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Post by Deleted on Jan 23, 2016 8:53:46 GMT -8
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Post by Karl L on Jan 23, 2016 10:54:05 GMT -8
I've done some calculations of convected and radiated heat from a bell.
For a surface at temperature T, convected heat is: Hc = h A DT
Hc = Convected heat h = Heat Transfer Coefficient A = Surface area DT = temperature difference between hot surface and air.
For a vertical plane surface, h is about 4-6 W/m2 K (I calculated this from a ~2Kw domestic radiator, guessing the effective surface area, but there are similar values listed on line).
Radiated power is:
Hr = S E (Th^4 - Ta^4)
Hr = Radiated Heat. S = Stefan-Boltzmann constant = 5.6×10-8 W/m2K^4 E = emissivity (This is about 0.75 for fire brick and for 'oxidised steel'.) Th = temperature of the hot surface Ta = the ambient temperature. '^4' means Raised to the forth power (I.e. squared, then squared again).
I made a simple spreadsheet to calculate convected and radiated heat for different temperatures, etc, assuming that the convecting surface area and the radiating surface area are the same, which would be true for a barrel or a bell, but is not true for a finned domestic 'radiator'.
The results are shown below for a Surface area of 1.0 m2, Heat Transfer Coefficient of 5, emissivity of 0.75, ambient temperature of 20C: T(C) C(kW) R(kW) R/C 50 0.15 0.15 1.0 100 0.4 0.5 1.3 150 0.7 1.0 1.6 200 0.9 1.8 2.0 250 1.2 2.8 2.5 300 1.4 4.2 3.0 350 1.7 6.0 3.6 400 1.9 8.3 4.4 450 2.2 11.2 5.2 500 2.4 14.7 6.1
The R/C column compares radiated power to convected power. As you would expect, radiation increases much faster with temperature than convection because it depends on T^4, not just T.
The front of my current (non-rocket) stove is usually at ~350C, so this area is radiating ~4 times as much as it is convecting. But all of the stove surface is hotter than 50C, so radiation is everywhere the dominant heat transfer mechanism.
Let me know if you want a copy of the spreadsheet.
Karl
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Post by Karl L on Jan 23, 2016 11:24:33 GMT -8
Thanks for that, Karl! -- I didn't see your post because I was so busy doing calculations. Your link above shows that HTC varies a bit with temperature, but my estimate wasn't too far out Karl L
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Post by satamax on Jan 24, 2016 1:59:43 GMT -8
Hi everybody.
Regarding my failure, or seemingly failure.
Friday, I lit it up around 11 in the morning. Then went to eat. Came back around quarter past two. Reloaded on the leftover embers, not many. Left to check a job. By the time I came back, about 6 hours and one reload after the first fire. The mass was warm to the touch. I have 8 inches of brick and concrete mixed. An air gap, a metal bell under all this. I think the bell most of the time peaks in the middle of the night. I think 4 burns a day would be all right in the coldest part of the year. I was expecting more. But, I didn't know what to expect realy.
It would be interesting to have a thermograph.
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Post by pyrophile on Jan 24, 2016 4:18:06 GMT -8
About different kinds of infrared radiations on human body confort, the interesting document from Herschel ( The Preferred wavelengths for Comfort Heating ) explains the reasons why : "Far Infrared is biologically more significant in comfort heating than shorter (hotter) heat wavelengths because skin content is 80% water which only absorbs heat wavelengths from 3 microns and lower (Far Infrared). Far Infrared is better absorbed by the skin than other wavelengths; is less “transmissive” and is not reflected – this is untrue for shorter wavelengths" And this : "Because Infrared Heat covers the whole spectrum of radiated heat (ranging from very intense heat from a light bulb at 2600°C to heat you’d feel from a glowing coal at 600°C, to a rock, warmed to 20°C by the sun), 3 correspondingly very different categories of Infrared have been defined, exactly fitting the above examples. These are “Near” Infrared (Also called Shortwave or IR-A); “Medium” Infrared (also called Middlewave or IR-B) and “Far” Infrared (also called “Longwave” or IR-C). And exactly along the above categories, from a point of view of Comfort Heating, the only acceptable waveband is Far Infrared (Humans reach optimal “Comfort” at around 21°C). Shortwave is too harsh. The body has developed certain protection mechanisms over millennia to avoid it (not looking directly at the sun; seeking shade from direct sunlight (Voke: 1999); the skin, indeed also reflects up to 35% of received shortwave and does not efficiently absorb the heat – indeed shortwave is transmissive into the skin producing discomfort (a signal to get out of its way). It is not well absorbed. (Schroeder et al 2008). Heat lamps emitting shortwave do not make appropriate Comfort heaters. Middlewave is better absorbed by skin and less reflected than shortwave, but in terms of Comfort heating of humans, emitters that output Middlewave heat are still strong (e.g. 600°C) and better adapted to industrial heating and drying processes than to provision of comfort. Longwave or Far Infrared on the other hand is the waveband at which water begins to absorb heat specifically well for the least input energy of the above 3 wavelengths. Far Infrared heat is optimally absorbed by the skin surface, where the warmth is readily absorbed by conduction into the tissue and blood and transported around the body. This is why Far Infrared is used in heating cabins and baby incubators and why Herschel specifically uses Far Infrared heaters." (in their section what is infrared heat?)
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Post by meusnierc on Oct 16, 2016 11:01:18 GMT -8
hello to everyone and everything Thank you to Peter for this great find and all of you to share your experiences on this forum. I am French (department 41 center) near the Chambord castle located for you. In my experience, I have a batch 180.the mass weighing 6 tons. (Clay, sand stone, straw) Of the fire door at the end of bench.measuring 9 meters. (Horizontal portion). The vertical part 7 meters. (Not yet lair). My bell is hexagonal from a diameter 66 centimètres.The top of the bell reached 510 degrès celsuis.Orange clear Thermal mass reached 100 degrees celsuis. I heat 650 cubic meters partially isolated. I'm very happy !!!! I added an air inlet which passes under the stove and finish in the heart of heaters. it accelerates the Venturi effect and lowers the flames.
4 air intake passing under the stove and going back along the riser.Operating it kills convection, but multiplied radiation. (Give a headache)
Hope
Cédric
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Post by satamax on Oct 16, 2016 14:32:40 GMT -8
Cedric, sorry mate. But this is nearly ununderstandable. Please, work on your english.
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Post by meusnierc on Oct 16, 2016 20:09:06 GMT -8
Sorry, I take the google translator
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