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30 Chapter 3 Heat exchanger design and construction where p is the porosity of the material, k it's conductivity, and the subscript p refers to the perforated material. For high porosity the numerical solution of Keller and Sachs[92] for p>0.5 for a square array of holes, shown in equation (3.9) may be used. k 2.==( n 1.95t1 (3.9) k [2 -(16p/n)0.5]0.5 3.4.4.3 Flow friction Flow friction data and empirical correlations for drag coefficient for perforated plate have been determined by several authors. The results of McMahon et al.[70], Mikulin et aI.[83], Shevyakova and Orlov[86], and Hubbell and Cain[87] all show that flow friction varies minimally with Reynolds number at Re>200, indicating that form drag predominates. Venkatarathnam[80-p98] in his thesis has presented a graph of drag coefficient versus plate porosity determined from the results of Shevyakova and Orlov, Mikulin et aI., and Hubbell and Cain. These are compared against values calculated from entry and exit pressure loss coefficients for multiple-circular-tube cores and apparent friction factor for the hydrodynamic entry length of a circular tube presented by Kays and London[71:Fig.5-2,pll1;Fig.6-23,p138]. He concludes that the values calculated from the data of Kays and London are substantially higher than those for real MHE's. However, for his calculations, he has used the apparent friction factor which already includes entry and exit loss, and he has ignored their recommendation of using values for Re=oo for interrupted fin surfaces. His data points attributed to Mikulin et aI., and Hubbell and Cain do not appear correct. Furthermore, the choice of a Re of 80 for comparison seems unreasonable since Mikulin et al.'s correlation is only valid for Re>100 above which ~ is independent of Re, and Shevyakova and Orlov's correlation applies to Re<160 once again above which ~ is independent of Re. Venkatarathnam's graph is re-plotted here as Figure 3.4, for Re=160 and 81d=1. Hubbell and Cain's data is not included since it is not for 8/d=1 and thus not directly comparable. 3.4.4.4 Geometric dependence of heat transfer and flow friction: lack of consistency in results The convective heat transfer and flow friction characteristics of perforated plate and wire mesh are strongly dependent on the shape and size of the perforations, porosity, plate and spacer thickness, alignment of perforations on adjoining plates, as well as the method of manufacture. Hubbell and Cain[87] show different heat transfer results for perforated plate manufactured by punching, with the perforation breakout material facing upstream, and downstream. This strong geometric dependence of heat transfer and flow friction characteristics is in keeping with the lack of agreement in the results of the various researchers, which is evident from Table 3.1 and Figure 3.4. The results ofPDF Image | CO2 removal from air for alkaline fuel cells operating with liquid H2
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