- Nov 6, 2023
- Reaction score
- Bonnie Scotland ye ken
Martin, you commented:
I also use the Stefan Boltzmann equation for radiation heat transfer. It needs modifying to account for emissivity as you state. Choosing those emissivity values for a given application is not easy. I assume a coal fire surface e = 1, but accept that may not be right. I would expect your ceramic burners to be near 1, otherwise they would not be widely used. But as for precise values?????
I think I must do some more work to get "the right answer" here.
I had to do "cooling" calculations on Power station electrical equipment being installed in Dubai, where the SUN heating could potentially heat the equipment above the maximum temperature where we wanted to run the plant. And at night, the air temperature could drop below freezing... So to stop the Sun we fitted sun-shades... but that meant we had a screen with sun (radiant) heating on one side, Hot air all around, and equipment trying to radiate heat as the air temperature was so high.... Emissivity factors were key to managing this problem.
So the table:
The "simple" analysis - as far as I can remember - is that a body cannot change its emissivity... -
- Aluminum (anodized) 0.77
- Aluminum (polished) 0.05
- Concrete 0.92
- Copper (polished) 0.05
- Copper(oxidized) 0.65
- Glass 0.92
- Gypsum 0.08
- Ice 0.97
- Sand 0.9
- Snow 0.8S
- oil (Dry) 0.92
- Soil (Saturated) 0.95
- Stainless Steel 0.59
- Water 0.95
- And from my memory...
- White painted (Aluminium) ~0.2,
- Grey painted ~0.5
- Matt black painted ~0.95
And once agreed how the material is performing you can do proper sums, but I can only GUESS as I have no other information.
- a White ceramic emissivity of "0.2" - if considered as a "white" surface - could really act like concrete at "0.92" which is close to the "Matt black" 0.95.
- But Coal ash on the surface of coal (cheap household coal or charcoal, "coal substitutes", etc.) may act more like Gypsum at 0.08 than Matt black at 0.95!
My GUESS of emissivity:
FOUND my fag-packet notes of my sums so I can do them properly for you if you need them? I have never actually verified any of these sums, but work simply on Gas power - with complete combustion - going "IN", and smoke-box gas temperature going "Out". Part radiant heating and part gas conduction (surface area) based to "text book" values...
- For coal = Part hot matt black coals burning at e = 0.95 and part Ash-coated (like Gypsum) at e = 0.1, insulating the coals behind and at a much lower temperature because adjacent to the Cooler gases adjacent to the copper inner surfaces of the boiler where heat is being conducted from the gases to the copper at ~200~250C - so maybe really around e = 0.8 for a loco fire?
- I used e = 0.8 for emissivity of WHITE ceramic: The surface is made of small cones, with gas burning alongside heating the cones, therefore a part of the surface is white at the gas/air temperature at the fuel inlet holes, part is probably over 900C, and the rest of the cone is somewhere between the two? - Maybe averaging 900C? (book max. value... I assume measured as a mean temp. of a larger area?).
- Similarly, I considered the copper in a boiler to be tarnished, so used e = 0.8 (a guess! - poor memory) but should perhaps have used 0.65? - For the fire-hole door, the heat assumed to shine on the inside, 80% absorbed by the door (Lost outside), and 20% reflected "for absorption elsewhere" to become heated water...
- For "stainless steel mesh" I used e = 0.95... (a guess?) but considered inner and outer wires to be heated by burning gas so the whole surface area at ~1220C. - Even though in photos the corners, and odd places are not even red hot.
Hope that clarifies my amateur ponderings?