This paper deals with experimental studies carried out to analyze heat transfer characteristics of Al 2 O 3 -, CuO-, TiO 2 -, and ZnO-water based nanofluids in a double-pipe, counter flow heat exchanger for different volume concentrations (0.025%, 0.05%, 0.075%, and 0.1%) of the nanofluids. The fabricated double-pipe heat exchanger is made up of two different materials, viz., copper as the inner tube and unplasticized polyvinyl chloride as the outer tube. The density, viscosity, and thermal conductivity were calculated, and were used to estimate dimensionless numbers, such as Reynolds number, Prandtl number, and Nusselt number, and also to estimate heat exchanger effectiveness.High-energy ball milling technique was used to prepare nanoparticles and were characterized using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Polyvinyl alcohol (3%) was used as a surfactant for making the nanofluids stable. It was observed from the experiment that with the increase in the volume concentration, thermal conductivity, viscosity, and friction factor increase, whereas the Reynolds number decreases. The experimentally observed data for Nusselt number were formulated into a correlation that matches the data for all these nanofluids within an error of 11.4%. It was found that the highest effectiveness was obtained while using TiO 2 -water nanofluids than other nanofluids. K E Y W O R D S Al 2 O 3 -, CuO-, TiO 2 -, ZnO-water nanofluid, double-pipe heat exchanger, heat exchanger effectiveness, nanoparticle synthesis, Nusselt number correlation
