Stable TiO 2 -H 2 O nanofluids are prepared and their stabilities are studied. An experimental set for studying the heat transfer and flow characteristics of nanofluids is established. Heat transfer and flow characteristics of TiO 2 -H 2 O nanofluids in a circular tube with rotating and static built-in twisted tapes are experimentally investigated and compared. An innovative performance evaluation plot of exergy efficiency is developed and the exergy efficiency of tube with rotating and static built-in twisted tapes filled with nanofluids is analyzed in this paper. The results indicate that the combination of rotating built-in twisted tape and TiO 2 -H 2 Onanofluids shows an excellent enhancement in heat transfer, which can increase the heat transfer by 101.6% compared with that of in a circular tube. The effects of nanoparticle mass fractions (ω= 0.1%, 0.3% and 0.5%) and Reynolds numbers (Re=600-7000) on the heat transfer and flow characteristics of TiO 2 -H 2 O nanofluids are discussed. It is found that there is a critical Reynolds number (Re=4500) for the maximum value of relative heat transfer enhancement ratio. The comprehensive performance of the experimental system is analyzed. It can be found that the comprehensive performance index of the experimental system firstly increases and then reduces with Reynolds number, and it can reach 1.519 at best. However, for the 2 performance evaluation of exergy efficiency, the coupling of rotating twisted tape and nanofluids deteriorates the exergy efficiency. Also, it can be found that the exergy efficiency of the circular tube with twisted tape is greater than that of circular tube under the same pumping power and pressure drop, but it shows deterioration under the same mass flow rate.

An experimental system is established to investigate the thermo-hydraulic performance of Fe3O4-water nanofluids in a corrugated tube under various magnetic fields. The influences of magnetic induction intensities (B=0 G, 100 G, 200 G, 300 G), nanoparticle mass fractions (ω=0.0%, 0.1%, 0.3%, 0.5%), electromagnet arrangement modes (one-side electromagnet and two-side staggered electromagnet), kinds of tubes (smooth tube and corrugated tube), Reynolds numbers (Re=800-12000) on flow and heat transfer characteristics are discussed. It is obtained that the augmentation of heat transfer is more sensitive to high nanoparticle mass fraction, high magnetic induction intensity, two-side staggered electromagnet and corrugated tube. A Comprehensive evaluation index is applied to estimate the thermo-hydraulic performance. It can be discovered that the comprehensive evaluation index increases with the increasing Reynolds number at first and then decreases, and the rough surface of corrugated tube delays the appearance of critical Reynolds number.

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