Skin temperature and latent heat release Elena Saltikoff FMI (material courtesy of prof. Timo...
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Transcript of Skin temperature and latent heat release Elena Saltikoff FMI (material courtesy of prof. Timo...
Skin temperature and latent heat release
Elena Saltikoff FMI
(material courtesy of prof. Timo Vesala, UH)
19.04.23Ilmatieteen laitos / PowerPoint ohjeistus 1
Good News
• Jochen: …if you have no lift, the hot and moist air will only make you sweat and wish for refreshment…
• Good news: we have the lift
• …taking you to 5th floor
• …for refreshments
• and optional hot and moist air
• (towels provided)
19.04.23Ilmatieteen laitos / PowerPoint ohjeistus 2
Hot and moist air: Sauna• Three million saunas in the world
• 4/5 of them are in Finland
• Smoke saunas and saunas heated by wood or electricity
• Originally the word ”sauna” has meant a pit in the ground or snow (especially dug by a bird to spend the night in it)
• Sauna of Sibelius in Ainola, photo from Wikipedia
19.04.23Ilmatieteen laitos / PowerPoint ohjeistus 3
Some quantitative facts*:• Temperature of air is 80-110 °C (175-230 °F)
• Dew point is 37-43 °C, that is 40 – 60 g of water per dry air kg, relative humidity is 10-20 %.
• Skin temperature is 39-43 °C
• * Teeri N., The Climatic conditions of the sauna. In: Sauna studies (Teir, Collan and Valtakari, Eds.), 1976.
Skin temperatures (Ladies):(Measured by IR-radiometer, 25.5.2005)• Before sauna: 34.4 (34.3-34.5) °C
• After sauna: 38.3 (35.7-40.7) °C
• After swimming in a lake 22°C (12.1°C, immediately after swimming)
• After cooling down 29.1 (27.1-30.6) °C
• Inside the sauna, lower level 41.6 °C
Some qualitative facts:• By throwing water on kiuas
(heart made of stones) the temperature and humidity are controlled
• Bather feels löyly (increase in humidity, not mist) as heat on the skin (why?)
• Bather perspires 1 – 4 litres per hour (normal rate is 1 litre per 24 hours)
• Feeling of “lacking oxygen” may occur (why?)
The essence of the matter:• Even a slight increase in humidity would suppress
vaporization from the skin (sweating)
• Vaporization might reverse to condensation (same process but opposite character)
• Energy is transformed in the form of enthalpy from the kiuas to the skin
We have experienced latent heat
Latent heat cooling, evaporation Condensation
19.04.23Ilmatieteen laitos / PowerPoint ohjeistus 8
Conclusions:• Latent heat
(including ceasing vaporization)
• Sensible heat
19.04.23Ilmatieteen laitos / PowerPoint ohjeistus 10
Postscript:• Oxygen content of sauna air corresponds to that at the
altitude of nearly 3 km.
• Heat exchange due to (black body) radiation is about 1000 J/s.
Latent heat seriously• Complex process of simultaneous forced
convection and non-steady diffusion around a bather is treated by means of a stationary semiempirical Sherwood number for a sphere corrected by a factor for non-steady pure diffusion:
• h(x,t) = f(x) g(t)
• According to Aristotle the re-evaluation by Kulmala, Vesala, Schwarz and Smolik (Mass transfer from a drop – II. Theoretical analysis of temperature dependent mass flux correlation, Int. J. Heat Mass Transfer, 1995) can be neglected and we write for the latent heat flux:
•
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