Experimenting combustion chamber and riser shapes

Hi all,

I'm new to the RMH seen and I have a few questions. I've been reading for a few months now and there are a few things I haven't found (clear) answers to.

For example, what does limiting the space between the riser and the top of the barrel do ? How does it affect the performance of the system. ?

Having a short riser cause an incomplete burn and not enough draft, what issues arises when the riser is to long ?

I haven't seen ports to the riser purpose built to generate a vortex in the riser. Instead the ports are centered and the gases hit the back of the riser straight on, why is that ?. Wouldn't a spiraling flow of the gases encourage a better burn ?

(edit)
*Using metal to build doesn't last long, but it can be put together very fast. So using metal to build experimental designs would be much faster. But how well does a metal system translates to a fire brick system ?
-I found that a metal system that is well insulated is usually close to the same system made of fire bricks but may need some tweaking.

*Is rock wool good to insulate a steel fire chamber and riser or is it going to melt ?
-Rock wool does work well of insulation a steel system in the short term. long term there are many factors that can affect the rock wool. For example, if the steel get way to hot the rock wool actually melts. Even tested that with the acetylene torches.

Thanks is advance,

Sidney

Hi again,

I'm still sifting threw the vast amount of information on this site. The information is spread out through many different threads. If I ask a question that has been answered before please feel free to point it out.

Thank you.

Sid,
If I can offer any more help, the fresh air from the burn chamber is supposed to enter the riser in a turbulent fashion so that it mixes with the unburnt fuel gasses and gets a chance to oxidize them.  

If you want a riser that can take any abuse you might want to throw at it (temperature-wise at least)  try this one David Searle made of soft refractory brick.
https://onedrive.live.com/?cid=373bc244e577ab46&id=373BC244E577AB46%21655&action=Share&v=3

Thomas Tipton wrote:Sid,
If I can offer any more help, the fresh air from the burn chamber is supposed to enter the riser in a turbulent fashion so that it mixes with the unburnt fuel gasses and gets a chance to oxidize them.  

Hi thomas,

That was my thought on forcing a vortex in the riser.

Thomas Tipton wrote:
If you want a riser that can take any abuse you might want to throw at it (temperature-wise at least)  try this one David Searle made of soft refractory brick.
https://onedrive.live.com/?cid=373bc244e577ab46&id=373BC244E577AB46%21655&action=Share&v=3

In this video, you can see the gasses start to swirl on there own. I've seen other videos where they don't. A small change in the port angle would force the swirling early on start up

Sid

I totally agree that encouraging the vortex formation has merit as it might create a tighter swirl and therefore, more time in the riser. It was my first thought when I saw various burn chamber aperature configurations.  Those who came before us generally speak of Time, Temperature, and Turbulence as being the most important factors in getting the best burn out of your riser. But is turbulence the best way to get that time, and incorporate the fresh, preheated air into the mix?
I'm sure someone has experimented with this.  It would be nice to know what they discovered.

BTW.  That octagonal stack of refractory brick is what I intend to use.  Though I may have to play with the geometry to get the size I want.  I expect I will never need to rebuild that.

Thomas, you can save on material building this octagonal stack if you cut them length wise. These soft brick are expensive. They insulate so well the building it like in the video is a little bit of an overkill. You could also add a layer of much less expensive material on the outside like an perlite mix with fire concrete or the likes.

You may well be right about cutting the insulating refractory bricks length wise.

Here's something regarding vortex induction you might be interested in.

Thomas Tipton wrote:You may well be right about cutting the insulating refractory bricks length wise.

Here's something regarding vortex induction you might be interested in.

https://www.youtube.com/watch?v=iTlOZdpMd6I&list=PLb2E9ll1ekcJcUuJqfkoykF5Q2R3Enuue

Yep, I saw that, this is the video that triggered the question if anyone here and tried this. I Tried to find the link to some instruction for the length wise cuts for the riser. I can't remember where I saw this. When I find it I'll post it

Sid.

I can't immediately point you to a reference, but I have seen researchers say that special "vortex-inducing" swirlers do not work measurably better than the standard burn tunnel to riser right angle. The sharp vertical right angle does effective mixing; a smooth swirling vortex might not even do that, depending on the minute details. The standard batch box port generates the "double ram's horn" which is supposed to give excellent mixing.

