Heat transfer and overall heat transfer in a double pipe heat exchanger fitted with twisted-tape elements and titanium dioxide nanofluid were studied experimentally. The inner and outer diameters of the inner tube were 8 and 16？mm, respectively, and cold and hot water were used as working fluids in shell side and tube side. The twisted tapes were made from aluminum sheet with tape thickness (d) of 1？mm, width (W) of 5？mm, and length of 120？cm. Titanium dioxide nanoparticles with a diameter of 30？nm and a volume concentration of 0.01% (v/v) were prepared. The effects of temperature, mass flow rate, and concentration of nanoparticles on the overall heat transfer coefficient, heat transfer changes in the turbulent flow regime , and counter current flow were investigated. When using twisted tape and nanofluid, heat transfer coefficient was about 10 to 25 percent higher than when they were not used. It was also observed that the heat transfer coefficient increases with operating temperature and mass flow rate. The experimental results also showed that 0.01% TiO2/water nanofluid with twisted tape has slightly higher friction factor and pressure drop when compared to 0.01% TiO2/water nanofluid without twisted tape. The empirical correlations proposed for friction factor are in good agreement with the experimental data. 1. Introduction Lehigh heat exchange devices are used in industry for heat transfer between two fluids. Heat transfer is a mechanism for movement. Viscous fluids such as water are used in the twin-tube exchangers. The tube exchangers are widely used for the following reasons: creation of a large surface for heat transfer, reduction in size, good mechanical design, well-established technique, and usability for a wide range of substances. The major disadvantage of these converters can be explained as follows: for large thermal load, two-way exchange of large volumes occupies. The unit price of heat transfer area is relatively high. Application of nanotechnology in classical thermal designs leads to the production of a new class of heat transfer fluids, that is, nanofluids (Choi [1]). Since conventional heat transfer (HT) fluids including water, oil, and ethylene glycol (EG) show relatively poor HT characteristics, NF has been introduced. NFs are formed by dispersing solid particles, fibers, or tubes of 1 to 50？nm length in conventional HT fluids. There are significant characteristics associated with NFs such as high HT rate, low fluctuation ability through passages, and thermal homogeneity. The advances in nanotechnology have resulted in the
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