Ponder appropriate rocket application utilize existing mass

[permiekev]

Hi Everyone!

I'm new to the forum, but not new to rocket stoves or at least the concept. I've built an 8 inch jtube using Matt Walkers plans out of ceramic fiber board, I have that hooked up to a recycled electric oven, makes pizzas real fast!

I've been studying batch rockets for a long time and have recently discovered the Double shoe box rocket 2 on batchrocket.eu/en/designs#dsr

Ok for the situation. I've got a 2000 sqft cape built in 1992, it has a 5' x 3' center chimney of brick. The chimney is in my living room with a cathedral ceiling, it stretches 16' at its tallest and 12' at it's lowest before going into the attic. Currently there is a wood stove(woodstock progress hybrid) hooked to the chimney.

The flu that it's hooked up to has excellent draft for I few reasons I believe: Center chimney, heated space, it's a metal liner thats slightly smaller than the 8" clay flu liner that it runs inside of, there is then perlite insulating between the metal liner and the clay flu. I think I might be able to oversize the rocket beyond 6" as the liner is bigger than 6" and should have excellent draw. The total height of the flu from living room to top of chimney is 20'

Now for my current thinking:
Build a DSR2 with an oven and a single bell, trying to keep it relatively light weight, build the bell walls and ceiling out of ceramic fiber board so that the energy is focused on charging the mass of the chimney(approx 10k lb of brick and concrete in the chimney in the living space.

[fiedia]

permiekev

The principle of bells is to extract heat from the fumes and store it. Hot smokes stay trapped in the top of the bell while cooler fumes exit at the bottom. With light boards, you will extract heat from hot fumes. But the heat can not be stored in a light material. It will be quickly radiated away, inside and outside the bell. That is why one wants to use heavy material for the bell.

Hot smokes in a vertical flue will make their way directly to the roof and outside. You will not get relevant heat exchange with a flue vertical brick walls and consequently no heat storage. The question is, can you transform this 10k lb chimney to a kind of bell?

[permiekev]

fiedia 

Thanks for the explanation of the bell. My understanding of how they operate is similar to yours. 

I want to clarify what I meant of having the bell use CFB. My wood stove sits on a brick hearth that is approx 3x5, the brick chimney is immediately behind this. I'm proposing build the rocket and bell on this hearth, have the bell walls be CFB except for the wall of the existing masonry chimney, leave this as existing masonry and join the bell to it. Since CFB is insulative it would I think send most of the heat towards the existing masonry chimney mass. I would of course skin the CFB with something light weight like tile.

Does that make more sense? If not I can try drawing a sketchup depicting what I'm envisioning.

Here is a sketchup link of what I'm trying to depict. The red is brick and the gray is the bell, the dsr2 is inside the bell but the door is sticking out. I left the top off of the bell in the drawing so that you can see inside that the wall of the bell that is shared with the existing masonry does not have cfb so as to allow more heat transfer into that masonry.

[fiedia]

looks like your link is not working. Can you try again ?

[permiekev]

Nov 10, 2022 23:58:05 GMT -8 fiedia said:

looks like your link is not working. Can you try again ?

I forgot to include it, here it is:

app.sketchup.com/share/tc/northAmerica/VEvHtMh9XRc?stoken=fFMomKpC6F-24cIYTltsOtpR8_mMxFMsxa-5GgAwcvFNB6FjJHIDA3xggCSIKtvL&source=web

[fiedia]

Got it,

I did some measurements with different bell thickness donkey32.proboards.com/thread/3820/thermal-conduction-heat-storage-thickness. It is limited to bricks and it will not tell you know how CFB will behave.   

My guess is that CFB will let heat flow away quite fast (within few minutes after the fire is dead). The small amount of heat taken during one run by the brick wall will not be enough to get the unheated sides to radiate (brick temperature will remain too low).

You will probably have to run the heater constantly to keep the room warm. In that case, the brick wall will store some heat during the day and give it back when you stop heating. But here again, I am afraid that your brick wall is too thick to get warm enough within one day. The curves on the lower corner show the temperature vs time behind brick walls of different thicknesses. The red line curve is the temperature behind 5cm brick + 1cm air gap + 20cm bricks.

Some heat stored in the bricks will be radiated back inside the box. If you close it after heating, the bell may give more heat back than the other sides of the brick wall.

Have you already try to build bricks with CFB? You may get leaks due to different material thermal behavior.

[permiekev]

CFB is a super insulating material so it's not going to radiate much if any heat. I think Matt Walker uses it for the tops of some of his bells.

I checked out your post on skin thickness and I think I understand your point that the brick isn't going to get hot enough. I should probably take your advice from your post and have my insulating material on the side of the chimney so I don't lose any heat there and instead have it radiate into the room.

"Therefore, I plan to optimize skin thickness for my next design: thin on the radiating sides of the bell and thick along the house wall. Thin skins will get warmer and radiate heat towards the room while the thick one behind the bell will store heat. I plan to build a thick inner skin in direct contact with the smokes and an insulating second skin against the house wall."

I've built a jtube out of CFB and the outter skin is bricks with a clay/sand mortar. This holds up well, but I'm sure the thermal changes on this are much different than a bell which is purposely absorbing heat.

I will research more on building bells before I try using any CFB in it.

Thanks for all your thoughtful replies fiedia!