2007, Piran, Slovenia

Environmental Ergonomics XII
Igor B. Mekjavic, Stelios N. Kounalakis & Nigel A.S. Taylor (Eds.), © BIOMED, Ljubljana 2007
496
A THEORETICAL CONSIDERATION OF SKIN WETTEDNESS
Masashi Kuramae, Takafumi Maeda and Shintaro Yokoyama
Hokkaido University, Sapporo, Japan
Contact person: kuramae@eng.hokudai.ac.jp
INTRODUCTION
The concept of skin wettedness (Gagge et al., 1977) is often used when calculating
evaporative heat loss or evaluating a hot environment. However, in past investigations in this
field, inappropriate handling or theoretical confusion of the term “wettedness” is observed as
a result of discussions that failed to correctly understand the relationship between the
fundamental difference in physical fields based on the difference in scale and macroscopic
laws. In this paper, the so-called skin wettedness is discussed from a different viewpoint to
clarify its true nature, by considering the heat and mass transfer phenomena on the human
body surface from a microscopic viewpoint.
METHODS
Skin wettedness might be interpreted as the ratio wA of the wet area Aw to the total surface
area A of a human body. This skin wettedness can not be measured directly from the human
body surface but can be calculated by the ratio wH of actual evaporative heat loss Esk to
maximum evaporative heat loss Emax However, there is an essential difference between wA and
wH from a physical viewpoint. The value of wH is a macroscopic quantity that requires no
information, such as which parts of the human body surface are wet or in what condition. On
the other hand, wA is a concept that can only be established from the viewpoint of
distinguishing the wet and dry parts of the human skin surface. No actual human body is , for
example, soaking wet in the upper half and completely dry in the lower half, but the body
usually appears to be uniformly wet uniformly, and it is therefore difficult to distinguish these
two conditions by the naked eye. Therefore, it is necessary to reduce the scale of
measurement to discuss wA from a microscopic viewpoint. While it is necessary from the
above viewpoint to clarify the relationship between the laws in a macroscopic field and the
heat and mass transfer mechanisms in the temperature and concentration fields of a
microscopic scale, where the wet and dry parts of the human body can be distinguished,
strictly theoretical handling is too difficult because of the extremely complex properties of the
actual human body surface. However, it is thought that a model of mass transfer from a
discontinuous source (Suzuki et al.., 1968) described below will be useful for understanding
the actual phenomena. Perspiration from the human body surface is thought to be a problem
of mass transfer from a surface consisting of a mixture of areas with mass transfer (wet area)
and those without mass transfer (dry area). A model that takes into account the above
problems ( see Fig.1 ) was used here to represent the condition of the human body from the
microscopic viewpoint. In this model, the semidry body surface is regarded as an assembly of
unit cells, each of which contains the skin and outside air layer. The circular area at the center
of the skin surface represents the wet surface, while the ring-like area surrounding it
corresponds to the dry area. This model can simulate the process by which sweat is released
from sweat glands and spreads over the surrounding area. Calculation of skin layer is handled
as only a heat transfer problem and that of the air layer as only a mass transfer problem based
on the assumptions that thermal conductivity on the skin layer is uniform and that molecular
diffusion occurs in the stationary film (thickness: δ) in the air layer.