Thermal performance of helical coils with reversed loops and wire coil inserts

•Modified helical coils with reversed loops and wire coil inserts exhibit unique flow and heat transfer characteristics.•The thermal performance of traditional helical coils can be improved by adding reversed loops with wire coil inserts.•New friction factor and heat transfer correlations for helical coils with reversed loops and wire coil inserts have been proposed. Flow and heat transfer characteristics of water in a newly designed helical coil heat transfer device were investigated in this experimental study. The conventional helical coil configuration was structurally modified aiming to improve its heat transfer performance. Specifically, 360° plastic tubing with or without wire coil inserts was added after each 180° of the main helical coil loop to enhance fluid mixing and redistribute the flow, which should have a direct effect on the thermal performance of the helical coil heat transfer device. Experimental results show that the structural modifications of the conventional helical coil configuration led to enhanced heat transfer in the test section while the pressure drop penalty increased slightly. Furthermore, the heat transfer performance of the overall test section improved by using wire coil inserts in the plastic tubing after every 180° of the main helical coil loop. The results reveal that the modified helical coil section offers a good trade-off between heat transfer enhancement and pressure drop penalty.