Comparative study between metal oxide nanopowders on thermal characteristics of nanofluid flow through helical coils

Nanofluids and helical coils are two different techniques to enhance thermal performance of highly efficient heat exchangers. By combining these techniques together, energy efficiency of equipments could be increased dramatically. The present study is an experimental comparison of heat transfer behavior between metal oxide nanofluid flows through helical coiled tube with uniform heat flux boundary condition. Nanofluids with required volume concentration of 0.25–1.0% were prepared by dispersing specified amounts of Al2O3 (35 nm) and TiO2 (50 nm) nanoparticles and appropriate amount of surfactants in distilled water. The experiments covered a range of Reynolds number from 500 to 4500. Experimental results indicated that a considerable heat transfer enhancement is achieved by both nanofluids. In addition, because of greater thermal conductivity and smaller size of Al2O3 nanoparticles compared to TiO2 nanoparticles, Al2O3/water nanofluid showed better heat transfer augmentation. Moreover, due to the curvature of the tube when fluid flows inside helical coiled tube instead of straight one, convective heat transfer coefficient and the pressure drop of both nanofluids grew dramatically. Besides, experiments indicated that the effect of coil pitch spacing is to some extent weaker than the curvature ratio effect on the heat transfer rate. Finally, it was studied that the experimental and predicted results of Nusselt number and pressure drop held reasonable agreement.