Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

11.3 Temperature Difference and Heat Transfer 327

body heat loss that occurs on a cold, windy day. However, keep in mind that the common

windchill correlations are based on a series of experiments performed in a very cold climate,

where the time that it took to freeze water in a plastic container under various surrounding

air temperatures and wind conditions was studied. A common correlation used to determine

the windchill index is

(11.21)

where

WCI windchill index (kcal /m

2

h)

Vwind speed ( m/s)

Taambient air temperature (C)

and the value 33 is the body surface temperature in degrees Celsius. The more common equiv-

alent windchill temperatureTequivalent(C) is given by

(11.22)

Note that in Equation (11.22),Vis expressed in kilometers per hour (km/h). Table 11.6 shows

the windchill temperatures for the range of ambient air temperature  30 C Ta 

10 C and

wind speed of 20 km/h V 80 km/h.

Radiation

All matter emits thermal radiation. This rule is true as long as the body in question is at a

nonzero absolute temperature. The higher the temperature of the surface of the object, the

more thermal energy is emitted by the object. A good example of thermal radiation is the heat

you can literally feel radiated by a fire in a fireplace. The amount of radiant energy emitted by

a surface is given by the equation

qesAT (11.23)

4

s

Tequivalent0.045 1 5.27V

0.5

10.450.28V 21 Ta 332  33

WCI 1 10.45V 101 V 2133 Ta 2

TABLE 11.6 Windchill Temperatures

Ambient Temperature (C)

Wind Speed

(km/h) 10 5 0  5  10  15  20  25  30

20 3.6 2.8 9.2 15.6 22.0 28.4 34.8 41.2 47.6

30 1.0 6.0 12.9 19.9 26.8 33.8 40.7 47.7 54.6

40 0.7 8.1 15.4 22.7 30.0 37.4 44.7 52.0 59.4

50 1.9 9.5 17.1 24.7 32.2 39.8 47.4 55.0 62.6

60 2.7 10.4 18.2 25.9 33.7 41.5 49.2 57.0 64.7

70 3.2 11.0 18.9 26.8 34.6 42.5 50.3 58.2 66.1

80 3.4 11.3 19.3 27.2 35.1 43.0 50.9 58.8 66.8

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