Computer Model for Temperature Curves

[Kvanbael]

Here's a graph from a computer simulation on how the heat travels through the thickness of the brick dome during a 2 hour fire, and 8 hours after putting out that fire.

The model's variables still needs tweaking (thermal capacity of the bricks, temperature of the fire, conductivity of the bricks, the air-brick layer, and the insulation as a whole), but nevertheless, it is already corresponding to several real-life-observations:

- If you measure with IR, then you might have a fairly quick drop after removing the fire (while the heat is still distributing more evenly through the thickness of the brick). This drop is determined by the duration of the feating fire.
- Once stabilised, the temperature drops much slower, in this phase, the drop-rate is determined by the exterior insulation.
- The heat really needs time to travel through the bricks: On the outside, the bricks won't reach their max temperature until well after putting out the fire

This shows why it makes sense to have a thermocouple one or two inches below the interior surface, as it will better predict at what temperature the oven will stabilize after putting out the fire.

[Karangi Dude]

Hi Kvanbael, you might be interested in reading this post I did a few years ago on this subject, it is interesting how these ovens heat up and how they cool down
https://community.fornobravo.com/for...586#post157586

Cheers Doug

[Kvanbael]

Interesting thread indeed. Your findings seem to support this simulation. The heat travels slower into the bricks than we think.
Next up is making measurements to tune the model’s parameters.

[Petter]

Your simulation looks very reasonable. Taking data from here, http://batchrocket.eu/en/building#bellsizing, they think it is suitable to burn 1-1.5 kg wood (4-6 kW minus efficiency) per m2 of inner dome surface area ( per hour) to charge the bricks. One can then calculate the amount of kWh needed to saturate a specific oven given its brick and floor mass to get the saturation time as a rough estimation. A faster burn only yields a great thermal gradient in the brick and heat loss though the chimney. It seems like very little wood though compared to videos and other recommendations how to fire the wfo.

[MarkJerling]

Here's a graph from a computer simulation on how the heat travels through the thickness of the brick dome during a 2 hour fire, and 8 hours after putting out that fire.

The model's variables still needs tweaking (thermal capacity of the bricks, temperature of the fire, conductivity of the bricks, the air-brick layer, and the insulation as a whole), but nevertheless, it is already corresponding to several real-life-observations:

- If you measure with IR, then you might have a fairly quick drop after removing the fire (while the heat is still distributing more evenly through the thickness of the brick). This drop is determined by the duration of the feating fire.
- Once stabilised, the temperature drops much slower, in this phase, the drop-rate is determined by the exterior insulation.
- The heat really needs time to travel through the bricks: On the outside, the bricks won't reach their max temperature until well after putting out the fire

This shows why it makes sense to have a thermocouple one or two inches below the interior surface, as it will better predict at what temperature the oven will stabilize after putting out the fire.

Thank you for your interesting computer model. I have created a graph of what happens inside my oven. This is a work in progress. I need to (when I fire it again) start measuring when I start the fire. In this instance, I only started measuring once I was ready to start cooking!

[Kvanbael]

This is great data. Just for fun, let me guess:

- Curve starts with steep drop, like the blue graph in my simulation. Hence: These are measured at the surface (with an infrared thermometer) (or just below the surface, in case of a thermocouple)
- Takes long (4 hours) for the heat balance through the depth of the dome. Hence: This is a fairly thick refractive dome (6" or more):
- Initial drop is significant: You used a relatively short but intense firing
- Once the heat is spread evenly, temperature drops with a halflife of about 10h: Your oven very decent insulation
- The first reading should probably be even higher: Your thermometer maxed out at 1000F.

[MarkJerling]

This is great data. Just for fun, let me guess:

- Curve starts with steep drop, like the blue graph in my simulation. Hence: These are measured at the surface (with an infrared thermometer) (or just below the surface, in case of a thermocouple)

Yup, you're right! Measured on surface with infrared thermometer. And yes, almost exactly like your blue line simulation.

