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The present work provides an empirical investigation on the thermal characteristics of Ag/oil nanofluids flow inside a sinusoidal tube under a constant heat flux boundary condition. Ag/oil nanofluids have been prepared in low-volume concentrations of 0.011%, 0.044%, and 0.171%. The average size of the nanoparticles was 20 nm. A heated coil was attached to the upper and lower surface of the tube that satisfied the constant thermal boundary condition of 204 W. The experiment has been pursued at low Reynolds numbers less than 160. A loop was designed to keep the flow hydrodynamically fully developed during the experiment. The test case was a sinusoidal tube. Upper and lower surfaces of the tube have been designed sinusoidally. Moreover, the width of the plates was long enough, so the problem was not considerably affected by the three-dimensional releasing effect. Convective heat transfer coefficient and Nusselt number were calculated. It has been observed that based on the acquired data of the present work, convective heat transfer coefficient increased up to 23% for the best case (nanofluid with a volume concentration of 0.171%) compared to the base fluid. This happened while the rise of the friction factor was very low. In addition, a comparison between the new results and the previous work by authors showed the positive performance of sinusoidal tubes in increasing the convective heat transfer coefficient (the average increase was calculated to be about 82%) compared to the annular tube. [ABSTRACT FROM AUTHOR]

This study deals with an empirical investigation on the convective heat transfer of Cu/oil nanofluid flow inside a concentric annular tube with constant heat flux boundary condition and suggests a correlation to predict the Nusselt number. The average size of particles was 20 nm and the applied nanofluid was prepared by Electrical Explosion of Wire technique with no nanoparticle agglomeration during nanofluid preparation process and experiments. The nanofluid flowing between the tubes is heated by an electrical heating coil wrapped around it. The effects of different parameters such as the flow Reynolds number, tube diameter ratio, and nanofluid particle concentration on heat transfer coefficient are studied. Using the acquired experimental data, a correlation is developed for the estimation of the Nusselt number of nanofluid flow inside the annular tube. This correlation has been presented by using the exponential regression analysis and least-squares method. The correlation is valid for Cu/base oil nanofluid flow with weight concentrations of 0.12, 0.36, and 0.72 in the hydrodynamically full-developed laminar flow regime with Re <140, which is applicable in mini- and microchannel heat exchangers, and it is in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]