Heat transfer enhancement in a tube fitted with helical blade rotors with grooves

• Rotors-assembled strand is a new type of passive insert.
• The heat transfer and friction factor characteristics are presented.
• Effects of rotor space, number and entrance distance are studied.

The experiments were carried out in order to evaluate heat transfer and friction characteristics of the circular tube fitted with helical blade rotors with grooves. Meanwhile, the influences of the space between rotors, rotor number in each group and entrance distance on heat transfer, friction factor and thermal performance factor were also investigated. An experiment using a smooth tube was carried out to calibrate the experimental system and data reduction method. The experimental results revealed that the helical blade rotors with grooves improved heat transfer with the Nusselt number increasing by 17.1–22.4% and friction factor increasing by 16.9–57.4% within the experimental Reynolds number range; meanwhile, the thermal performance factor of the rotors inserted tube was higher than 1.0, which validated the practical heat transfer enhancement function of the rotors. Furthermore, the comparison results of different assembled modes indicated that both the Nusselt number and friction factor increased with the decreasing space between rotors, rotor number in each group and entrance distance; the thermal performance factor improved with the decreasing space between rotors and entrance distance; and five rotors in each group offered a better thermal performance and generated higher heat transfer enhancement than both one rotor in each group and ten rotors in each group.