Abstract
A heat transfer correlation for opposing mixed turbulent convection in vertical ducts was obtained utilizing surface renewal theory. The correlation was found to be Nu D b = 0.0115 Re D h 0.8 Pr 0.5 1 + [1 − 696 Re 0.8 D h + 8300 Gr D b Re 2.6 D h (Pr 0.5+1) ] 0.39 The correlation fit data to within 7% over a parameter range of 0.7 < Pr < 7, 1 × 10 4 < Re D h < 2 × 10 4, and 1 × 10 6 < Gr D h < 2 × 10 9. The mean residence time, characterizing the time a clump of fluid resides on the wall, was found to decrease as both Gr D b Re 2.6 D h (Pr 0.5+1) and Re D h increase. This explains the enhanced heat transfer due to buoyancy in opposing mixed turbulent flows. This heat transfer enhancement was also reflected in a decreasing thermal boundary layer thickness with increasing Re D h , Gr D h or Pr.
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