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An experimental work had been carried out to investigate the heat transfer and friction factor characteristics of Al2O3 /water nanofluid with circular ring inserts of various pitch to diameter ratios at constant heat flux condition. Al2O3 /water nanofluids with two different volume concentrations were used as the working fluid. The experiments were performed in the turbulent flow regime of 5000 to 25 000 Reynolds number. The results showed that the heat transfer characteristics of Al2O3 /water nanofluids were intensified compared to water and it increases when utilised with circular ring insert. The Nusselt number enhancement of 0.3% volume concentration of nanofluid in the tube without insert was 8.61% and with insert was 38.71% compared to water. Based on the investigation, two correlations among various parameters namely, Nusselt number (Nu), friction factor(f), Prandtl number (Pr), pitch to diameter ratio (p/d), Reynolds number (Re) and particle volume fraction (f) were presented. Over the range investigated, the maximum thermal performance factor of 1.26 is found with the simultaneous employment of the insert and Al2O3/water nanofluid at 0.3% volume concentration. [ABSTRACT FROM AUTHOR]