Peter van den Berg

Gerry Parent wrote:Peter,  I think I remember reading something about you saying something over at Proboards.com (which I can't find right now) that flames that come out of the riser are not the most ideal situation. If this was you that said this and is a correct statement, could you elaborate on it a bit and what could be done to make it more 'ideal'.

We are entering fundamentals here, largely untrodden territory I might add. In an ideal situation combustion is all over and done within the confinement of the riser. Flames out of the riser isn't a good thing: compare it to a candle flame. What happens there is that the oxydizing of the combustible material only takes place at the outside of the flame. Inside there's no oxygen so therefore no burning.

Flames out of the riser usually are oscillating, in a split second it comes up, goes down and up again. Looks like it's doing right, after all it isn't like a steady candle flame is it? But outside the riser the temperature is much, much lower and therefore one of the conditions in order to achieve complete combustion isn't met anymore. I didn't come up with that by myself, the Testo teached me that flames out of the riser aren't as good as one might think. Lots of carbon monoxide and some smoke as well are released into the outside air. Apart from the fact that smoke is wasted fuel it also becomes a pollutant, we shouldn't allow that to happen.

And no, the flames won't be that high when the barrel is in place, since coaching the gases into a 180 degree change of direction means friction. The larger the top gap, the less friction seems to be a good rule of thumb.

Now how to counteract this effect, adding friction isn't something I am particularly fond of, quite the opposite. But during development of the DSR2 I learned another thing: the end of the riser, expansion room or whatever you like to call it can be made smaller than system size. Just a bit more csa as the riser port seems to do the trick, any smaller than that and the whole thing became very sluggish. The maximum gas velocity in the riser port seems to be limited by the end port, combustion goes on in a less violent way and CO production is very low.

This effect of the above is largely not understood yet, it has something to do with expansion of gases between riser port and end port. While fiddling with the DSR2 I tried restricting the top of the extremely short riser and the whole of the thing stopped working the correct way entirely. It looks like there need to be some sort of expansion room between those two ports. The overall effect is an afterburner flame that stays largely inside the riser stub. Running full tilt the flames are entering the expansion space but tend to stay at the rear half due to the stumbling block halfway the expansion room's ceiling.

In case this end port thing is one of those effects dictated by laws of physics it might work on a straight riser as well.
I need to stress this is pure speculation, I didn't try this with a straight riser as yet so the waiting is for somebody to try it out.

Gerry Parent wrote:How close can the wood be placed to the stub? Right up to it or best to leave a gap for air flow?

Right up to the stub is OK no problem, air flow is going around it anyway. Left and right of the stub the fuel could be shoved in right up  to the rear wall although a small gap would work slightly better so the fire is able to work itself down close to the port first.

What I do as a habit: I shove logs in until it touches the rear wall and pull it back about an inch. Please don't be too religiously regarding this, it's just a way to make sure there's some space there. Sometimes a piece is too long for this method so I let it touch the rear wall anyway.

Shoving a piece, however small, into the port will disrupt proper gas flow in port and riser which will inevitably lead to smoke out of the chimney stack. Visable smoke inside the firebox isn't uncommon, as long as the afterburner flame is raging it will be taken care of by that.

Gerry Parent wrote:1) Do you load your entire batch of wood in (to within about 2" to the ceiling) then reach way to the back to light it with a long match? What I found is that its takes a while before the fire catches the wood below it when lit from the top vs starting it at the base of the stub.

What I do is loading the entire batch for that moment. Biggest pieces first, somewhat smaller on top of that and kindling on top of that. I tend to leave a lower level in the middle, where I place a handful of small tinder and light that with a single barbecue lighter. Most of the time 2" from the ceiling isn't enough to reach the back so in practice I light the fire halfway. This method allows for stacking the entire load, as opposed to starting low in front of the port and adding more fuel while the fire grows.

Gerry Parent wrote:2) If the door is put on prematurely after lighting from the top back (before lets say 10 minutes) does the fire die down almost to the point of going out?

