Experimental investigation of water based nanofluid containing copper nanoparticles across helical microtubes

This experimental work mainly focuses on heat transfer and fluid flow characteristics of a nanofluid flowing in the helical microtube (HMT). Experiments encompass Reynolds numbers from 700 to 2100, HMT coil diameters of 0.96 × 10− 2, 1.43 × 10− 2, 2.13 × 10− 2, and 2.87 × 10− 2 m, and Cu–water nanofluid weight concentrations of 0.0%, 0.1% and 0.3%. Each HMT is made up of copper microtube with diameter of 787 μm and contains twenty curvatures. Firstly, the experimental setup and procedure are validated using a straight microtube. The results show that the thermal performance of the HMTs is better than the straight microtube. For the same curvatures, both Nusselt number and friction factor increase as the coil diameter of the HMTs decreases. It is also found that the addition of small amounts of Cu nanoparticles to the water augments the heat transfer coefficient of the HMTs with a slight pressure drop. Likewise, the nanofluid flow has higher effects on the thermal–hydraulic performance of the HMT with lower coil diameter. The 0.3% nanofluid gets higher values of the considered performance evaluation criterion (PEC) compared to the base fluid and 0.1% nanofluid. At the studied ranges, the highest value of the PEC of 2.24 is obtained for the 0.3% nanofluid inside the HMT with the coil diameter of 0.96 × 10− 2 m. Finally, correlations are developed for the Cu–water nanofluid flow inside the HMTs as function of nanoparticle weight fraction, curvature ratio, Reynolds number, and Prandtl number.