ABSTRACT Heat transfer enhancement and flow friction were experimentally investigated for a swirl flow that enters tangentially in a tube roughened by a helical wire. The effects of tube roughness intensity, tangential to total momentum flux ratio and Reynolds number were studied. Measurements were conducted in a horizontal tube roughened with helical-wires at two different diameter to pitch ratios of 0.025 and 0.050, respectively, within a range of Reynolds number from 10103 to 60103, and at different values of the tangential to total momentum flux ratios of 1.73, 2.94, 3.46 and 5.88, respectively. An enhancement in the heat transfer coefficient accompanied with excess friction was observed within the investigated ranges of the different parameters. Also, it was found that the efficiency index is critically dependent on the roughness Reynolds number, momentum flux ratio and tube roughness intensity and it is almost less than that for the rough plain tubes. New correlations for Nusselt number enhancement ratio and the friction factor ratio were obtained as functions of momentum flux ratio, d/p ratio and Reynolds number .