Heat Transfer Enhancement of Low Volume Concentration of Carbon Nanotube-Fe₃O₄/Water Hybrid Nanofluids in a Tube With Twisted Tape Inserts Under Turbulent Flow

In this paper, it is estimated the heat transfer coefficient and friction factor for fully developed turbulent flow of carbon nanotube (CNT)-Fe₃O₄/water hybrid nanofluids flow through a tube with twisted tape inserts at constant heat flux conditions. The nanocomposite of CNT-Fe₃O₄ was prepared by in situ method; which contains dispersion of carboxylated-CNTs in distilled water followed by mixing of ferrous chloride and ferric chloride in the molar ratio of 2:1. Sodium hydroxide was used as reducing agent to form CNT-Fe₃O₄ nanocomposite. The detailed surface morphology and magnetic properties were performed by X-ray diffraction and scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM). The stable hybrid nanofluids were prepared by dispersing nanocomposite in distilled water, and the heat transfer and friction factor experiments were conducted for particle volume concentrations of 0.1% and 0.3%. The results indicate that a maximum of 31.10% enhancement in Nusselt number with a penalty of 1.18-times increase of pumping power was observed for particle concentration of 0.3% at a Reynolds number of 22,000 as compared to base fluid data. The Nusselt number is further enhanced to 42.51% for 0.3% nanofluid flow through a tube with twisted tape of H/D = 5 at a Reynolds number of 22,000 compared to base fluid data. The empirical correlations were proposed for the estimation of Nusselt number and friction factor to match well with the experimental data.