Under conditions for which the same room temperature is maintained by a heating
ID: 2993954 • Letter: U
Question
Under conditions for which the same room temperature is maintained by a heating or cooling system, it is not uncommon for a person to feel chilled in the winter but comfortable in the summer. Provide a plausible explanation for this situation (with supporting calculations) y by considering a room whose air temperature is maintained at 20 Dagree C throughout the year, while the walls of the room are nominally at 27 Dagree C and 14 Dagree C in the summer and winter, respectively. The exposed surface of a person in the room may be assumed to be at a temperature of 32 Dagree C throughout the year and to have an emissivity of 0.90. The coefficient associated with heat transfer by natural convection between the person and the room air is approximately 2 W/m2 Middot K.Explanation / Answer
a) The emissivity of the person's skin is 0.9
Thus the power lost due to radiation from the human to the room's walls during summer at 27 degrees celcius is (sigma epsilon (305^4 - 300^4 = 28.25 W) whereas during winter is (sigma epsilon (305^4 - 287^4 = 95.38 W)
This huge difference in radiation losses from the body accounts for the chill!
b) One more explanation is that during winter the humidity ratio of the air is higher than that during summer. Hence the specific heat of the air or in lay man's terms the felt air temperature is lower in winter which accounts for the "chill"
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