Forced Convective Heat Transfer Enhancement by the Turbulent Promoter Generating Streamwise Vortices

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The flow characteristics and the heat transfer performances of such pipes have been reported by many authors, but all of these reports treated the repeated-rib roughness where the ribs were set perpendicular to the pipe axis. Since the flow in the pipe with this promoter is greatly accelerated in the front part of the promoter, the pressure loss also increases. In the light of reducing the pressure loss and getting higher heat transfer performance, the inclined repeated-rib roughness are proposed in this paper, namely the ribs are set not perpendicular but inclined to the pipe axis. The pipe with this type of promoter is considered to have two kinds of advantages. One advantage is smaller flow contraction ratio of the cross sectional area due to the promoter, compared with that of the normal repeated-rib roughness. The other is that the inclined repeated-rib produces the secondary flow in the pipe which enhances the heat transfer performance of the pipe.The mechanism of the generation of secondary flow was quantitatively investigated from the measured velocity profile by considering the contribution of the advection terms of the N.S. equation. The intensity of the secondary flow did not depend on the Reynolds number and the maximum velocity of the secondary motion was over 45 percent of the mean axial velocity of the pipe in the Reynolds nember region from 3500 to 20000. The detailed structure of the flow was claified by the LDV, and it is concluded that the measured Reynolds stress is accurately correspond to the velocity gradient and the eddy diffusivity assumption is applicale.The flow resistance of the pipe with the inclined repeated-rib roughness evaluated by the roughness fuction was lower than that with normal repeated-rib, which means that the inclined repeated-rib roughness has great adavatage in a practical usage.