Radiation thermal

Thermal radiation is provided by heat lamps which emit large amounts of infrared light. Besides keeping food warm at smorgasbord-type restaurants, heat lamps can be used to warm and/or dry samples. 

Although heat lamps cannot provide great amounts of heat, they have the advantage of being able to work at a distance. Because heat lamps do not require contact or air to heat materials, they can be used to heat materials in a vacuum. 

Do not stare at the light of a heat lamp because it can bum your retina. Similarly, do not leave your hands exposed to its light for more than a few seconds at a time because it will sunburn and damage your skin. 

Gases and liquids are partly transparent to thermal radiation. Therefore emission and absorption of radiation takes place inside the gas or liquid space. In gases and liquids emission and absorption are volumetric effects. In contrast, on the surface of a solid object, radiation is completely absorbed within a very thin layer (a few micrometres). Radiation from the interior of a solid body does not penetrate the surface, emission is limited to a thin surface layer. It can therefore be said that emission and absorption of radiation by a solid body are surface effects. This means that it is allowed to speak of radiating and absorbing surfaces rather than correctly of radiating solid bodies. 

There is an upper limit for the emission of heat radiation, which only depends on the thermodynamic temperature T of the radiating body. The maximum heat flux from the surface of a radiating body is given by 

This law was found 1879 by J. Stefan10 as a result of many experiments and was derived in 1884 by L. Boltzmann11 from the electromagnetic theory of radiation using the second law of thermodynamics. It contains an universal constant, known as the Stefan-Boltzmann constant a, which has a value of 

10 Josef Stefan (1835-1893) became Professor of Physics at the University of Vienna in 1863. He was an excellent researcher and published numerous papers on heat conduction and diffusion in fluids, ice formation, and the connection between surface tension and evaporation. He suggested the T4-law after careful evaluation of lots of earlier experiments on the emission of heat from hot bodies. 

11 Ludwig Boltzmann (1844-1906) gained his PhD in 1867 as a scholar of J. Stefan in Vienna. He was a physics professor in Graz, Munich, Leipzig and Vienna. His main area of work was the kinetic theory of gases and its relationship with the second law of thermodynamics. In 1877 he found the fundamental relation between the entropy of a system and the logarithm of the number of possible molecular distributions which make up the macroscopic state of the system. 

An element in a thermally radiative environment absorbs, reflects, refracts, diffracts, and transmits incoming radiative heat fluxes as well as emits its own radiative heat flux. Most solid materials in gas-solid flows, including particles and pipe walls, can be reasonably approximated as gray bodies so that absorption and emission can be readily calculated from Stefan-Boltzmann s law (Eq. (1.59)) for total thermal radiation or from Planck s formula (Eq. (1.62)) for monochromatic radiation. Other means of transport of radiative 

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. 

Thermal radiation is electromagnetic radiation covering wavelengths from 2 to 16 p,m (infrared). It is the net result of radiation emitted by radiating substances such as HjO, CO2, and soot (often dominant in fireballs and pool fires), absorption by these substances, and scatter. This section presents general methods to describe 

In the point-source model, it is assumed that a selected fraction (/) of the heat of combustion is emitted as radiation in all directions. The radiation per unit area and per unit time received by a target (q) at a distance (x) from the point source is, therefore, given by 

It is assumed that the target surface faces toward the radiation source so that it receives the maximum incident flux. The rate of combustion depends on the release. For a pool fire of a fuel with a boiling point above the ambient temperature (Tg), the combustion rate can be estimated by the empirical relation  

The solid-flame model can be used to overcome the inaccuracy of the point-source model. This model assumes that the fire can be represented by a solid body of a simple geometrical shape, and that all thermal radiation is emitted from its surface. To ensure that fire volume is not neglected, the geometries of the fire and target, as well as their relative positions, must be taken into account because a portion of the fire may be obscured as seen from the target. 

