Abstract
One reason human beings wear stockings is to warm their legs. Ordinary textile materials are thermally insulative, which prevents body’s heat from dissipating. In contrary to this common sense, it was discovered that some knitted stockings made up of them permanently promote heat release and cool body. This non-intuitive phenomenon emerges when micro-size yarns are knitted to form wide spacing between neighboring yarns. However, the reason why they cool body was unclear because conventional principles of cooling garments cannot account for it. Here, in the basis of fluid-solid conjugate heat transfer analysis of natural convection, we have clarified the cooling mechanism originates from relative relationship between their geometric structure, a periodic alignment of minuscule ribs, and thermal boundary layer. Our novel finding revealed that sufficiently small ribs on the surface are exposed to steep temperature gradient within thermal boundary layer. Thereby, thermal conduction via ribs is enhanced complementarily as they are separated to guide cooler flow onto the surface. Our study provides a general insight into understanding permanent cooling mechanism on micro-size ribbed surfaces in contrast to conventional theory for heat sink, which is applicable not only to other clothes, but also to artificial devices or natural structures.
Highlights
One reason human beings wear stockings is to warm their legs
The analytical solution of natural convection on a vertical isothermal surface was formulated by Squire and Eckert[16], for which local heat transfer coefficient h and thermal boundary layer thickness δ are respectively expressed as, 1Production Technology Center, Asahi Kasei Corporation, Kawasaki, Kanagawa, 210-0863, Japan. 2R&D Laboratory for Applied Product, Asahi Kasei Corporation, Moriyama, Shiga, 524-0002, Japan. *email: yamamoto.tt@om.asahikasei.co.jp www.nature.com/scientificreports/
Comparison of experiment and simulation in the ribbed case shows quantitative agreement over a broad middle area, we have successfully validated the quantitative capability of our method to predict heat transfer coefficients in the area
Summary
We have numerically elucidated that cooling mechanism of the stockings in natural convection along micro-size ribbed surfaces This intriguing result is greatly owing to the enhancement of heat release through minuscule ribs. According to the Eq (2), thermal boundary layer thickness ranges 5.1 mm to 17.1 mm in the upper 99% area on the plate, which means almost all micro-size ribs are sufficiently smaller than the thickness (e δ) Addition of such small ribs is regarded as a perturbation with respect to the solution of natural-convection flow on a vertical isothermal surface, whose temperature and temperature gradient profiles are analytically described as[16], T(x) = T ∞ + (T w − T ∞)(1 − x/δ)[2 ],. Minuscule protrusions are often observed in self-organized structures in nature, in a similar fashion, some natural creatures might wear their own cooling garments to maintain life activity[18,19,20]
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