- Takes long (4 hours) for the heat balance through the depth of the dome. Hence: This is a fairly thick refractive dome (6" or more):

The dome is 115mm (roughly 4.5 inches) thick and very dense, heavy nightstore heater fire bricks.

- Initial drop is significant: You used a relatively short but intense firing

Yes, you're right. I made a hot fire for about two hours. Next time, I'll try a smaller fire over a longer period.

- Once the heat is spread evenly, temperature drops with a halflife of about 10h: Your oven very decent insulation

Yes! I have 1-2 inches of furnace insulation over the whole thing and then regular fibreglass insulation over the top of that, about 6-8 inches, then an industrial thickness aluminium foil layer. I went by the idea that you can never have too much insulation! Under the floor, I have 75mm aerated autoclaved concrete as insulation under a sand layer which supports the floor bricks. The floor bricks are only 45mm thick.

This is great data. Just for fun, let me guess:
- The first reading should probably be even higher: Your thermometer maxed out at 1000F.

Yes, you're absolutely right!

Thank you again for your great graph and thank you for your (spot on!) comments. I'll do more measuring next time and will try to heat the oven over a longer period.

[david s]

[QUOTE=MarkJerling;n431877]

"Yes! I have 1-2 inches of furnace insulation over the whole thing and then regular fibreglass insulation over the top of that, about 6-8 inches, then an industrial thickness aluminium foil layer. I went by the idea that you can never have too much insulation! "



From my experience the foil layer is a mistake (unless it's perforated) because it acts as a vapour barrier with steam simply condensing on its underside only to be reabsorbed by the fibre insulation again. A path for its escape must be made or the oven will take way longer to purge itself of moisture.

[MarkJerling]

From my experience the foil layer is a mistake (unless it's perforated) because it acts as a vapour barrier with steam simply condensing on its underside only to be reabsorbed by the fibre insulation again. A path for its escape must be made or the oven will take way longer to purge itself of moisture.

True! As the foil is not the perforated type, I layered the foil so that the top part of the dome's foil overshoots the lower foil layer working on the assumption that steam will go to the highest point, condense and then run down the underside of the (upper) foil. So, the theory goes that the moisture can get out over the lower "wrap" of foil. But then, the dome did have a week of sunny weather to dry out between completing the dome and adding the insulation so hopefully that helped with the water vapour trapping issue.

But, the bigger consideration for me is that I don't want moisture which may get through the plaster layer, get to my insulation. In that regard, even if the construction takes loinger to dry out, the foil will not allow exterior moisture, through hairline cracks in the plaster or whatever, to get to the insulation. So, from a waterproofing perspective I think it's a good addition.

[david s]

A lot depends on weather conditions. We live in the tropics and my oven is exposed to the weather. In extreme humidity even if it hasn’t rained, moisture gets into the oven from the atmosphere and accumulates in the insulation. I covered my insulation with foil and being concerned about it acting as a vapour barrier, perforated it in two placed about 5 sq in each. Consequently when the oven gets damp the outer shell becomes hot to the touch in those two places where steam is escaping from underneath th foil. I wish I had perforated it all over.

[MarkJerling]

A lot depends on weather conditions. We live in the tropics and my oven is exposed to the weather. In extreme humidity even if it hasn’t rained, moisture gets into the oven from the atmosphere and accumulates in the insulation. I covered my insulation with foil and being concerned about it acting as a vapour barrier, perforated it in two placed about 5 sq in each. Consequently when the oven gets damp the outer shell becomes hot to the touch in those two places where steam is escaping from underneath th foil. I wish I had perforated it all over.

Now, that's very interesting. The first few times I fired my oven, I wondered why the oven felt warm in the area where my foil layers overlap and therefore "vent". It must have been steam. The last few times I've fired it, those areas were cold, like the rest of the plaster. I can only assume that all the (or most of the) steam has now "left the building"!

[MarkJerling]

A more detailed temperature profile this time, and graphed from cold to hot, to almost cold again.