Combined Free and Forced-Convection Heat-Transfer for Fully-Developed Laminar Flow in Horizontal Tubes (Experiments)

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Experimental investigations have been made of the forced-convection heat-transfer for a fully developed laminar flow in horizontal circular tubes and concentric annuli in order to clarify quantitatively the effect of natural convection on their heat-transfer characteristics. When the Grashof number Gr=βgLrTr/Ur², defined using Tr=WGLr²/α, Lr=(d²₂-d²₁)(q₁+q₂)/(d₁q₁+d₂q₂), Ur= ν/Lr as the characteristic temperature, length, and velocity, respectively, is introduced, the heat-transfer characteristics of the secondary flows induced by natural convection can be expressed by the following equations; for inner tubes, Nui=0.44Gr^0.20Pr^0.28(d₂/d₁)^0.35 for outer tubes, Nuo=0.38Gr^0.20Pr^0.28 These two equations are applicable to circular tubes and annuli, regardless of heating amounts of inner and outer tubes.