Effect of decaying swirl flow on tubular turbulent heat transfer enhancement by using short length helical tapes

This study deals with an experimental investigation on turbulent thermal-hydraulic performances in a plain tube (PT) mounted with swirl flow devices of short length helical tapes (SL-HTs). The heat transfer experiments were conducted at constant heat flux wall conditions while the pressure drop tests were performed at iso-thermal wall conditions. Effects of Reynolds number (Re) at 5000-19,000, tape length ratios (L/D) at 31.034-46.552, hole diameter ratios (W/D) at 0.276-0.414 and pitch length ratios (P/D) at 1.034-1.724 were explored by employing air as working fluid. The main findings are that the Nusselt number (Nu) and friction factor (f) in the PT inserted with SL-HTs could be enhanced by around 1.58-2.64 times and 4.72-16.26 times respectively in comparison with the alone use of PT. The obtained information also provides that both the Nu and f increase with increasing L/D, decreasing W/D and decreasing P/D while the PEC increases as L/D and P/D increase. In addition, the maximum PEC of about 1.17 is gained by the SL-HT at Re = 5971, L/D = 46.552, W/D = 0.276 and P/D = 1.724. The comparisons with some previous studies indicate that the SL-HTs hold certain competitive advantage. At last, the thermal-hydraulic characteristics correlations are developed with Re, L/D, W/D and P/D and the deviations from the experimental values are within ±5% for Nu and ±9% for f.