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Somebody explain this please

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by johnstra, Feb 7, 2011.

  1. johnstra

    johnstra Feeling the Heat

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    I've experienced this on several occasions now, so it's not a fluke. Tonight I needed to put 2 medium splits on to kill time before loading for the ovenight burn. So I put 2 pine splits on a bed of hot coals with the air wide open. My soapstone stovetop temp doesn't really respond to this... it went from 400 to about 450, but what is surprising is how much heat pours off the stove with these small, wide open loads. It's 13F outside right now and my stove room is 78 - i mean it's hot!

    I'd like to understand this better. It really feels like the stove is radiating more heat on these short, small cycles than it does with big loads. Is there some thermodynamic magic happening? Does all the fuel in a big load absorb some of the radiant energy, where with the small loads it all gets directed at the walls of the stove?

    Just curious.

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  2. begreen

    begreen Mooderator Staff Member

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    Big stove with a lot of radiant surface. A 50 deg increase is not trivial. There's a lot more heat radiated. BK will probably shortly arrive with the actual math.
  3. henkmeuzelaar

    henkmeuzelaar New Member

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    This is one of these paradoxes that make combustion processes so interesting to me. Here you are using LESS fuel (only two splits) and probably burning in a less efficient manner by sending more hot air up the stack and yet you appear to be getting MORE heat output! What's going on??

    Because two critical combustion parameters have changed drastically, namely air-to-fuel ratio and wood surface-to-volume ratio you are getting a much hotter burn.

    Because of the higher combustion temperature more radiant heat will be sent through the stove's big ceramic glass window (energy is being radiated as the fourth power of absolute temperature AND this energy is being radiated at shorter wavelengths that pass more easily through the window).

    Moreover, as already mentioned by BeGreen, your whole stove surface will be hotter and produce more heat by radiation (plus spontaneous convection).

    Finally, your entire flue pipe will become hotter because of the higher temperatures and air flows, thereby also producing more heat by radiation plus convection.

    Yet, when all is said and done you have probably LOST BTUs because the high temperature burn has been much shorter AND you have lost more BTUs up the stack.

    In other words, your two splits went up in a blaze of glory!! In the process you traded in long-term heat output potential for short-term gains while shifting the radiant heat vs. convective heat balance of your stove's output to the radiant side (making it more like a fireplace IMHO).

    Henk
    Jon1270 likes this.
  4. soupy1957

    soupy1957 Minister of Fire

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    Phew! A fella could get downright SWEATY reading thru that.........lol.

    I burn wood, it gets warm in my house, I keep burnin wood so's I don't get cold.........

    -Soupy1957
    Hearth Mistress, Pallet Pete and loon like this.
  5. johnstra

    johnstra Feeling the Heat

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    Me too, Soup, but this is interesting stuff.

    Thanks Henk. So it is a hotter fire and I'm getting the benefit of lots of radiant heat - particularly with the Mansifield's big window. Ok. What the heck is spontaneous convection? I've never heard that.

    It would be interesting to know if there is some threshold at which a high heat burn with a small amount of fuel does not vent more BTUs up the flue than it imparts on the surroundings. Seems there has to be a tipping point.
  6. henkmeuzelaar

    henkmeuzelaar New Member

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    Spontaneous convection caused by heat gradients ( especially along vertical surfaces) as opposed to forced convection driven by fans (along any surface or through any tube).

    A "big bang" burn (let's say a cloud of saw dust particles exploding in 100 % oxygen) where radiation is released almost instantly and at very high temperatures would be most efficient, simply since there is no time for loosing BTUs up the stack :) You just might need a new stove, of course ;)

    In short, there is no real tipping point from a combustion reaction perspective but there will automatically be maximum operating temperatures annex combustion rates from a materials properties and oxygen supply perspective.

    I have been thinking for some time already about the design of an efficient fireplace configuration with lots of radiant output while still made of reasonably affordable materials. Hope to post the results one of these days.

    Henk
  7. Jags

    Jags Moderate Moderator Staff Member

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    Also keep in mind that heat output by degrees is not a linear increase. Meaning that 400 going to 450 will yield MORE than the 12.5% increase in temp to BTU's into the home. (if that makes any dang sense?)
  8. henkmeuzelaar

    henkmeuzelaar New Member

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    You're quite right, of course, since radiation output increases as the fourth power of temperature (when written in degrees Kelvin), as mentioned in my post.

    Thus, the radiation output increase at 450F versus 400F is {[(450-32) x5/9] +273}e4 / {[(400-32) x5/9] +273}e4 x 100 % = 125.38 %. Roughly a 25 % increase, all other things being equal.

