primary flame competing with secondary flame?

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mar13

Feeling the Heat
Nov 5, 2018
466
California redwood coast
I spend a lot of time watching and enjoying a fire, especially with being at home due to Covid lock down. I've also been experimenting a lot this winter with different woods and sawdust logs( e.g. pres-to-log). I'm still learning the "science" and "art" of a good fire. One thing I can't quite figure out lately is what are the conditions best for creating a secondary flame display and if that display is over rated. Here are some conditions that leave me a bit unsure of what's going on with respect to the science:

Situation 1. Strong yellow/orange (some may call "sooty") flames, such as produced when a sawdust log expands and burns vigorously. A lot of heat is produce, lots of flame, but no real classic display of secondary flame. (I suspect an inefficient use of the log's potential BTUs.) Mind you, this is when the box wasn't full of many other wood or deep in coals because I didn't want to risk over firing with a sawdust log. I'm wondering if the primary flame is consuming most of the oxygen and not leaving much oxygen for the secondary burn?

Situation 2. This may be related to the first situation, but with a twist. This afternoon I just had a couple small splits of oak and a sawdust log, as I just try to maintain the house temperature when it's in the 40s outside. They ignited from some coals raked forward and I let it burn for quite a while with the air fully open. The flue probe was reading upper orange (650-700?) and there were strong yellow flames but no obvious secondary. To experiment, I shut the air all the way down and I get the classic secondary flame show with much less primary flame. Knowing that it wasn't sustainable, I added more air to get what I'd consider a more sustainable mix of primary and secondary flames. My question here is, why does the secondary take off when the primary air is turned down so low? Is most of the air now coming from the PE's baffle secondary air and thus giving the gases first chance at the oxygen?

I'm working with a PE T5, series D, which has the secondary air opening linked proportionally to the primary air opening, if that matters.

All thought and guesses are welcome. I know there have been two other related posts going about over the past couple of days.
 

BrianVA

New Member
Oct 28, 2020
48
Central VA
I believe this is at least partially due to the fact that when you suddenly close the air down all the way, the fire becomes very smoky, and because the heat was so intense just prior to closing it down, that smoke is being burned very efficiently by the secondary combustion tubes. I'm sure there are other factors as well.
 
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NoGoodAtScreenNames

Feeling the Heat
Sep 16, 2015
422
Massachusetts
The draft is caused by the temperature in the firebox. Once you close it down fast that negative pressure doesn’t change too much. But since you shut the primary down that pressure now pulls from the secondary. More oxygen at the top then tips the flames to come from there.

I often get secondaries by themselves but often they are mixed with a little primary as well. Often it’s like a small candle that is igniting the robust secondaries at the top.
 

nph3

Member
Jan 8, 2019
1
New Hampshire
What you saw in situation #1 is common with saw dust logs or any super-dry wood like that. Typically those things are kiln-dried so most of the stuff in them that would off-gas and fuel the secondaries is already gone. What’s left is the quick burning stuff the primaries go nuts over so they give you that quick blast of heat and then poof, just some powdery ash is all that’s left.
 
My question here is, why does the secondary take off when the primary air is turned down so low

I think its simply a matter of where you are burning your combustible gasses. If you consume all these gasses in the primary flame, there is none left for the secondary. If you kill the primary flame and conditions are hot enough, all that gas burns in the secondary.

Usually i get a mix of both but my new 2020 T5 will occasionally do all of one or the other. So long as there is no smoke out of my stack I don't really care which is which.