Thermal Mass Musings

[JoeT62]

I know there has been a lot of discussion regarding thermal mass vs. heating time, but I had a thought last night as I was drifting off to sleep...would slightly more thermal mass at the apex of the dome provide for longer "stay hot" times without sacrificing too much in the way of rapid heat up? Here's my logic (such as it is when falling asleep after a night on call): the top of the oven seems to be the place where the oven goes clear first (makes sense - heat rises, and it is often in direct contact with the flame), and so would also be the place that starts to leech out heat into the insulating layer first. If more of this were captured by thermal mass (while the rest of the oven continues to go clear) might it not provide more reflective heat later?

I was thinking this could be done with either a slightly thicker level of refractory mortar at the top, or even mortaring small fragments to the outside of the dome at the apex (say the top 3 or 4 rows). Nothing that would compromise structural integrity.

Of course, the other argument is that the extra mass would be robbing the heat from the side walls which might be getting some of it via brick to brick contact).

I know Father McMahon (my HS Physics teacher) is rolling over in his grave right now.

[Hendo]

Re: Thermal Mass Musings

Joe,

I'm relieved that someone else out there has these musings too, before/during/after sleep! For me it's a morning thing from around 6am.

What you suggest to me is entirely logical. I was thinking recently of possible 'parabolic' heat distribution in the dome and (like you) whether it would be more efficient to have a greater mass on top of the dome, tapering down to standard half-brick thickness at the floor.

As to the question of whether increased thermal mass at the top would 'rob' lower courses of heat, I'll leave that to the thermofluid dynamic experts out there.

Cheers, Paul.

[wlively]

Re: Thermal Mass Musings

Interesting discussion. More thermal mass on top may be a way of storing more heat "on the cheap" as it were. But then again it may not. The heat stored in the top would only be useful if transmitting said heat energy to the walls by conduction or useful in cooking thru radiation. Convection would be going out the chimney.

My oven is the opposite of your proposal. On ring 8 I went from 1/2 bricks to alternating 1/2 and 1/3 bricks. And ring 9 up is all 1/3 brick. I also added extra insualtion in the form of 2 inches of perlcrete on top. My thermocouples are in the center of each brick. As expected the top heats up faster, true in any thickness I am sure. After the big fire goes out it takes about 2 hours for the temps to generally equalize with the middle and bottom zones increasing in temp for the 2 hrs, while the top falls.

I am sorry I have not made a very thorough measurement run yet. I have been waiting to see if the heat up times will continue to decrease as they have so far. As soon as I get a fully instrumented run together I will post the data.

[Dutchoven]

Re: Thermal Mass Musings

Just off the top of my humble head the idea seems to make sense but, what are you thinking with the additional mass. Longer bake times? If so soaking heat into the floor would seem like a better idea to me. Also from a structural standpoint you would probably need to buttress the walls somehow to deal with the potential increase in thrust. Temember that most ovens fail right in the topmost rings of the dome. Increasing the weight in that area would only make gravity more of an enemy. I think I like thwe way that you build your oven with the shortening of the bricks as the height increased. That is a built in buttressing and reduction of the thrust. I have seen ovens constructed with standard firebricks in the lower walls and after about 4 or 5 courses switched to "splits" cut in half and then down to thirds for the upper area of the dome. Seems to make construction without forms a bit easier given each brick is lighter.
Anyway, I wake up with ideas too!
Best
Dutch

[maver]

Re: Thermal Mass Musings

I think Dutch is asking the right question - what is the reason for increasing thermal mass? What performance gain are you looking for. While I think you are right that if you are to add thermal mass, the dome apex is the safest place to add it thermodynamically, the first question must be answered first. For pizza making you could argue that a larger heatsink in the apex might stabilize oven temperatures better, the hearth is still where it's the biggest issue, and at the apex you are getting pretty far from the hearth. In breadmaking it may be a help, but then again, you probably wait longer for the oven interior to equalize with higher mass in the hotter part of the oven. Then there's that whole concern with it caving in... I still think the idea is intriguing, this might be a good question to pass on to my friend the rocket scientist.

[wlively]

Re: Thermal Mass Musings

Yes Maver, that is what I was trying to elude to. Convection and conduction heat, I think are more useful in brick ovens than the IR. If high mass is what you want it would be better "spent" in the floor. But that is where heat absorbsion is the slowest.

Good point Dutch. I forgot about the structural concerns. But that was my thinking when I thinned out the upper 1/3 of my dome. I was also thinking I had enough mass already and less on the top would probably not be very noticeable. Especially when well insulated.

It would be very informative if those of us with instrumented ovens can post mass profile along with temp profile. I guess I had better get on that.

[maver]

Re: Thermal Mass Musings

OK, who's going to be the first to build a thicker hearth and thinner dome with copper heat pipes running through the dome and the hearth?