal in this one as I had it to hand, but won’t hold up in the finished version. Also untested with a bell and a chimney.

One thing I noticed was in the firebox it seemed to favour two ports to one side and would sometimes flick from one to another, but rarely use all four at once. I think this might be the air flow coming from the front, in the final one I’ll try equally spaced metal tubes inside the firebox under the ports, so they each get the same dose of air. But it might be simply having 4 exit ports causes this, so might be room to optimise.

I have found that with all the short, or no riser designs, a tall insulated chimney will make a world of difference.
I have only recently discovered just how much a good chimney can make, most of my experimenting has been done with fairly short chimneys of around 1.8-2.4m but I have now added an extra meter and what a difference!
It is the same with cold or warm weather!
My partyhouse vortex J tube runs all year around as I use it for cooking in the summer, in mid summer with no wind and 25c air temp, it is lazy to start up and rarely sends the cooktop above 400c.
In the winter, 10c air temp with a stiff breeze, it will burst into life and sore up to 550 in a few minutes!

I tried a few iterations of the 4 port version, but they were all a bit volatile. Quick to start and mostly clean, but the flames were jumping around all over the firebox switching from port to port and sometimes causing puffs of smoke out the top or even backdrafts through one or two of the ports as they jostled with each other. I concluded that any more than one port in the firebox causes mayhem!

Luckily, I seem to have stumbled on something better. Two tests in and its burning clean in around 5 minutes and then stays very clean throughout, even on refuel...

Video

Basically, it uses a small chamber above the main fire chamber, to inject gasses into a central riser, which form a double vortex at the bottom. Pretty much like Peters Shorty, but instead of the gasses exiting through the rear, here they exit through the top at the front into a small chamber where the gasses change to a horizontal flow, then enter the riser...





Having the firebox port at the front like this is unusual, but I think it has some advantages. On startup, flames and smoke go directly up this port, without having to travel to the back of the firebox and cool down and instead of the flames rushing over fresh unburned wood and casting it to smoke, the heat radiates out and draws the fuel to the front through the hottest part. It takes some getting used to, but I think it's helping contribute to a really quick and clean start. Versions I've made that roar to the back right away often churn out quite thick smoke until secondary combustion kicks in, but here you only get very thin smoke when you light it, which quickly clears up.

Here are the dimensions:  

- Port 1: 55%
- Mini chamber: CSA just over 100% of chimney CSA
- Port 2: 55%
- Riser: 113% CSA (100% for the bottom third where the vortex is formed) .

I think the riser could probably be 100% CSA, flames were rarely going higher than the bottom third of the riser. The volume of the riser is notably less than the Shorty or a standard J tube, so whatever's going on in the firebox, mini chamber and bottom of the riser seems to mean less space is needed later. Time to stop tinkering and build the thing, in a room, with a chimney!