The upside down firing method requires an emphasis on primary air for the first 10 or so minutes of the burn. That's why I leave the firebox door open about half an inch (one finger thick). When the door is closed too early, in some instances I've seen the fire goes down to the point that the afterburner flame pops off, literally. Results in this sense means what the Testo is making of it, on average significantly better than the other method over the course of ten burns each.
Just weld the threshold to the floor channel's feed and don't try to empty all the ashes out when you are at it. I almost never do unless I want to take the floor channel out to have a look at the bottom of the riser.

Thomas, I didn't realize you meant the pink type, those are quite good. I've used those in the past at Paul Wheaton's and those are soft but relatively firm. Given the fact you being the only user would be a workable situation. When you are aware of the limitations of the material, please use it and stay careful. As a material for a firebox from a pyro point of view, those Skamol bricks couldn't be any better.

The kind of glass that says "Schott" and "Ceran" in one of the upper corners is glass ceramic, certainly not tempered glass. I have cut one at Paul Wheaton's in 2017, in fact it is the one of the rocket cook top in Allerton Abbey. And I did some small clear pieces at home, also using a tile wet saw. Very important: support the glass with a piece of plywood or the like and cut it in two or three passes. I never tried to cut it with an ordinairy glass cutter, by the way.

Yes, I am listening.
And no, you'll need a larger size in order to have it in proportion. Let's see... that would be 1.85" square by 1/8" thickness. One of 1.75" square by 14 ga or 0.083" thickness being the closest according to MetalsDepot. Don't be scared because of the lesser thickness, heavier steel doesn't necessarily last longer.

Last night the total of unique visitors of the site exceeded 150,000. At the same time the total views counted 409,777. It took 14 months to go from 100,000 to 150,000.

At the moment translations aren't doing well. Russian has grind to a halt, Chinese, German and Portuguese didn't come to light at all and the Italian translator can't be reached anymore. I am lagging behind myself with the publishing of the Double Shoebox Rocket developments, it's high time to start writing a longish article. Both in Dutch and English, the latter one being the base for all the other translations.

One good thing to mention: I am going to conduct a workshop in Belgium, from 1st through 7th of May. A DSR2 design, bell and bench construction similar but not quite the same as the Mallorca build in 2017. It's announced on Facebook in Dutch, but Google knows what it'll look like in English, more or less.

Miles Flansburg wrote:If I build a large underground wofati, with up to 10 rooms under the larger roof, each room having it's own mass heater.  Is it OK to tie the chimneys together at some point so that there is only one exit hole out of the large common roof?

Miles, it won't work unless there's only one heater running at any given time. All the others should be closed very tight. If not, the heater that's running will be hampered by other heaters feeding air (or worse, exhaust gases) into the main outlet. Two fires burning at the same time will influence each other. There's also a large risk of one that runs well and the other feed small amounts of smoke (and carbon monoxide!) into the living space. It's not allowed in building code, and there's some reason to it.

Imagine one great house in rural England, all rooms having a fireplace and separate chimneys.
Those 18th and 19th century people weren't stupid, they knew combined chimneys are dangerous, asking for trouble. Please don't go that route.

Rune Dahlgreen wrote:Now regarding the newest version (since Peter mentions that the the feed/horisontal part is now larger and "The feed is close to twice as large as the stub, csa-wise") is it safe for me to conclude that the horizontal part should be close to 10 % riser CSA and the vertical part of the newest floor channel should stay in the same range, close to 5 % of the riser CSA?

Also back in the days of the P-channel coming down from the 'ceiling' of the fire box Peter emphasized that the width of the P-channel should be as wide or slightly wider than the width of the port. Can we now totally disregard this rule for both the horisontal and vertical part of the newer floor channel?

@Rune,
It would also be convenient to use two 1.5" square ducts side by side as Thomas already mentioned.

The newer floor channel is indeed another animal, other rules apply so the stub is narrower and the feed could be much wider than the port.

Thanks for replying Thomas.
I think there's a way to view Sketchup files irrespective of the operating system, though. It's called SketchUp Free and runs inside a browser. Apart from that, there seems to be a SketchUp viewer app as well for mobile devices.

Edit: Now I see Gerry already provided the same link.