    Henk
  9. Jags

    Jags Moderate Moderator Staff Member

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    Yeah, yeah - thats exactly what I was thinking.
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  10. Milton Findley

    Milton Findley Feeling the Heat

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    I know what prompted this post, but you gotta love that sort of understanding and the long long hours of skulduggery it took him to get that amount of expertise. I have been chasing around the net trying to find a place where it gets explained simply, and I keep getting sent back here to his posts. :) That says something.
  11. henkmeuzelaar

    henkmeuzelaar New Member

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    Awww shucks, Just saw that I managed to contradict myself BIG TIME!!

    Johnstra is right, there may well be a tipping point somewhere.

    I got so excited by my own "big bang" scenario that I failed to consider that the hot gases produced along with the big radiation output, still should go up the stack, rather than be released into the house (although the latter outcome is alas more likely... ;) ).

    So, I promise to keep thinking about Johnstra's question and see if I can come up with a better answer.

    Henk
  12. Jags

    Jags Moderate Moderator Staff Member

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    Hey - I brought it up. A little credit please. :coolsmirk:

    :lol: :lol: :lol:

    It should be evident who the brainiac is on this subject. Cu-dos to PyMS.
  13. Dakotas Dad

    Dakotas Dad Minister of Fire

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    I don't need no stinkin' math. I am just a simple man.

    I know that the feel of the heat output difference from cold stove to 200 is nothing like the difference from 400 to 550. Much above 500 and I truly can't stand to be near the thing.

    and as a simple man.. "I feel warm, therefore, I am"


    ;-)


    EDIT TO ADD: those of you with steel stoves running 6-7-800+ degrees.. I don't see how you stand it. I was at a stove shop last week they had a new StepTopSomething running in a corner, the thermo on top said 725 or so, I could only stand to be close enough to read it for a short time, but my buddy would have sat on it I think. He grew up with some giant steel stove in the basement in upstate NY..
  14. RNLA

    RNLA Minister of Fire

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    I envy the math skills. I too am a simple man, it is unfortunate, I could have done so much more in life if I could only get over my lack of education in math. This is not meant to be a funny post, I suck at math it makes me nervous! So for anyone who has little ones around take it from me, make math fun & a priority of learning. You still can not beat wood heat. That is a simple fact you can not deny! :gulp:
  15. Battenkiller

    Battenkiller Minister of Fire

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    It's simply a conversion of degrees Fahrenheit to the required degrees Kelvin. Easy to do with most scientific calculators. Lacking one, if you have a smart phone there are many apps available to do this conversion (and hundreds of others) automatically. The only thing you need to do is to use a calculator and take the fourth power of each temperature in degrees Kelvin and compare. The results are in Joules, not BTUs, but since you are just comparing the temps relative to each other, there is no need to convert to SI units.

    So, now maybe Henk can explain why I notice the same thing in my stove... which just so happens to be as windowless as a dungeon.
  16. Battenkiller

    Battenkiller Minister of Fire

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    Trust me, it is never too late to learn. Many folks are mathphobic, especially adults who never delved into it. The numbers and symbols are alien looking and frightening to many. I was in my mid 40s when I went back to school. I had forgotten everything I once knew, but before long I was tutoring in the Math Lab. It was there that I discovered how much people would recoil from the simplest concepts just because they were filled with fear. I was able to help numerous returning adult students clear the hurdle of their fears and continue on with the subject.

    I pursued mathematics through Calculus, and it wasn't all that hard (and I'm no natural brainiac), but for most people, just a little basic algebra and trigonometry will allow them to solve numerous real life problems that had previously eluded them.
  17. henkmeuzelaar

    henkmeuzelaar New Member

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    You still get the same increased heat production from a hot little burn at relatively high air-to-fuel and (effective) surface-to-volume ratios that Johnstra does.

    All that changes is that the piece of metal that sits where your stove window should be ;) is quite a bit less effective in passing on radiant heat to your room environment.

    The energy "saved" there, however, should translate into somewhat higher stove and flue temperatures than you would have gotten with a proper window (just yanking your chain, of course, because I happen to be a sucker for fireplace-style radiant heat output :) ) and thus increased radiant + convective heat output to the room.

    All in all, the balance between radiant and convective heat output from "blaze of glory" events in your stove probably doesn't tip as far toward radiant as Johnstra's does.

    Henk
  18. henkmeuzelaar

    henkmeuzelaar New Member

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    +1

    Believe it or not; I don't particularly like math! When it comes to mathematical laws and formulas I have always suffered from CRS (Can't Remember S*@t) syndrome unless I really needed to do a calculation in order to help interpret lots of tiresome measurements or to estimate how best to minimize a newly planned measurement series. Just call me lazy, I guess ;)

    In the old days of big slide rulers, there was almost no escape from having to learn all sorts of boring rules and manipulations. Also, it took a lot of time to look up formulas and procedures one had forgotten. With the internet, all that has become a lot easier, of course.

    In fact, the main threat to young geeks is that they may never get away from behind the computer long enough to learn how to put their pants on straight, let alone operate a wood stove......

