Heat transfer and pressure drop characteristics of water flow in horizontal tubes with/without longitudinal inserts used as a heat exchanger tubing was experimentally studied. Testing was performed on bare tubes and tubes with square and rectangular as well as crossed-strip inserts with aspect ratios $AR=1$ and 4 and varied ratios of inlet mixed mean temperature to wall temperature of 0.88 to 0.97. The Reynolds number ranged from approximately 250 to 1750 for flow visualization and from 1700 to 4000 for the pressure drop and heat transfer measurements. Flow visualization, using a dye injection method, revealed a highly complex flow pattern including a secondary flow formed in the cross section for crossed-strip inserts. The thermal entrance length was found and correlated in terms of Re for this type of inserted tubes. The enhancement of heat transfer as compared to a conventional bare tube at the same Reynolds number based on the hydraulic diameter was found to be about a factor of 16 at Re⩽4000, while the friction factor rise was only about a factor of 4.5 at Re⩽4000. [S0022-1481(00)01303-7]

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