I believe that Peter van den Berg, in his early experiments, tried various mixing methods and found that the straightforward ones that are now the standard worked as well or better than any fancy versions. You could find some of those discussions in the forums at donkey32.proboards.com.

Sid,
I meant to ask you if you had seen the BB Rocket as done by ABC Acres?



This is the burn box I am aspiring to.  What do you think of this setup?  I can't really tell if the preheated air is being released in the bottom or top rear of the burn chamber.  I have the plans from them, but they are a bit incomplete.

If I can get the parts cut for me, and if I can learn to weld well enough to put it together. I have a 2000 square foot basement this will go in that will help make a nice living space.  The basement also has a fireplace with dysfunctional flue damper that I will use for the exhaust.  But first I'll need to cut out some of the steel from the existing 1950's pre-fab firebox to facilitate a SS liner, and then maybe I'll be ready to start putting something together.

Thomas,

Peter van den Berg is the one that seems to have gotten the most efficient burns of this type of batch burn fire boxes. I think that he came up with the idea.

For very detailed information and some options check out this site: http://batchrocket.eu/en/

BTW, these old fireplaces should have a fair amount of firebrick in them. If you are condemning the fire place you could re-use those.

Sid

Thomas,

For the length wise cuts for the riser, check in the applications section and the water boiler. That is how they build the riser.

Sidney Beauchamp wrote:Hi all,

I'm new to the RMH seen and I have a few questions. I've been reading for a few months now and there are a few things I haven't found (clear) answers to.

For example, what does limiting the space between the riser and the top of the barrel do ? How does it affect the performance of the system. ?

Having a short riser cause an incomplete burn and not enough draft, what issues arises when the riser is to long ?

At first I started writing out how to calculate the area of outflow from the riser, but this post was getting a bit too lengthy. If you want I can post it in here later.

If the area of outflow is significantly smaller than the area of the riser then it will create a bottleneck which can cause draft problems and lead to smoke-back. Similar to how a bottleneck at the manifold connecting the barrel to the piping can cause issues. It seems critical to not choke the flow area of those points, so figuring out those numbers is important.

I believe you can use a much larger gap with less point heat on top for cooking and more of a stratification chamber to extract more heat quickly. This could be useful for places such as a workshop where you want quick heat and are less interested in having a large thermal mass or keeping it warm all the time. I'd imagine at a certain point having a large enough chamber to heat input ratio would reach a point where draft issues could arise depending on the design, but I've seen a couple pretty large ones that appear to work fine.

I think the riser length has a similar principle to the gap; Going a little longer doesn't seem to hurt anything but going too short can cause problems. If it is longer then it should draft better. I remember seeing a paper on rocket stoves where someone tested a couple types with 1, 2, and 3 foot risers where there were measurable yet diminishing gains in draft and more complete combustion using taller risers. This was just for stoves, so there are other considerations when designing a heater. I don't think it would matter too much how long you make it with an insulated riser with little thermal mass. Making it several times longer than needed with a large mass, such as non-insulating castable refractory or brick, could sap enough heat from the riser to cause further issues. With an open chimney it isn't a big deal because of other factors, but trying to get the air to flow back down a really large barrel after being cooled off that much might not draft properly. I've had pretty good luck getting things to burn completely using a thin hotface of refractory on the inside of an insulated riser.

I learned a lot about refractory and insulation from backyard metal casting sites and forums. Some of the stuff from there and ways to calculate performance automotive exhaust can be directly applied to rocket stoves and heaters. I even found a fuel injector calculator worked perfectly for measuring gravity fed drip irrigation output using rain barrels! It's really enjoyable to learn about all of this stuff! Good luck with your project and share some pictures if you get a chance.

Thank you Daniel for that clarification. It confirms my thoughts. The gap calculations are most important to maximize the heat transfer to a surface like a cook-top without negatively affecting the flow of the gases.

And I totally agree with the learning part. With everything going on in the world and the greed that keeps crushing the masses, I decided to be less dependent on the market and more self sufficient so I've been learning a lot lately.

I have a few idea's of my own, but I want to get a better understanding of the basics first.