Stable TiO

2

-water nanofluids are prepared by a two-step method

stabilities of nanofluids are investigated by precipitation method and transmittance method respectively

and thermal conductivities and viscosities are also measured. An experimental system for studying the heat transfer enhancement of nanofluids is established

and heat transfer and flow characteristics of TiO

2

-water nanofluids in heat exchanger systems with a triangular tube and circular tube are experimentally studied. The effects of nanoparticle mass fractions (ω=0.1 wt%-0.5 wt%) and Reynolds numbers (Re=800-10000) on the heat transfer and flow performances of nanofluids are analyzed. Fitting formulas for Nusselt number and resistance coefficient of nanofluids in a triangular tube are put forward based on the experimental data. The comprehensive performances of nanofluids in a triangular tube are investigated. It is found that nanofluids in a triangular tube can significantly improve the heat transfer performance at the cost of a small increase in resistance coefficient compared with that in a circular tube

especially the resistance coefficients are almost t

he same between different nanoparticle mass fractions at turbulent flow. It is also found that the comprehensive evaluation index η decreases with Reynolds number at laminar flow but a critical maximum value appears at turbulent flow.