Radiation direct exchange areas

Evaluation of the AS" s that charac terize an enclosure involves solution of a system of radiation balances on the surfaces. If the assumption is made that all the zones of the enclosure a re gray and emit and reflec t diffusely, then the direct-exchange area ij, as evaluated for the black-siirface pair A and Aj, applies to emission and reflections between them. If at a surface the total leaving-flnx density, emitted phis reflected, is denoted by W (and called by some the radiosity and by others the exitance), radiation balances take the form ... 

The circumferential strip of the kiln freeboard (Figure 7.13) is isolated and subdivided in N small area elements. The portion of the kiln freeboard, approximately three kiln inside diameters in each axial direction viewed by these area elements, is subdivided into volume zones such as Vj comprising the freeboard gas, and the surface zones such as Aj that form the exposed bed and wall surfaces. The exchange areas between each of the zones and the elements of the circumferential strip are then evaluated and used to calculate the radiation streaming to these elements from the various volumes and surface zones, either directly or after undergoing one or two reflections. 

This is the area within the porous preform where the complex heat and mass exchanges occur. In this multiphase system the heat is transferred outwards through convective and radiation mechanisms. The precursor gases are supplied from the outside towards the reactive zone while the exhaust gases are transferred in the reverse direction after the chemical reactions. 

The solid angle is defined as the ratio of infinitesimal area normal to Cl, to the square of the distance between two infinitesimal surface elements exchanging radiation (see Fig. 7.2). Note that radiation intensity may vary as a function of location r= r(x, y, z), direction Cl = ft(0, <()), time t, and wavelength X therefore, it is a function of seven independent parameters. For most calculations of interest, the transient nature of radiation intensity is not critical. Recently, however, with the advances in femto- and picosecond pulsed lasers, the transient radiative transfer applications have started becoming important (see, e.g., Ref. 4) nevertheless, we will not cover these advances in this chapter. 

Area is a derived, or secondary, physical quantity. It pltQ a significant role in many engineering problems. For example, the rate of heat transfer from a surface is directly proportional to the exposed sur ce area. That is why a motorcycle engine or a lawn mower engine has extended surfaces, or fins as shown in Figure 7.7. If you look closely inside buildings around your campus, you will also see heat exchangers or radiators with extended surfaces under windows and... 

Currently, TLD and OSL dosimeters are the most commonly used dosimeters at nuclear power plants. These badges are exchanged and read on frequencies determined by the anticipated dose received by the wearer. For most workers not expected to receive doses near the limits, this frequency is quarterly or annually. For workers performing tasks in high radiation areas or expected to receive more significant doses, the frequency may be shortened to monthly. If information regarding a worker s dose is needed more often than that, direct-reading dosimeters are used in addition to provide the necessary real-time information. 

One can derive the net radiative exchange between two surfaces separated by a nonabsorbing medium by considering Figure 7.11 which shows elemental areas dAj and (M2 that form portions of the finite areas and A2. If we define as the net intensity of radiation leaving surface 1 in the direction Sj, then the total energy intercepted by surface (M2 is... 

The direct radiation exchange between a freeboard surface zone such as Aj and an area element A,-, that is, exposed wall-to-exposed bed on the circumferential strip may be calculated using the expression... 

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