TAILIEUCHUNG - Heat Transfer Handbook part 63

Heat Transfer Handbook part 63. The Heat Transfer Handbook provides succinct hard data, formulas, and specifications for the critical aspects of heat transfer, offering a reliable, hands-on resource for solving day-to-day issues across a variety of applications. | RADIATIVE EXCHANGE BETWEEN SURFACES 613 where Figure Arrangement of parallel or concentric radiation shields. R-1j - 77 7 -1 a The analysis of radiation shields is one of the few applications where analysis of specularly reflecting surfaces is relatively simple and may lead to substantially different answers for concentric shields with strongly varying radii. For a specularly reflecting shield Aj with Aj-1 being specular or diffuse the radiative resistance becomes Rj-1j - - - 1 Aj specular Vj-1 7 Aj-1 Note that it is desirable to make shields highly reflective low e and this tends to make them specularly reflecting also desirable because it also increases the resistance . Further simplifications arise if all shields are of identical material e2 e3 eN-1 on the other hand eqs. through remain valid for shields with different emittances on both of its sides different values for j in Rj- j and Rj j 1 . While the network analogy can and has been applied to configurations with more than two surfaces seeing each other this leads to very complicated circuits because there is only one resistance between any two surfaces . For such problems the network analogy is not recommended and the net radiation method eq. should be employed. 614 THERMAL RADIATION Radiative Exchange between Diffuse Nongray Surfaces In a number of important engineering problems the assumption of gray surface properties may not provide adequate accuracy when properties exhibit strong spectral variations across the important range of the spectrum . To deal with such effects two simple models known as the semigray approximation and the band approximation will be described. Semigray Approximation Method This method employs the principle of superposition The radiative flux at any given point is the sum of the contributions from the various emitters in the enclosure each one acting independently. In some applications there is a natural division of the radiative energy within

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