04c1aa93-06ea-4a79-8e47-c6cc5f7b8054.ppt

Chapter14 Radiation Heat Transfer Radiation Definition: It may be considered to be energy streaming through space at the speed of light, may originate in various ways. Some types of materials will emit radiation when they are treated by external agencies. All substances at temperatures above absolute zero emit radiation that is independent of external agencies. Radiation that is the result of temperature only is called thermal radiation. Fundamental facts concerning radiation Radiation moves through space in straight lines, or beams, and only substances in sight of a radiating body can intercept radiation from that body. Radiation as such is not heat, and when transformed into heat on absorption, it is no longer radiation. Some definitions: The fraction that is absorbed is called absorptivity α. The fraction that is transmitted is called transmissivity τ. The fraction of the radiation falling on a body that is reflected is called reflectivity ρ. The sum of these fractions must be unity, or ρ+ α + τ =1 (14-1) The maximum possible absorptivity is unity. A body which absorbs all incident radiation is called a black body. The maximum possible reflectivity is unity. A body which reflects all incident radiation is called a white body. Emission of radiation The radiation emitted by any given mass of substance is independent of other material in sight of , or in contact with the mass. The net energy gained or lost by a body is the difference between the energy emitted by the body and that absorbed by it from the radiation reaching it from other bodies. When bodies at different temperatures are placed in sight of one another inside an enclosure, the hotter bodies loss energy by emission of radiation faster than they receive energy by absorption of radiation from the cooler bodies