EXPERIMENTAL STUDY ON TURBULENT HEAT TRANSFER, PRESSURE DROP, AND THERMAL PERFORMANCE OF ZnO/WATER NANOFLUID FLOW IN A CIRCULAR TUBE by Ahmad Reza SAJADI a , Seyed Soheil SADATI b , Masoud NOURIMOTLAGH c , Omid PAKBAZ a , Dariush ASHTIANI a* , and Farshad KOWSARI a a School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran b Department of Mechanical Engineering, Damavand Branch, Islamic Azad University, Tehran, Iran c Young Researchers Club, Dareshahr Branch, Islamic Azad University, Iran Original scientific paper DOI: 10.2298/TSCI131114022S In this experimental study heat transfer and pressure drop behavior of ZnO/water nanofluid flow inside a circular tube with constant wall temperature condition is in- vestigated where the volume fractions of nanoparticles in the base fluid are 1% and 2%. The experiments' Reynolds numbers ranged roughly from 5000 to 30000. The experimental measurements have been carried out in the fully-developed turbulent regime. The results indicated that heat transfer coefficient increases by 11% and 18% with increasing volume fractions of nanoparticles, respectively, to 1 vol.% and 2 vol.% . The measurements also showed that the pressure drop of nanofluids were, respectively, 45% and145% higher than that of the base fluid for volume fractions of 1% and 2% of nanoparticles. However experimental results revealed that overall thermal performance of nanofluid is higher than that of pure water by up to 16% for 2 vol.% nanofluid. Also experimental results proved that existing correlations can accurately estimate nanofluids convective heat transfer coefficient and friction fac- tor in turbulent regime, provided that thermal conductivity, heat capacity, and vis- cosity of the nanofluids are used in calculating the Reynolds, Prandtl, and Nusselt numbers. Key words: nanofluid, convective heat transfer, turbulent flow, friction factor, overall thermal performance Introduction Nanofluid as a promising technology is supposed to enhance the heat transfer capabili- ties of conventional liquids both conductively and convectively [1-3]. Greater amount of work- ing fluids' heat transfer will help thermal systems in many industries, such as energy, electron- ics, and transportation to be designed smaller and/or more efficient. Also the conventional working fluids which possess low thermal conductivity coefficients can no longer meet the re- quirements of high-intensity heat transfer duties. The conventional methods to improve the heat transfer rate include passive techniques with application of extended or rough surfaces and swirl flow, and active techniques with sur- Sajadi, A. R., et al.: Experimental Study on Turbulent Convective Heat Transfer, ... THERMAL SCIENCE: Year 2014, Vol. 18, No. 4, pp. 1315-1326 1315 * Corresponding author; e-mail: dariusha63@gmail.com