Abstract
Natural-convective heat-transfer in vertical concentric pipe annuli is investigated both numerically and experimentally for a fluid of Prandtl number of 0.7. Calculations for three cases different in the heating condition for pipes (heated inner pipe, heated outer pipe, heated both pipes) are made of laminar flows for varying the diameter ratio of inner to outer pipe di/do from 0.2 to 0.8. For each case, the length of thermal entrance region x/b is well correlated with the modified Grashof number Gr* and the ratio of length to clearance of annulus L/b at the Grashor numbers Gr*=102∼5×105. Local Nusself numbers at thermally fully-developed region show some constant values dependent on the diameter ratio di/do, regardless of Gr* and L/b. Average Nusselt numbers for the thermal entrance region are also independent of Gr* and L/b.
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