Accurate measurement of nanofluid thermal conductivity by use of a polysaccharide stabilising agent

•Measurement of liquid thermal conductivity complicated by convection currents.•The polysaccharide agar used as a gelling agent to prevent convection currents.•Transient hot-wire measurement precision significantly improved by gelling agent.•Measurement method validated for water and water/ethylene glycol reference fluids.•Method was used to measure thermal conductivity of alumina and copper nanofluids. Measuring the thermal conductivity of low viscosity fluids such as aqueous nanofluids is challenging due to the formation of convection currents. In the current work, a modification of the transient hot-wire thermal conductivity measurement technique was investigated to address this problem. The polysaccharide agar was used as a gelling agent to prevent the formation of convection currents, thereby enabling measurement of thermal conductivity. The experimental method was validated by comparison of experimentally measured thermal conductivity values with published reference values over a range of temperatures for two reference fluids stabilised by agar: water and an ethylene glycol/water solution. The precision of thermal conductivity measurements was found to be significantly improved by use of this gelling agent. These findings indicate that agar, or a similar gelling agent, can be used to enable accurate measurement of the thermal conductivity of aqueous fluids. This measurement technique was utilised to accurately measure the thermal conductivity enhancements of copper and alumina aqueous nanofluids with low nanoparticle concentrations, over a range of temperatures. The thermal conductivities of these nanofluids were found to be within ±2% of those predicted by the Maxwell model.