Aqueous carbon nanotube nanofluids and their thermal performance in a helical heat exchanger

•Viscosity and thermal conductivity were measured for MWCNT nanofluids.•Pressure drop and heat transfer of nanofluids in helically coiled tubes.•Previous correlations can reproduce the thermal behavior of MWCNT nanofluids.•No heat transfer enhancement based on fixed flow velocity and fixed pumping power.

This work experimentally investigated the hydraulic and thermal performance of aqueous multi-walled carbon nanotube (MWCNT) nanofluids in a double-pipe helically coiled heat exchanger. Measured viscosity and thermal conductivity values of the MWCNT nanofluids, instead of literature values or values calculated from correlations, were used for data analyses and performance evaluation of the nanofluids. A transient plane source method was adopted to measure thermal conductivity. The increase in viscosity of the nanofluids is much larger than the thermal conductivity enhancement. For example, the relative thermal conductivity is only 1.04 while the relative viscosity is 9.56 for a 1.0 wt% MWCNT/water nanofluid. Pressure drop and heat transfer characteristics were experimentally studied for aqueous MWCNT nanofluids of weight concentrations 0.02 wt%, 0.05 wt% and 0.1 wt% inside the helical heat exchanger. By using the measured nanofluid properties, the Wu et al. correlation and the Seban and McLaughlin correlation can reproduce the thermal behaviours of the tested MWCNT nanofluids for laminar flow and turbulent flow very well, respectively. Possible MWCNT effects, e.g., Brownian motion and thermophoresis, on the thermal performance in helically coiled tubes are probably unimportant. No heat transfer enhancement was found as a fixed flow velocity and a fixed pumping power were considered.