By: Jeff NussME 340

Fall 2010

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Average surface area of the body = 1.9 m^2

Average heat generated by the body = 2200 Cal/day = 107 W

Flux = 107 W/1.9 m^2 = 56.32 W/m^2

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Assume a cool day with no wind (0 oC, hair = 25 W/m^2*K) and thickness, t, of 0.005 m

Create a thermal circuit from the skin to the outer air

q”conv = h * (Tunderwear - Tair) Tunderwear = (q” / h) + Tair = 2.253 oC

q”cond = -k * (Tskin - Tunderwear) / t Tskin = (q”*t/-k) + Tunderwear

q”conv = q”cond = q”

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100% Polyester k = 0.14-0.4 W/m*K 50% Viscose 50% Polyester k = 0.1-0.23

W/m*K 100% Polypropylene k = 0.12-0.25

W/m*K 100% Cotton k = 0.06 W/m*K 100% Neoprene k = 0.15-0.45 W/m*K

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100% Polyester

50% Viscose 50% Polyester

100% Polypropylene

100% Cotton

100% Neoprene

k(W/m*K 0.14 0.4 0.1 0.23 0.12 0.25 0.06 0.15 0.45Tskin(C) 4.26 2.96 5.07 3.48 4.60 3.38 6.95 4.13 2.88

On their own, the thermal underwear types considered would be inadequate to provide ample protection on a cold day. However, when used in parallel with other forms of insulation, they provide an excellent supplement and an additional layer that can be removed if one gets too hot. As an avid skier, I am deeply interested in the ability of a thin fabric to provide ample insulation, but be easy to take off if I get too hot.

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These fabrics perform at about the same level for the same thickness. However, there are several other factors to consider such as breathability, water resistance, and weight. For example, when cotton gets wet, it is much worse at insulating. Also, neoprene is the material used in wet suits and is not very breathable. Polyester can be good because it is fairly cheap, while the military uses polypropylene because of its flame resistance in case a soldier is subjected to fire or high heat. The bottom line is that common materials used in thermal underwear perform fairly similarly as far as insulation is concerned. The main factors that will determine which one is best are dependent on individual application and preference.

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