critical relative dimensions

Hello all,
Ive searched the forums quite a bit before posting this so I probably missed a clear answer. I've read to keep the CSA constant throughout the whole system, but I think that would be really tough in the bell between the outer diameter of the heat riser and the inner diameter of the bell. I've also read the exhaust should be the same size or larger than the feed.

My system is 5x5 and worked really well until I put my bell on. I then read that the gap should be pretty small. Currently its like 4 inches, but I calculated the gap should be 1.25 to keep the same CSA.

Any advice on tuning the system?


Thanks!

welcome to the forum, Steve.

you want to tell us a bit more about your heater? materials and dimensions would be a good start to help the folks here help you. photographs, drawings, and google sketchups are all very helpful, too.

Hi Steve

your calculations are correct and, yes, you're going to have to adjust the gap down to get things moving along properly.

think about the fluid dynamics at that point: you've got screamin' hot gases and flames that are being 'pushed' up by the draft engine of the system, your heat riser, but you want them to head in the opposite direction. with a gap of 4", you are giving the gases too much room to whirl and whorl before they reach the outer edge of the bell and start heading down. all this fluffing about makes the gases lose their velocity and get, well, confused. the small gap is needed to get your difference engine to actually do the extra work of pulling your gases down. instead of letting them swirl about for a bit, the small gap sort of 'peels' open the flow and makes it spread evenly across the top-- kinda like the end of a trumpet. this makes the gases flow smoothly to the outer edge and start flowing down the sides where they cool off and continue to fall. the small gap helps these falling gases 'pull' the system along.

Hey there.
Thanks for the replies. I'm going to post photos asap.

My materials are 5x5 steel box tubing, a 5 gallon propane tank, a 10 gallon propane tank for the bell, and pearlite for insulation.

I will lower the gap this weekend and report my findings. I'll try to post photos before then.

Thanks again!

Also,
What if my CSA of the bell minus the CSA of the heat riser/insulation is larger than the CSA of the rest of the system? Will that make much of a difference?

Hello all,
I have some photos for you. I lowered my riser gap down to roughly 1.5 inches and I did notice an improvement but it's still not sucking in air and rocketing like it did before I put the bell on, so what gives? There was quite a bit of smoke-back and it just seems like the system isn't performing like it should.
Any feedback is greatly appreciated.

First attempt that didn't even sort of work. Made this L tube and welded it into the tank behind it. Just thought I'd throw this in for laughs.


I cut that all apart when I got some 5x5 box tubing and welded that all up and filled with with pearlite:




This by it self did not work until I just set a piece of duct on top to make a stack. Then it worked great. Lots of updraft and rockety sounding.



The tubes sticking out the sides are actually air intakes which help a lot but allow for some smoke-back so I welded them shut. Hot coals could also fall out of them.



I welded on my heat riser and did a test burn with the ports welded shut and no change; very rockety:



I then set to work insulating my riser. I used a piece of the duct shown above and made the diameter smaller. I glued the bottom on with mortar to keep the pearlite from falling out and folded the top over. More mortar on top of the pearlite just in case the thing gets tipped over.





Another test burn and no change. All good so far. It then occurred to me that the top of the propane tank was getting very hot because the heat riser is directly welded to it and could release quite a bit of heat into the bell creating a back draft so I cut the top off another propane tank and made another chamber filled with pearlite:



I was pleased that it looks like one tank like I had intended.

This is where I ran into problems. The bell heater seems to really be messing things up. This photo is before I cut it open again and lowered my gap:




The view from the back. I thought the same sized exhaust port was causing problems so I cut it to about 6.5x6.5 and just left it as a gaping hole; no horizontal tube. The lowered gap made a difference but not that significant. Any ideas?

It also occurred to me tonight that there is a small slit in the elbow of the feed tube where I was unable to fit my mig welder. You can see it in the 3rd photo. I have an oxy/acetylene torch that should fit, I just cant imagine it would make any difference being a tiny slit across the top. I will weld it shut anyway.

CRD, we might never know until the "powers that be" help by starting a sticky "FAQ/best practices/basic design section" and we as members help by taking good data and uploading to the aforementioned: this has to be a team effort - too many variations on a theme with too many variables; unless we and the moderators all share the work involved we'll continue to see the same questions asked.

I try to answer some questions as I am able, but so much of this is repetitive.......or outside my expertise.
.
WE NEED TO BUILD A FAQ THREAD. I don't have the permissions to do so, but I will contribute.

respectfully,

Andor

I'm also thinking I'd like to build an insulation box around the feed box. It gets over 600 degrees and I figure any heat radiating from the front and not going up the heat riser makes the system less efficient. I just need to get the materials.

Steve McCluskey wrote:I'm also thinking I'd like to build an insulation box around the feed box. It gets over 600 degrees and I figure any heat radiating from the front and not going up the heat riser makes the system less efficient. I just need to get the materials.

I would guess that this would help you a lot. I'm used to seeing rocket cores made either of regular bricks and insulated well or insulating refractory bricks (probably not the best spot for such fragile material), but I've been seeing a lot of uninsulated metal burn tubes recently. in my experience, that part needs to be really hot for good performance, and it just won't get hot enough without insulation of some sort.