Estimation of Heat Transfer Coefficient and Thermal Performance Factor of TiO2-water Nanofluid Using Different Thermal Conductivity Models

Background: Enhancement of heat transfer rate is one of the most important aims in industrial applications. The conventional fluids including oil, water and ethylene glycol have poor thermal properties compared to those of most solids. With this information, it leads to the idea that, if we disperse very small particles and let them suspend stably in base fluids, thermal conductivities of that base fluids should be higher. In previous study of the authors, the effects of different thermal conductivity models on heat transfer and pressure drop of nanofluids were not carried out. In view of that consequence, this article is aimed at reporting the effect of different thermal conductivity models on the prediction of convective heat transfer coefficient and thermal performance factor of nanofluids experimentally. Method: An experimental study was performed for TiO2-water nanofluid with a volume fraction between 0.002 and 0.02 and Reynolds number (Re) from 8,000 to 51,000. The experimental apparatus is a horizontal double tube counter-flow heat exchanger. Results: It shows that by growing the Re or nanoparticle volume fraction value, the Nusselt number enhances for all models studied. All equations used to calculate the thermal conductivity of nanofluid show same trends regarding the Nusselt number when Re or nanoparticle volume concentration changes. Some models could show more variation or low changes in the Nusselt number when the Re or nanoparticle volume concentration changes, however. Meanwhile, all nanofluids have a higher Nusselt number compared to distilled water. Conclusion: The nanofluid Nusselt number significantly enhances with growing Re and volume concentration for all thermal conductivity models studied in this work. By applying the nanofluid at a 0.02 nanoparticle volume fraction and Re equal to 47,000, the maximum thermal performance factor of 1.86 is found, based on Yu and Choi's and Jang and Choi's models. At low Re, all models show approximately same Nusselt numbers for all nanoparticle volume concentration. For moderate and high Re, the difference between the Nusselt numbers calculated by different models enhances. The thermal performance factor is higher than the unity for all Re and all volume concentration in this study, based on every thermal conductivity model.