optimum efficiency for flues in brick ovens
Hello all,
I came across your forum and was wondering if you could help me understand or guide myself in the right direction for some questions perhaps basic in your field and knowledge but I am having trouble understanding.
I am building a brick oven with a flue, one of those pizza ovens. My oven is a pure semicircle with a diameter of 128cm and a dome height of 64cm. My 2 main issues are about the dome entrance size and flue size.
1) Dome Entrance: I'm sure there is a exact formula to the science behind it all im sure and Im guessing that the archway the dome entrance should fit into a percentage of the whole volume of an oven to work at its optimum)
However we have managed to get the hight of the arch to an overall size due history and peoples experiences.
In relation to the heigh of the arch generally most people who build brick ovens say that it's approximately 63% = +/- 5% to the height of the top of the inner dome. If it’s too high their will be too much heat loss, too low and not enough air flows through not heating it sufficiently. O.K. so how did they come up with figure? After all the research I haven’t got a clue! A valid reason was that of all the oven measured including the ones still standing today after 2000 years it seems that it’s around the 63%. Good enough for me but still it bugged me as I had a itch to find why? I found out that it could be a empirical ratio. An empirical formula of a mathematical equation that predicts observed results, but is derived from experiment or conjecture and directly from first principles. perhaps a empirical ratio that is ultimately related to the properties of pizza and brick? There is no fundamental heat transfer law that governs this. It is simply an empirical ratio, boring as that may be. Could it have to do with the “Venturi effect” as cold air is drawn in and hot air expelled from the flue?
Perhaps It could be the “stoichiometric mixture" formula since it's a Air-Fuel ratio much like a car. There is a optimal efficiency to the combustion of the air/fuel mixture. So if the stoichiometric formula is correct and I am using the normal chemical equation for wood CxHyOz + O2 ------> CO2 + H20 ) which I don't understand as I pulled off the net. Could we work out the optimum size of the entry arch and would this equate to the 63% that has been averagely measured over 2000 years of pizza ovens (some which still stand, like in Sicily Italy)?
If you were feeding your oven with a regulated amount of gas or oil then the stoichimetric ratio might apply, but if you have a pile of wood of varying composition, size and surface area, then the ratio is not applicable. and could perhaps be moot.
The only number that I could come up with was of the fibonacci level of 61.8% of natural flow and expansion. Does it somehow naturally effect the growth of heat at its maximum capacity without the loss of heat (so its maximum efficacy so to speak)?
2) Flue Design: What would be the optimum flue size for maximum efficiency, Would it have to do with a percentage of the oven size? A lot of people are saying a minimum of 8inches but I'd like to find out the formula to this or how they come to that figure apart from the bigger the more draw,
Any help and guidance would be greatly appreciated and credit and reference will be stated in a book that I will publish on building ovens.
Thanks in advance