A study on the characteristics of carbon nanofluid for heat transfer enhancement of heat pipe

•The rate of thermal conductivity increase of oxidized MWCNT nanofluids is about 46% higher than that of MWCNT nanofluids.• The thermal conductivity of longer-length MWCNT particle is higher than shorter-length MWCNT particle.•The viscosity of the oxidized CM-95 MWCNT nanofluids was about 11% lower, indicating that it had been reduced by oxidization.•The viscosity at 90 °C, significantly, had decreased by about 65.2% compared to that at 10 °C.•The thermal conductivity and viscosity tended to increase and decrease, respectively, with increasing temperature.

For the purposes of enhancing the heat-transfer utility of the heat pipe in a solar collector, the present work attempted to improve nanoparticle dispersion stability by means of a chemical reformation process wherein nanofluid is formulated with hydroxyl radicals combined with oxidized multi-walled CNTs (MWCNTs). Experiments entailing measurements of thermal conductivity and viscosity in distilled water as functions of temperature were carried out to determine the best nanoparticle mixture ratio. The thermal conductivity increased with the increasing volumetric ratio of the oxidized MWCNTs and with increasing temperature. The viscosity also increased, slowly, until its concentration reached 0.01%, and then steeply increased, and was lower at high temperature.