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

Abstract

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=0.44Gr0.20Pr0.28(d2/d1)0.35 for outer tubes, Nuo=0.38Gr0.20Pr0.28 These two equations are applicable to circular tubes and annuli, regardless of heating amounts of inner and outer tubes.