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

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 , ( N Rc ) from 380 to 6,900, ( N Gr ) from 0.18 × 10 6 to 7.3 × 10 6 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 ( N Nu )‐vs‐( N Gr ) curve occurred around a Grashof number of 10 6 .