Should I shorten the burn tube? Or not?

Ernie Wisner wrote:Actually your rocket stove should be considered as a system. the J tube part is only a piece of the whole When the barrel/bell goes on things change then when the thermal mass goes on things change again.
you put drag on the exhaust you put back pressure on the J tube. your fire slows down, an essential thing to consider is time, temp and turbulence. this catches lots of folks who want to modify the stove. in order to burn up most of the smoke you need it to be in the flame path for a period of time you need the gasses to mix and you need to have ignition temp.

What this means is you build a core that roars and the flame path is near the top of the heat riser. put a barrel on it the flame path is a little further down the heat riser then put the ducting in the thermal mass with the general couple corners that lowers the flame path a bit further. the shape of the feed and burn tunnel and the texture of the brick act with the wood to provide a good mixing then the residue gasses get mixed a second time when the stream from the heat riser hits the inside of the bell this puts the little bit of O2 in contact with the little bit of pyrolitic gasses remaining and re-burns. consuming all the wasted fuel (smoke) what remains is CO2 and water with a couple gasses that wont burn unless you get a real high heat going. I am not for creating a super stove that wont work half the time I want my systems to be robust and work all the time. so I consider the system rather than a component. Something for the super rockety folks to think on, time temp turbulence.

Not sure whether I should be starting a new thread with this, but my questions seem directly relevant to this statement, by Mr Wisner, that I've quoted above.

The combustion unit for my RMH/stove is now mortared together, but I can't decide whether to rip some bricks off and shorten the horizontal burn tube or let it ride, because at this stage (as yet uninsulated and without the barrel/bell), the flame is only just about coming around the corner and up the heat riser, say 5" or 6" at most. This certainly doesn't conform to the idea of the flame being near the top of the heat riser.



The bricks are high spec refractory, as is the heat riser. The white bricks at the very bottom are high spec, very light and insulative. I was thinking that maybe when all the insulative material is in place around core and particularly the heat riser, and all the brick/ceramic material is up to temperature, that this might bring the flame up, but it seems that the words from the wise suggest the opposite. I don't want to increase the height (or diameter) of my heat riser, so I figure the only option is to shorten the horizontal burn tube.

The reason I went ahead with the mortaring even though the stacked bricks performed the same, was that I had assumed that the heavy leaking of air between the unmortared bricks were contributing to the poor performance, which I thought sealing would rectify. Anyone have any further words of advice, before I start de-constructing the core?

Thanks!

bump

Hello J Black. First off,I have to tell you, I am in no way qualified to give you ANY advice. I have not built a heater myself. The only reason I am commenting, is you haven't heard from anyone yet, and I didn't want you to become discouraged. I've been studying awhile, and I am just asking a couple questions that MIGHT help a more experienced individual diagnose your build. From the picture, I'm wondering if your dimensions are all in proportion.

What does the inner diameter of your heat riser measure, and what does the feed tube opening measure ? Then, what is the length of your burn tube (horizontal) from the center of each vertical tube ? If the feed tube where you drop in your wood is 6" x 6", then the burn tunnel and heat riser should measure the same.

Did you build with the same cross - sectional area the whole wAy thru, and did you keep tunnel lengths 1:2:3 proportions ? If your feed tube is 1 foot long, the burn tube should be 2', and the heat riser should be 3'.

I'm sorry if I'm telling you things you already know. I don't mean to imply you've done it wrong. It's easy sometimes to just use whatever we find, and miss a simple (but critical) measurement.

Best wishes on getting your rocket running. Hopefully, one of the skilled builders will catch sight of your thread and give you more help.

I appreciate yr reply, Mr Dearborn, and yr absolutely right. I should really give more details.

Using the measuring points from the book, it is as follows:

Feed tube is 12.2cmx12.2cm and 23cm long (ash pit goes another 10cm down)

Burn tunnel is 11.9cmx12.2cm and 33cm long

Heat riser is 14cm in diameter and 80.6cm long

Thanks!

J Black wrote:I appreciate yr reply, Mr Dearborn, and yr absolutely right. I should really give more details.

Using the measuring points from the book, it is as follows:

Feed tube is 12.2cmx12.2cm and 23cm long (ash pit goes another 10cm down)

148.84

J Black wrote:IBurn tunnel is 11.9cmx12.2cm and 33cm long

. 145.18

J Black wrote:IHeat riser is 14cm in diameter and 80.6cm long.

. 153.94 cm

J Black wrote:IThanks!

So your Cross Sectional Area numbers ARE off, but to my inexperienced thinking, they aren't off by much. Again, someone more experienced can better evaluate that issue. The other thing that grabs my attention, is your mention of the ash pit. Original designs did include them, but it seems to have been phased out. Would you be willing to try an experiment ? Try filling in the pit with some dirt or gravel, then do another burn to see if anything changes. Make sure your wood is VERY dry, and give it sufficient time for all parts of the unit to heat up. As the mass achieved thermal saturation, the burn should improve.

One last thing, have you installed a "p" channel ? There are threads regarding these. Since I'm new and unsure how to link directly to them, I'll simply find a thread with pics, and comment on it <bump> to bring it up on the main page so it's easy to find.

Best wishes, Bob

Sorry, more thoughts are coming to mind. Are you splitting your fuel into numerous small pieces, rather than a few large solid chunks ? Part of the success of the RMH is the thorough mixing of oxygen with the gases being released from the fuel. The small spaces between the individual wood pieces act like numerous mini chimneys, accelerating air down through the fuel, giving adequate turbulent mixing.

Thanks, Bob! I've just checked out the P-channel, on yr recommendation, over at the Donkey forums and it's something that I hadn't tried, but could easily get together. Especially, as it doesn't involve ripping any bricks off the existing unit! Wood-wise, I'm using sticks (straightish branches and small trees, with bark on), mostly, as that is the wood that I have most free and easy access too. Mostly, they are pretty dry. I could find better wood, I'm sure, but realistically, I will mostly be burning these sticks in my regular use. Oh, the ash pit, I can easily put a brick in, too.

My main confusion is that I don't know how high I should be expecting the flame to come up the heat riser. I've singed a fair bit of my hair trying to get a look down the tube at various configurations that I've been playing with over the last few days, so there's definitely heat coming up, just not visible flame any more than a maximum of 1/4 the way up the heat riser with any combination, thus far

J Black : The story of the three blind men who find an elephant, one grasps the tail and says '' this animal is like a vine''! another grasps a leg and says
'' this Animals like a tree'' and the third feels the trunk and says '' this animals like a big snake !" we are all blind and trying to help, none of us are wrong
and all of us are right !

so! I am assuming that you have only tried to visualize the flame patterns during the day and not after full dark ! Try again using very dry thin split wood
with more raw wood surface than bark ! Then I want you to take cell phone pictures, getting a few different cell phones into the mix ! Most cell phones take
pictures well into the far infrared and ultra violet !

I expect that you will find your cell phone Camera will tell you you have great more flame than you think you have. While sneaking pictures down the Heat
Riser, you may want a tallish step ladder and then have some one hand you the camera when you are set! Please, Please, do not drop the camera !

For the Good of the Craft! Think like Fire! Flow like a Gas! Don't be the Marshmallow! As always your comments and questions are solicited and Welcome !
Big AL

Thanks, Big Al! I will try to keep all that in mind