Combined free‐and‐forced convection: II. Heat transfer in opposing flow

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Abstract
The rate of heat transfer to air flowing downward at low velocities through a heated tube was studied. The apparatus has been described in a previous work. The investigation covered a flow rate ranging from 85 to 930 lb./hr.ft.2, (NRc) from 380 to 6,900, (NGr) from 0.18 × 106 to 7.3 × 106 and nominal wall temperatures of 100, 500 and 950°C.The local Nusselt number could be correlated with the Grashof number independently of its axial position, over a range of Grashof numbers not previously investigated. In particular, it was noted that a marked change in the slope of the (NNu)‐vs‐(NGr) curve occurred around a Grashof number of 106.