tailieunhanh - Heat Transfer Handbook part 43

Heat Transfer Handbook part 43. 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. | 412 FORCED CONVECTION INTERNAL FLOWS Nuq-x 1 3 3 x x x x The Nusselt number definitions are NUx q Ph k Tq x - Tm x NUq-x q Ph k ATavg where ATavg is furnished by eq. . It is worth repeating that x. represents the dimensionless longitudinal coordinate in the thermal entrance region eq. which in the case of the parallel-plate channel becomes x x Ph Reph Pr All of the results compiled in this section hold in the limit Pr rc . Thermally and Hydraulically Developing Flow When the Prandtl number is not much greater than 1 especially when XT and X are comparable the temperature and velocity profiles develop together at the same longitudinal location x from the duct entrance. This is the most general case and heat transfer results for many duct geometries have been developed numerically by a number of investigators. Their work is reviewed by Shah and London 1978 . Here a few leading examples of analytical correlations of the numerical data are shown. Figure shows a sample of file flntte-Pr data available lor a round tube with an isothermal wall. The recommended analytical expressions for the local Shah and Bhatti 1987 and overall Stephan 1959 Nusselt numbers in the range Pr 1000 in parallel-plate channels are 1 17x-Q-64 - NUx 2 5 1 64 2 Nuq-x 7 -y Q x 1 17x-Q 64 The pressure drop over the hydrodynamically developing length x or XP P 0 - P x can be calculated using Shah and London 1978 P 13 74 x 1 2 1-25 64x - 13-74 x 1 2 1 pU2 1 x -2 OPTIMAL CHANNEL SIZES FOR LAMINAR FLOW 413 On the right-hand side x is the dimensionless coordinate for the hydrodynamic entrance region x D x RDd which also appears on the abscissa of Fig. . Note the difference beween X and x . Equation can be used instead of the Cf 0-x ReD curve shown in Fig. by noting the force balance nD2 AP To x nDx 4 or AP 4x .

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