Visualization experiment and thermal-hydraulic behaviour of assembled self-rotating twisted tape in double-pipe heat exchangers

To improve heat transfer and reduce higher flow resistance, we proposed an assembled self-rotating twisted tape inserted in the tube, which is characterized by automatic rotation when flowing through. The visualization experiment performed hydraulic performance under different flow rates, and passive motion was solved by combining dynamic mesh and Six Degrees of Freedom method to obtain the force distribution in frequency and time domain. The results show that angular velocity linearly increases with an increase in inlet velocity, and both lift/drag force augments. By the fast Fourier Transformation, the lift force at higher harmonic is smaller than that at the fundamental or second frequency, and the fluctuation is much stronger at higher inlet velocity. Moreover, Power Spectral Density of lift force at the second harmonic for different inlet velocities increases by an average of 2.48 times against that of fundamental frequency. Furthermore, thermal-hydraulic performance comparison shows that the Nusselt number of assembled self-rotating twisted tape increases by 22.67% and friction factor decreases by 26.25% than that of stationary twisted tape, and comprehensive correlations of Nusselt number and friction factor integrated with angular velocity and Reynolds number were obtained. The research is significant for designing and selecting inserts in the tube.