Heat transfer enhancement in a tube with twisted tape inserts placed separately from the tube wall

► Heat transfer enhancement in a tube inserted with separately twisted tapes was examined. ► The results were presented for five twist ratios and two clearance ratios. ► The highest heat transfer rate, friction factor and enhancement efficiency were obtained for y/D = 2 and c/D = 0.0178. ► Empirical correlations were also given to predict Nusselt number, friction factor and heat transfer enhancement.