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Heat transfer augmentation by dual-helical twisted tapes (D-HTTs) and triple-helical twisted tapes (T-HTTs) is presented in comparison with that by single-helical twisted tape (S-HTT). The effects of tape width ratio (w=D ¼ 0.1, 0.15, and 0.2) on heat transfer, friction factor, and thermal performance are also reported. The experiments were performed using air as a working fluid for Reynolds number between 6000 and 20,000. At similar conditions, the use of D-HTTs and T-HTTs leads to the increase of Nusselt number by respectively 15.6% to 17.6% and 19.5% to 23.4%, the increase of friction factor by respectively 83% to 206% and 143% to 335%, and the decrease of thermal performance factor by 3.9% to 20.3% and 8.3% to 26.2% compared to those obtained from the use of S-HTT=promoter. As the width ratio increases, both Nusselt number and friction factor are considerably increased. However, due to the poorer trade-off between the increases of Nusselt number and friction factor, lower thermal performance factor is consistently obtained at the conditions possessing higher Nusselt number. At similar conditions, S-HTTs consistently give a higher thermal performance factor than D-HTTs and T-HTTs; the maximum thermal performance factors given by S-HTTs with w=D of 0.1, 0.15, and 0.2 are 1.34, 1.31, and 1.29, respectively (at Reynolds number of 6000).

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