Heat transfer enhancement of TiO2/water nanofluid in a heat exchanger tube equipped with overlapped dual twisted-tapes

Titanium dioxide (TiO 2 ) in water as nanofluid was employed for heat transfer enhancement together with overlapped dual twisted tapes (O-DTs). The study encompassed Reynolds numbers from 5400 to 15,200, O-DTs with overlapped twist ratios ( y o / y ) of 1.5, 2.0 and 2.5 and nanofluids with TiO 2 volume concentrations ( ϕ ) of 0.07%, 0.14% and 0.21%. The experimental and numerical results indicated that O-DTs with smaller overlapped twisted ratio delivered a stronger swirl intensity and higher turbulent kinetic energy (TKE). The use of O-DTs at the smallest overlapped twist ratio of 1.5 enhanced heat transfer rates up to 89%, friction factor by 5.43 times and thermal performance up to 1.13 times as compared to those of plain tube. In addition, heat transfer increased as TiO 2 volume concentration of nanofluid increased, owing to the increases of contact surface and thermal conductivity . The simultaneous use of the O-DTs having twist ratios 1.5 with the nanofluid with TiO 2 volume concentration of 0.21% resulted in heat transfer enhancement around 9.9–11.2% and thermal performance improvement up to 4.5% as compared to the use of O-DTs alone. The empirical correlations of heat transfer rate ( Nu ), friction factor ( f ) and thermal performance ( η ) in a constant wall heat flux tube equipped O-DTs at different overlapped twist ratios ( y o / y ) and volume concentrations of TiO 2 nanoparticles ( ϕ ) are also reported for heat transfer applications.