Thermal radiation view factor

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. 

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. 

Thermal radiation includes the visible spectrum Use your eye to help with concept of view factor If an object cannot be seen by a heat source,... 

The surfaces of a two-surface enclosure exchange heat with one another by thermal radiation. Surface 1 has a tempetature of 400 K, an area of 0.2 m, and a total emissivity of 0.4. Surface 2 is black, has a temperature of 600 K. and an area of 0.3 ntl If the view factor is 0.3, the rate of radiation heal transfer between the two surfaces is ( ) 87W (h) I35W (c) 244W... 

Krishna, S.M. 1987. Geometric view factors for thermal radiation hazard assessment. Fire Safety Journal... 

Equation (3.60) assumes that all radiation arises from a single point and is received by an object perpendicular to this. This view factor must only be applied to the total heat output, not to the flux. Other view factors based on specific shapes (i.e., cylinders) require the use of thermal flux and are dimensionless. The point source view factor provides a reasonable estimate of received flux at distances far from the flame. At closer distances, more rigorous formulas or tables are given by Hamilton and Morgan (1952), Crocker and Napier (1986), and TNO (1979). 

As mentioned above, small-scale photoreactions are quite often carried out in quartz or Pyrex tubes, by external irradiation. However, this is certainly not an optimal solution for maximizing the exploitation of the emitted radiation. Internal irradiation is obviously better from the geometric point of view, but (relatively) large-scale preparations must take into account all of these factors and achieve optimal light and mass transfer. These elements are not taken into account in exploratory studies or small-scale syntheses, just as is the case for thermal reactions, where the optimization is considered at a later stage the essential requirement is that the explorative study is carried out under conditions where occurrence of the reaction is not prevented. Thus, it is important that the source is matched with the reagent absorption, the vessel is of the correct material, and the solvent does not absorb competitively (unless it acts also as the sensitizer). Figure 1.7 and Table 1.1 may help in this choice, in conjunction with the U V spectra of all of the compounds used (it is recommended that the spectra are measured on the actual samples used, in comparison with those taken from the literature, in order to check for absorption by impurities). 

When a detector is cooled to 4.2 K with a background temperature of 300 K, it produces a reduction in the room-temperature NEPbr by a factor of l/ /2, while NEPbr (cm, 4.2 K, 1 Hz) deduced from expression (4.3) is 1.4 x 10 15W. This is the reason why, under laboratory conditions, the background radiation (BR) incident on low-temperature thermal detectors is strongly attenuated by filters cooled at the detector temperature, which cut the medium IR background and provide a low value of Tback- An improvement is also observed by reducing the field of view of the incident radiation. 

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