Effects of trapezoidal and twisted trapezoidal tapes on turbulent heat transfer in tubes

Trapezoidal and twisted trapezoidal tape elements which are used for heat transfer enhancement in tubes are experimentally investigated. The effects of the tapes on heat transfer, pressure drop, and overall performance factor are analyzed. Three different thread heights are used in the experiments, which are obtained by dividing the tape width by 4, 6, and 10. Pitch length is kept constant for the trapezoidal tapes, while the constant twisting ratio is used for the twisted trapezoidal ones. Tapes were held in the tube with no wall contact by stretching mechanically. Experiments were conducted under constant heat flux boundary condition and for Reynolds number interval of 5000-25,000, targeting turbulent flow at which overall performance factor values reported in literature are relatively small comparing to laminar flow. The experimental results show that trapezoidal tape elements enhance heat transfer and increase pressure drop with changing rates depending on thread heights. Overall performance factor in the tested interval is detected over unity for two twisted trapezoidal inserts, making them significant alternatives comparing to the literature. The best thermal performance of 1.164 is obtained with the twisted trapezoidal tape element with the maximum thread height, while Nu is 90.52 and friction factor is 0.12. A maximum Nu of 118.46 is achieved with the proposed twisted trapezoidal tape, which is 1.9 times higher than the plain tube case and 1.57 times higher than the conventional twisted tape case. Since the twisted trapezoidal tapes perform better than the plain trapezoidal ones, it is concluded that the combination of the twisted and trapezoidal forms may be favorable in terms of heat transfer enhancement and overall performance factor regarding combination of swirl flow effect and periodical flow disturbance effect.

Automated summary

The trapezoidal and twisted trapezoidal tape elements used for heat transfer enhancement in tubes were experimentally investigated, analyzing their effects on heat transfer, pressure drop, and overall performance factor. The experimental results showed that the twisted trapezoidal tape elements had excellent heat transfer performance and overall performance factor.