Effect of radiation on perceived temperature
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The “radiation effect” results from radiation heat exchange between human bodies and surrounding surfaces, such as walls and ceilings. It may lead to phenomena such as houses being cooler in the winter and warmer in the summer at the same temperature. For example, in a room in which air temperature is maintained at 22° Celsius at all times, but in which the inner surfaces of the house is estimated to be an average temperature of 10° Celsius in the winter or 25° Celsius in the summer, heat transfer from the surfaces to the individual will occur, resulting in a difference in the perceived temperature.
We can observe and compare the rate of radiation heat transfer between a person and the surrounding surfaces if we first make a few simplifying assumptions:
- The heat exchange in the environment is in a “steady state”, meaning that there is a constant flow of heat either into or out of the house.
- The person is completely surrounded by the interior surfaces of the room.
- Heat transfer by convection is not considered.
- The walls, ceiling, and floor are all at the same temperature.
For an average person, the outer surface area is 1.4 m², the surface temperature is 30° Celsius, and the emissivity (ε) is 0.95. Emissivity is the ability of a surface to emit radiant energy compared to that of a black body at the same temperature. We will be using the following equation to find out how much heat is lost by a person standing in the same room in summertime as compared to the winter, at exactly the same thermostat reading temperature: Q ̇=εσA_s (T_s^4-T_surr^4) Where Q ̇ is the rate of heat loss (W), ε is the emissivity (or the ability of an objects surface to emit energy by radiation) of a person, σ is the Stefan-Boltzmann constant (5.670x〖10〗^(-8 )W/m2∙K4), As is the surface area of a person, Ts is the surface temperature of a person (K), and Tsurr is the surface temperature of the walls, ceiling, and floor (K). Please note that this equation is only valid for an object standing in a completely enclosed room, box, etc.
In the winter, the amount of heat loss from a person, when the inner surfaces of the room were 10 degrees Celsius, was found to be 152 Watts. (Q ̇=(0.95)(5.67x〖10〗^(-8) )(1.4)[(30+273)^4-(10+273)^4 ]=152) ̇
In the summer, the amount of heat loss from a person, when the inner surfaces of the room were 25 degrees Celsius, was found to be 40.9 Watts. (Q ̇=(0.95)(5.67x〖10〗^(-8) )(1.4)[(30+273)^4-(25+273)^4 ]=40.9) ̇ Thermal radiation is the form of radiation emitted by bodies because of their temperature. It differs from other forms of electromagnetic radiation such as x-rays, gamma rays, microwaves, radio waves, and television rays that are not related to temperature. Scientists have found that all bodies at a temperature above absolute zero emit thermal radiation. People are constantly radiating their body heat, but at different rates. From these values, the rate of heat loss from a person is almost four times as large in the winter than in the summer, which explains the “chill” we feel in the winter even if the thermostat setting is kept the same.
- Çengel, Yunus A., Afshin J. Ghajar, and Mehmet Kanoglu. Heat and Mass Transfer Fundamentals and Applications. New York: McGraw Hill Higher Education, 2011. Print.
- "Emissivity: Definition and Influence in Non-contact Temperature Measurement." / 13.04.2013. N.p., n.d. Web. 12 Apr. 2013.
- Siegel, Robert, and John R. Howell. Thermal Radiation Heat Transfer. New York: Taylor & Francis, 2002. Print.
- "The University of British Columbia." Thermal Radiation. N.p., n.d. Web. 12 Apr. 2013.