Heat transfer intensification in tubes containing multiple twisted-tape bundle inserts was experimentally investigated. The experimental parameters included the tape numbers (N) of 2, 4 and 6 pieces, tape twist ratios (y/w) of 4.0, 5.0 and 6.0) and two different tape arrangements: co-swirl tapes (CoT) and counter-swirl tapes (CT). The tests were carried out using air as the working fluid under a constant heat flux. The experimental results showed that all multiple twisted tape bundles caused higher heat transfer rates, pressure losses and thermal hydraulic performance than a typical twisted tape. The heat transfer rate (Nu) and friction factor (f) increased with the number of tapes (N) and decreased with the twist ratio (y/w). At similar operating conditions, the counter-swirl flow twisted tape sets (CT) provided higher heat transfer (Nu) and friction factors (f) than the co-swirl flow sets (CoT). The highest thermal hydraulic performance (η) of 1.33, was offered by the twisted tapes having y/w = 4.0 and N = 6 at Re = 6000.
