Pressure drop and convective heat transfer of water and nanofluids in a double-pipe helical heat exchanger

Pressure drop and convective heat transfer characteristics of water and five alumina/water nanofluids of weight concentrations from 0.78% wt. to 7.04% wt. were experimentally investigated for both laminar flow and turbulent flow inside a double-pipe helically coiled heat exchanger. Effect of nanoparticles on the critical Reynolds number is negligible. A new correlation was developed for laminar flow in helically coiled tubes, which can predict the experimental heat transfer data very well. For turbulent flow, the Seban and McLaughlin correlation can accurately predict the thermal behavior of water and nanofluids when nanofluid properties are taken into account. For both laminar flow and turbulent flow, no anomalous heat transfer enhancement was found. The heat transfer enhancement of the nanofluids compared to water is from 0.37% to 3.43% according to the constant flow velocity basis. Figure of merit based on the constant Reynolds number can be misleading and should not be used for heat transfer enhancement comparison. Additional possible effects of nanoparticles, e.g., Brownian motion, thermophoresis and diffusiophoresis, on the convective heat transfer characteristics of the nanofluids are insignificant compared to the dominant thermophysical properties of the nanofluids. No multiphase phenomenon was found and the tested alumina nanofluids can be treated as homogeneous fluids.