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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:
•
3
1
,
,
,
)(6.02
)(
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airw
airw
Aairw
L
D
DSh
tD
Dtg
where
tgShAxxD
LcDQ