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

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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

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Conclusions:• Latent heat

(including ceasing vaporization)

• Sensible heat

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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

)(

)()(

airw

airw

Aairw

L

D

DSh

tD

Dtg

where

tgShAxxD

LcDQ