Thermal conduction and heat storage vs bell skin thickness

[fiedia]

An ideal bell should transfer easily heat from the fireplace into the room as well as store a lot of heat to create constant comfortable temperature inside the house. Therefore, one would like to use thin walls to conduct heat as well as heavy walls as heat storage.
I built a 5” batch rocket + bell with different skin thicknesses. Skin1 is 5 cm thick all around the bell as well as on top. Skin 2 is 5cm thick on two sides and 20cm thick on other sides. Skin2 top is 12cm thick. There are three different thickness combinations (skin1+skin2): 5+5; 5+12; 5+20. Temperature inside the bell was monitored thanks to more than 50 thermocouples.


- Thermal conduction: As expected, outside walls of thinner skins get warmer
o Behind the 5+5 cm skins, the maximum temperature reached during the season was 100°C.
o Behind the 5+20 cm skins, the maximum temperature was 60°C.

- Thermal inertia: Thicker walls require more time to warm up and cool down:
o 5+5cm skins: half peak temperature after 1.5h, peak temp at 6h and back to half peak temp 18h after burnt start.
o 5+20cm skins: half peak temp after 6h, peak at 12h. Temperature remains above half peak after 24h.
- Below are two pictures showing TC location and labels for the following graphics

- The six graphics below show a 24h temperature recording of a 3kg wood single burn :
o Top left: inside HR (bottom, middle and top) + bell exit (flue) + fire box (face toward skin 1)
o Top middle and right: skin1 toward the fire box and skin2 respectively
o Lower graphics: skin2 toward skin1, inside 20cm thick skin2 walls, toward the room
- It shows clearly the short burn peak (30mn width) and the heat wave propagating through the different skins.

- Looking at temperatures inside the bell (graphic 2): once the air inlet closed, 5+5 skin temperature (cyan) drops much faster than the 5+20cm skin temperature (magenta). It shows that heat is flowing from the warmer thicker walls towards the colder thinner ones.
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.

[bbdoc]

Hi, don't you think that the way the stove is working is a flow of heat passing through the walls an coming from the hot rocket, instead of "stocking" heat.
May be it is worth to consider materials heat conductivity.

[fiedia]

bbdoc,

There are two points in your post:

1) materials thermal conductivity: Indeed we want the heat to flow from the rocket through the skins and for this purpose, high thermal conductivity materials are better.

2) conducting heat instead of storing it:
Standard stoves such as my former Godin with light thin skins (therefore high thermal conduction) stay warm one or two hours. You have to make a new fire quite often to get constant room temperature.
One big advantage of mass heaters is that you just have to make one or two burns a day. The mass around the fire place stores heat and remains warm for 12 to 24h.

If you use a thin skin with high thermal conduction, you will have less mass and therefore less heat storage. Even for materials such as steatite which have both high thermal conductivity and capacity, you will have to choose the right skin thickness to balance between heat conduction and storage.

It is the purpose of this study, how can I optimize the geometry for a cheap, available and mostly used material : bricks.

[kolben]

Really useful measurements.

Did you build a bell with insulating bricks on one side?

Are the 5cm bricks on the inner layer the ordinary non-insulating firebrick, or any particular temperature rating, or composition?

THANKS!

[fiedia]

All bricks are hard bricks (not insulating).

Inner skin bricks were bought from local manufacturers (21st century made). Most of them are refractory. High alumina content exclusively for the HR bricks.

Outer skin bricks come from older buildings (probably 19th century made).

There is a 5cm insulating aerated concrete layer between the bell west side (20 cm + 5 cm) and the house wall.

Regarding brick temp rating, I did not measure very high temperatures inside the bell. Brick destruction may also come from chemical etching as pointed out here : donkey32.proboards.com/thread/3909/firebricks-refractories-fail-silica-flux. Looks like hard bricks are safer.

Happy to share more information if you need any.

[bbdoc]

May 25, 2021 6:29:20 GMT -8 fiedia said:

bbdoc ,

If you use a thin skin with high thermal conduction, you will have less mass and therefore less heat storage. Even for materials such as steatite which have both high thermal conductivity and capacity, you will have to choose the right skin thickness to balance between heat conduction and storage.

It is the purpose of this study, how can I optimize the geometry for a cheap, available and mostly used material : bricks.

May be I should have write resistivity instead, track "le coeur battant" of Xelyx a mass heater concept in this excessively long tread  construction réaliste d'un poele de masse (in french)

[fiedia]

bbdoc,

I tried to find the mass heater from xelyx, but did not find a description of his stove after 15mn around page 150. Sorry but it will be to much time consuming to read through the 300 pages (what a mess!).

Found some posts about "coeur leger". It looks like Xelyx is also playing with skin thickness to get flat outer skin temperature. Is it what you want to point out ?

 

[bbdoc]

Dec 10, 2022 2:16:10 GMT -8 fiedia said:

bbdoc ,

I tried to find the mass heater from xelyx, but did not find a description of his stove after 15mn around page 150. Sorry but it will be to much time consuming to read through the 300 pages (what a mess!).

Found some posts about "coeur leger". It looks like Xelyx is also playing with skin thickness to get flat outer skin temperature. Is it what you want to point out ?

 

try here also. l'archve du site web.  et une part d'explication