Turbulent heat transfer enhancement by counter/co-swirling flow in a tube fitted with twin twisted tapes

In the present study, the influences of twin-counter/co-twisted tapes ( counter/ co-swirl tape) on heat transfer rate ( Nu), friction factor ( f) and thermal enhancement index ( η) are experimentally determined. The twin counter twisted tapes ( CTs) are used as counter-swirl flow generators while twin co-twisted tapes ( CoTs) are used as co-swirl flow generators in a test section. The tests are conducted using the CTs and CoTs with four different twist ratios ( y/ w = 2.5, 3.0, 3.5 and 4.0) for Reynolds numbers range between 3700 and 21,000 under uniform heat flux conditions. The experiments using the single twisted tape ( ST) are also performed under similar operation test conditions, for comparison. The experimental results demonstrate that Nusselt number ( Nu), friction factor ( f) and thermal enhancement index ( η) increase with decreasing twist ratio ( y/ w). The results also show that the CTs are more efficient than the CoTs for heat transfer enhancement. In the range of the present work, heat transfer rates in the tube fitted with the CTs are around 12.5–44.5% and 17.8–50% higher than those with the CoTs and ST, respectively. The maximum thermal enhancement indices ( η) obtained at the constant pumping power by the CTs with y/ w = 2.5, 3.0, 3.5 and 4.0, are 1.39, 1.24, 1.12 and 1.03, respectively, while those obtained by using the CoTs with the same range of y/ w are 1.1, 1.03, 0.97 and 0.92, respectively. In addition, the empirical correlations of the heat transfer ( Nu), friction factor ( f) and thermal enhancement index ( η) are also reported.