Heat Chap12-074 - Chapter 12 Radiation Heat Transfer...

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Chapter 12 Radiation Heat Transfer Special Topic: Heat Transfer from the Human Body 12-74C Yes, roughly one-third of the metabolic heat generated by a person who is resting or doing light work is dissipated to the environment by convection, one-third by evaporation, and the remaining one- third by radiation. 12-75C Sensible heat is the energy associated with a temperature change. The sensible heat loss from a human body increases as ( a ) the skin temperature increases, ( b ) the environment temperature decreases, and ( c ) the air motion (and thus the convection heat transfer coefficient) increases. 12-76C Latent heat is the energy released as water vapor condenses on cold surfaces, or the energy absorbed from a warm surface as liquid water evaporates. The latent heat loss from a human body increases as ( a ) the skin wettedness increases and ( b ) the relative humidity of the environment decreases. The rate of evaporation from the body is related to the rate of latent heat loss by Q m h fg latent vapor where h fg is the latent heat of vaporization of water at the skin temperature. 12-77C The insulating effect of clothing is expressed in the unit clo with 1 clo = 0.155 m 2 . C/W = 0.880 ft 2 . F.h/Btu. Clothing serves as insulation, and thus reduces heat loss from the body by convection, radiation, and evaporation by serving as a resistance against heat flow and vapor flow. Clothing decreases heat gain from the sun by serving as a radiation shield. 12-78C ( a ) Heat is lost through the skin by convection, radiation, and evaporation. ( b ) The body loses both sensible heat by convection and latent heat by evaporation from the lungs, but there is no heat transfer in the lungs by radiation. 12-79C The operative temperature T operative is the average of the mean radiant and ambient temperatures weighed by their respective convection and radiation heat transfer coefficients, and is expressed as T h T h T h h T T operative conv ambient rad surr conv rad ambient surr 2 When the convection and radiation heat transfer coefficients are equal to each other, the operative temperature becomes the arithmetic average of the ambient and surrounding surface temperatures. 12-60
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0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 V h Chapter 12 Radiation Heat Transfer 12-80 The convection heat transfer coefficient for a clothed person while walking in still air at a velocity of 0.5 to 2 m/s is given by h = 8.6 V 0.53 where V is in m/s and h is in W/m 2 . C. The convection coefficients in that range vary from 5.96 W/m 2 . C at 0.5 m/s to 12.4 W/m 2 . C at 2 m/s. Therefore, at low velocities, the radiation and convection heat transfer coefficients are comparable in magnitude. But at high velocities, the convection coefficient is much larger than the radiation heat transfer coefficient.
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