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Before testing, the blades have to be painted black to ensure a symmetrical thermal radiation. Painting is done in an additional cabin. After the measurement the blades or vanes are cleaned in an ultrasonic bath. 

Fig. 1. Schematic for thermographic imaging. The ambient thermal radiation is imaged on the focal plane which converts the infrared to an electrical signal for display on a video monitor. Sensors with uncooled focal planes are now the size of a minicam and cost 5 to 10 times as much.

Blackbody Radiation Engineering calculations of thermal radiation from surfaces are best keyed to the radiation characteristics of the blackbody, or ideal radiator. The characteristic properties of a blackbody are that it absorbs all the radiation incident on its surface and that the quality and intensity of the radiation it emits are completely determined by its temperature. The total radiative fliix throughout a hemisphere from a black surface of area A and absolute temperature T is given by the Stefan-Boltzmann law ... 

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). 

Total Radiation Pyrometers In total radiation pyrometers, the thermal radiation is detec ted over a large range of wavelengths from the objec t at high temperature. The detector is normally a thermopile, which is built by connec ting several thermocouples in series to increase the temperature measurement range. The pyrometer is calibrated for black bodies, so the indicated temperature Tp should be converted for non-black body temperature. 

Impact of release on people, property, or environment thermal radiation... 

Fireballs Giant hazardous fireballs result from large BLEX s. Several formulas for BLE physical parameters and thermal radiation hazards have been summarized by the Center for Chemical Process Safety (CCPS) of the American Institute of Chemical Engineers and by Pruffh. (See AlChE/CCPS, 1989 Prugh, 1994.) For the maximum fireball diameter, in meters, CCPS has selected... 

This accident has the potential to seriously injure 50 people because of blast overpressure and thermal radiation effects. 

Figure 4.29. Sample assembly for optical shock temperature measurements. The sample consists of a metal film deposited on a transparent substrate which serves as both an anvil and a transparent window through which thermal radiation is emitted. Rapid compression of gases and surface irregularities at the interface between the sample film and the driver produce very high temperatures in this region. The bottom portion of the figure illustrates the thermal distribution across through the assembly. (After Bass et al. (1987).)...

Luminosity is the amount of chemical energy in the fuel that is released as thermal radiation. 

To give additional protection from thermal radiation from any cylinder stack fire the minimum separation distance to any nearby building housing a vulnerable population should, for quantities >400 kg, be 8 m or as in Table 9.1 7, column 3, whichever is the greater. This may be reduced to those in Table 9.17, column 4, by the installation of fire-resisting separation or a fire wall. 

The fireball resulting from ignition of a cloud of flammable vapor may be relatively long lasting (2-5 seconds), and represents a thermal radiation hazard to those close to the cloud CCPS (1994b). 

If a large amount of a volatile flammable material is rapidly dispersed to the atmo vapor cloud forms. If this cloud is ignited before the cloud is diluted below its lower flammability limit, a UVCE occurs which can damage by overpressure or by thermal radiation. Rarely are UVCEs detonations it is believed that obstacles, turbulence, and possibly a critical cloud size are needed to transition from deflagration to detonation. 

The thermal radiation received from the fireball on a target is given by equation 9.1-31, where Q is the radiation received by a black body target (kW/m ) r is the atmospheric transmissivity (dimensionless), E = surface emitted flux in kW/m", and f is a dimensionless view factor. 

Hymes, 1., 1983, The Physiological and Pathological Effects of Thermal Radiation, UKAEA Safety and Reliability Directorate, Report SRD R275, Culcheth U.K. 

Roberts, A. F., 1981, Thermal Radiation Hazards for Releases of LPG from Pressurized Storage, Fire Safety Journal 4, p 197-212. 

When there are only two surfaces in the hollow, the net thermal radiation is... 

The air Temperature senaor shall be offeccivcly protected from -my effects of thermal radiation cxmiing from hot or C(>ld walls. 

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