    Henk
  19. DanCorcoran

    DanCorcoran Minister of Fire

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    I think there's a big difference between being comfortable with math and being good at math. I've always prided myself at being comfortable with it, in the sense that I could understand it if was explained slowly and I could ask questions. I can also work out math problems, if given the time to remember or look up the principles.

    I worked with folks who were GOOD at math, though, and that's a whole different story. They'd either be doing things in their head that I needed to work out on paper, or they'd be solving problems on paper in 5% of the time it took me.

    I think children (and adults who are interested) can be made to feel comfortable with math, but being good at math is, I think, a gift.
  20. johnstra

    johnstra Feeling the Heat

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    I can keep up until you throw double or triple integrals at me... at that point, something in my brain says "STOP OR IT'S GOING TO HURT!!"
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  21. Battenkiller

    Battenkiller Minister of Fire

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    I don't look at it like a higher air-to-fuel ratio per se so much as an improved mixing of the air and the fuel that is available. Many stove designs would pass the excess air too far away from the hottest part of the fire, providing no increased burn efficiency, but definitely creating a cooling effect on the fire and flue. With enough turbulence inside the firebox, however, the entire cavity becomes filled with a more evenly mixed mixture of gases and air. With a full load, the effect of turbulence isn't as pronounced, so there will be fuel-rich areas between the splits that just don't get enough oxygen in the primary burn area. Therefore, I believe that burning smaller amounts of wood promote more complete combustion, but it certainly is difficult to heat that way unless the smaller amounts of wood are being dropped into a burn pot in pellet form.

    I thing the OP was right on when he mentioned the presence of the wood in the full load shielding the walls and glass from the direct effects of IR radiation. Still, if IR through the glass was largely responsible for this burst of felt heat, it fails to explains why the actual air temperature he is experiencing rises so quickly. IR radiation passing through air heats it up very little, so I think that convection may be a larger part of the picture in the case he is mentioning - both coming from the stove itself and coming from the other radiating surfaces in the room.

    Which brings up and even more curious phenomenon I have experienced but can't quite explain. I can get my stove up to a certain temperature, but when I close the bypass I get an instantaneous wave of heat that really just hits me in the face. When I play the IR gun over all of the stove and pipe surfaces, I see no instantaneous rise in surface temperature (which, of course, is what I would expect). My feeling is that I must be feeling heat from the stove via convection vs IR radiation, but I still don't understand how this would occur without immediately increased surface temps.
  22. Battenkiller

    Battenkiller Minister of Fire

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    Heh, heh! I know what you mean. I don't look at any of this level of mathematics as being a gift, though. It usually comes from long hours with paper and pencil in hand.

    My calc professor told me the she took an advanced grad level course in heat transfer at RPI and got an A in the course. She didn't understand a word of what was said about the actual topic, but she had a very decided advantage over the real engineering students because she could integrate just about anything my hand, whereas the engineering students were forced to use a book full of integration tables because they never worked hard at integration by trig substitution, etc. She learned how to set up the problems when she saw them, then got the answer and was out of the test room before the engineers even cracked the integration books.

    Now, if you're talking about dictating 40 pages of partial differential equations from memory, and then going back and saying, "Wait... on page 23 near the bottom... that should have been the absolute value of ø, not a negative sign".... well, that's a gift. :cheese:
  23. krex1010

    krex1010 Minister of Fire

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    I think he forgot to carry the 1,
    1+1=3, yeah I think that's right.
  24. Battenkiller

    Battenkiller Minister of Fire

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    As I said... not that difficult at all. :roll:
  25. henkmeuzelaar

    henkmeuzelaar New Member

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    +1
    Basically we are both saying that transport limitations are responsible, with me emphasizing (1) transport of oxygen to the fuel (i.e. air-to fuel ratio in a well-mixed system), and (2) transport of oxygen into plus heat and pyrolysis/combustion products out of the wood (i.e. surface-to-volume ratio or, better, effective particle size) and you emphasizing (3) mixing (in case of a non-homogeneous environment). When trying to build combustion models that include transport limitations (1) and (2) are ALWAYS needed and (3) is necessary in a poorly mixed environment.

    Agreed. Perhaps you could generalize the pellet example to any kind of continuous hopper/auger driven feed system (chips, corn, etc)


    Note that I was careful NOT to say that. I am just pointing out that for a given surface area the window will radiate far more heat than the stove or the flue. However, since there are many more square feet of stove surface and (presumably) flue pipe surface, the total (radiant plus convective) heat production from these sources is likely to be "largely responsible" instead. In fact, I could not even have tried to explain your similar (as the one reported by Johnstra) "blaze of glory" experience if I had indeed committed myself to the idea that Johnstra's stove window was "largely responsible".

    Sorry, but I first will need to try and get my @ss out of the sling with regard to the glaring holes in my "big bang" combustion theory (expounded several posts ago) before I can risk getting myself further into trouble here :) :) :)

    Henk

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