Heating thermal radiation

Radiative heat transfer or thermal radiation is a distinctly separate mechanisms from conduction and convection for the transport of heat. Thermal radiation is associated with the rate at which energy is emitted by the material as a result of its finite temperature. The mechanism of emission is related to energy released as a result of oscillations or transitions of the many electrons that constitute the material. These oscillations are, in turn, supported by the internal energy for this reason the temperature drops. The emission of thermal radiation is thus associated with thermally excited conditions within the matter. 

The net heat flux B for chemically nonreacting flow represents the net flux of heat into the fluid element due to volumetric heating (thermal radiation) and temperature gradients (thermal conduction). It is mathematically defined as... 

Fig. 3. Probes and signals in surface spectroscopy. Those that are applicable ex situ only are printed in Italic type fV, heat, thermal radiation H, magnetic field E, electric field e , electrons ions or neutral...

The reaction with fluorine occurs spontaneously and explosively, even in the dark at low temperatures. This hydrogen—fluorine reaction is of interest in rocket propellant systems (99—102) (see Explosives and propellants, propellants). The reactions with chlorine and bromine are radical-chain reactions initiated by heat or radiation (103—105). The hydrogen-iodine reaction can be carried out thermally or catalyticaHy (106). 

Resiilts for a large number of other cases are given by Hottel and Sarofim (op. cit., chap. 2) and Hamilton and Morgan (NACA-TN2836, December 1952). A comprehensive bibliography is provided by Siegel 3.nd [owe Thermal Radiation Heat Transfer, McGraw-HiU, 1992). 

Thermal turbulence is turbulence induced by the stability of the atmosphere. When the Earth s surface is heated by the sun s radiation, the lower layer of the atmosphere tends to rise and thermal turbulence becomes greater, especially under conditions of light wind. On clear nights with wind, heat is radiated from the Earth s surface, resulting in the cooling of the ground and the air adjacent to it. This results in extreme stabihty of the atmosphere near the Earth s surface. Under these conditions, turbulence is at a minimum. Attempts to relate different measures of turbulence of the wind (or stability of the atmosphere) to atmospheric diffusion have been made for some time. The measurement of atmospheric stabihty by temperature-difference measurements on a tower is frequently ntihzed as an indirect measure of turbulence, particularly when climatological estimates of turbulence are desired. 

We have estimated the likely heat that may be generated by a particular size of conductor and enclosure for a certain current rating and then have counterchecked whether the conductor and the enclosure so chosen can dissipate this heat by radiation and natural convection, and reach a state of thermal stability within permissible limits or we may have to increase the size of the conductor... 

Siegel R., and Howell J. R. Thermal radiation heat transfer. Washington, DC, Philadelphia, PA, London Hemisphere Publishing, 1992. 

Accident scenarios leading to vapor cloud explosions, flash fires, and BLEVEs were described in the previous chapter. Blast effects are a characteristic feature of both vapor cloud explosions and BLEVEs. Fireballs and flash fires cause damage primarily from heat effects caused by thermal radiation. This chapter describes the basic concepts underlying these phenomena. 

In general, when a flammable vapor cloud is ignited, it will start off as only a Are. Depending on the release conditions at time of ignition, there will be a pool fire, a flash fire, a jet fire, or a fireball. Released heat is transmitted to the surroundings by convection and thermal radiation. For large fires, thermal radiation is the main hazard it can cause severe bums to people, and also cause secondary fires. 

A flash fire is the nonexplosive combustion of a vapor cloud resulting from a release of flammable material into the open air, which, after mixing with air, ignites. In Section 4.1, experiments on vapor cloud explosions were reviewed. They showed that combustion in a vapor cloud develops an explosive intensity and attendant blast effects only in areas where intensely turbulent combustion develops and only if certain conditions are met. Where these conditions are not present, no blast should occur. The cloud then bums as a flash fire, and its major hazard is from the effect of heat from thermal radiation. 

In order to compute the thermal radiation effects produced by a burning vapor cloud, it is necessary to know the flame s temperature, size, and dynamics during its propagation through the cloud. Thermal radiation intercepted by an object in the vicinity is determined by the emissive power of the flame (determined by the flame temperature), the flame s emissivity, the view factor, and an atmospheric-attenuation factor. The fundamentals of heat-radiation modeling are described in Section 3.5. 

Consequently, if none of these conditions is present, no blast effects are to be expected. That is, under fully unconfined and unobstructed conditions, the cloud bums as a flash fire, and the major hazard encountered is heat effect from thermal radiation. 

A massive amount of propane is instantaneously released in an open field. The cloud assumes a flat, circular shape as it spreads. When the internal fuel concentration in the cloud is about 10% by volume, the cloud s dimensions are approximately 1 m deep and 100 m in diameter. Then the cloud reaches an ignition source at its edge. Because turbulence-inducing effects are absent in this situation, blast effects are not anticipated. Therefore, thermal radiation and direct flame contact are the only hazardous effects encountered. Wind speed is 2 m/s. Relative humidity is 50%. Compute the incident heat flux as a function of time through a vertical surface at 100 m distance from the center of the cloud. 

In the case of thermal insulation that primarily reduces thermal radiation across air spaces, the term k, is not used. This type of insnlation is called reflective insulation, and R is not always directly proportional to thickness. The R-value of a reflective system is the temperature difference across the system divided by the heat flux. 

Thermal radiation takes place by the emission of electromagnetic waves, at the velocity of light, from all bodies at temperatures above absolute zero. The heat flux from an... 

The heat flux radiated from a real surface is less than that from an ideal black body surface at the same temperature. The ratio of real to black body flux is the normal total emissivity. Emissivity, like thermal conductivity, is a property which must be determined experimentally. 

Heat energy supplied by the emission of rays. Thermal radiation travels at the speed of light (186,000 miles per second). 

Electrically-heated carbide elements, JO mm in diameter and 0.5 m long, radiating essentially as black bodies, are to be used in the construction of a heater in which thermal radiation from the surroundings is negligible. If the surface temperature of the carbide is limited to 1750 K, how many elements are required to provide a radiated thermal output of 500 kW7... 

Siegel, R. and Howell, J. R. Thermal Radiation Heat Transfer, 2nd edn (McGraw-Hill, New York, 1981) Sparrow, E. M. and Cess, R. D. Radiation Heat Transfer (Hemisphere Publishing, New York, 1978) Taylor, M. (ed.). Plate-fin Heat Exchangers Guide to their Specification and Use (HTFS, Harwell, 1987). Tohloukian, Y. S. Thermophvsical Properties of High Temperature Solid Materials (Macmillan, New York. 1967)... 

Siegel, R- and Howell, J.R. Thermal Radiation Heat Transfer (McGraw-Hill, New York, 1981)... 

Tien, C.L. Thermal Radiation Properties of Gases in Hartnett, J.P. and Irvine, T.F., eds Advances in Heat Transfer. Volume 5 (Academic Press. New York. 1968)... 

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