2007, Piran, Slovenia

Measurement methods
and calculated heat loss based on the assumption that the skin surface is completely wet.
Although to validate this method, an analogy must be established among the physical
quantities. However, it is thought that there is no linear relationship between the wet area and
evaporative heat loss in a microscopic field where the wet and dry areas of the human body
surface can be distinguished. It can thus be seen that this handling of considering the mass
transfer coefficient as constant has an intrinsic problem. As long as a macroscopic point of
view is adopted, Esk/Emax can not be considered to be the ratio of an area but a correction
factor for the mass transfer coefficient and cannot have more significance than the ratio of
heat loss or of evaporation. In this sense, it is thought to be inappropriate to call Esk/Emax the
skin wet area ratio, and it might be more reasonable to use a term such as evaporative
efficiency or heat loss ratio. In fact, this inappropriate naming is thought to be a cause of
theoretical confusion. Meanwhile, when considering the human body surface from a
microscopic point of view, because evaporative heat loss becomes smaller than the maximum
value only when the real skin wet area ratio is extremely close to 0, evaporative heat loss is
thought to be close to the maximum value when the actual skin surface is felt even slightly
wet. This also provides validity for the assumption that evaporative heat loss smaller than the
maximum value is due to recession of the evaporation surface to the interior of the skin layer
or other reasons rather than to a decrease in the true skin wet area ratio. There is usually a
considerable variation in results of plotting the value of Esk/Emax macroscopically found from
the amount of weight loss against atmospheric temperature and relative humidity. This is
thought to mean that, in addition to problems associated with measurements, Esk/Emax itself is
susceptible to a local imbalance of the temperature and vapor concentration fields because it
is subject to the above-mentioned mechanism of microscopic transfer phenomena and that it
is not determined univalently from average characteristics, such as the outside air temperature
and relative humidity. Considering all these aspects, there may be certain limits in the use of
so-called skin wettedness when discussing thermal sensing indexes or other matters.
DISCUSSION
In this investigation, so-called skin wettedness was discussed to clarify its true nature by
considering the heat and mass transfer phenomena on the human body surface from a
microscopic viewpoint. The mainresults are as follows:
The value of Esk/Emax is thought not to change greatly and to remain close to 1 even if the real
wet area ratio Aw/A decreases under usual ambient thermal condition, by considering the skin
surface from microscopic viewpoint distinguishable between wet and dry area.
If evaporative heat loss is smaller than the maximum value, this is thought to be due to factors
other than decrease in the true skin wet area ratio, for example, recession of the evaporation
surface to the interior of the skin layer.
There may be certain limits in so-called skin wettedness, so some notifications should be paid
to use of the skin wettedness when discussing thermal sensing indexes or other matters.
REFERENCES
Gagge, A. P. and Nishi, Y., 1977. Heat exchange between human skin surface and thermal
environment. Handbook of Physiology Section 9 Reactions to Environmental Agents (Ed.
by D.H.K.Lee), Am. Physiol. Soc., Bethesda, Maryland
Kuno, Y., 1956, Human Perspiration. Charles C Thomas, Springfield
Suzuki, M. and Maeda, S., 1968, Mass transfer from discontinuous source, J. Chem. Eng.
Japan, 1, 26-32
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