This paper presents experimental and theoretical determination of the effective thermal conductivity of various base fluids and nano TiO₂ composition. Ultrasonically assisted sol–gel method was used for synthesising anatase TiO₂ nanoparticles and dispersing them into base fluids using sonication for the synthesis of nanofluids. It is observed that thermal conductivity enhancement is significantly higher than that of base fluid. The thermal conductivity shows an increment with the addition of nanoparticles and confirms a 22% enhancement achievable in base fluids. The effect of base fluids is a complex idea and difficult to understand; lower base fluid viscosities were supposed to contribute higher in enhancement of thermal conductivity, but another important factor; i.e. fluid nanoparticles surface interaction, nanoparticles crystal type also contributes in enhancement. In the further study, as the sonication time increases; an improvement in the thermal conductivity of nanofluids is also observed. Except water-based nanofluids, all others show reasonably good agreement with the data predicted by Bruggeman model and the prediction is in the range of 5%. This study is important since it covers base fluids with a wide range of thermal conductivity and viscosity.

Concentric tube heat exchanger is a low cost, which increases reliability by restricting mixing of fluids exchanging heat. Concentric tube heat exchanger has potential application such as heat recovery from engine cooling circuit, oil cooling, desuperheating in refrigeration and air conditioning, dairy, and chemical industry, pharmaceutical industry, refinery, etc. This paper concentrate on an experimental study on concentric tube heat exchanger for water to nanofluids heat transfer with various concentrations of nanoparticles in to base fluids and application of nanofluids as working fluid. The experimental results on this type of heat exchanger configuration and water nanofluids could not be located in literature. Overall heat transfer coefficient was experimentally determined for a fixed heat transfer surface area with different volume fraction of nanoparticles in to base fluids and results were compared with pure water. It observed that, 3% nanofluids shown optimum performance with overall heat transfer coefficient 16% higher than water