Three new heat transfer fluids consisting of combinations of multi-walled carbon nanotubes (MWCNT) and microencapsulated phase change materials (MPCMs) were formulated, and tested in a turbulent flow heat transfer loop. Stable nanofluids have been prepared using different sizes of multi-walled carbon nanotubes and their thermal properties like thermal conductivity, viscosity and heat transfer coefficient have been measured. Microencapsulated phase change material slurries containing octadecane as phase change material have been tested to determine their durability and viscosity. A blend of MPCM slurry with MWCNT nanofluid has also been prepared to form a new heat transfer fluid that exhibits unique thermophysical properties including non-Newtonian viscous behavior. Heat transfer experiments have also been conducted to determine heat transfer coefficient and pressure drop values of the MWCNT nanofluids, MPCM slurries, and blends of MWCNT with MPCM under turbulent flow and constant heat flux conditions. The heat transfer results of the MPCM slurry containing octadecane was in good agreement with the published results. A maximum thermal conductivity enhancement of 8.1% was obtained for MWCNTs with diameter of 60-100 nm and length 0.5-40 mu m. Heat transfer results indicate that MWCNT nanofluid exhibits a convective heat transfer enhancement in the range of 20-25% in turbulent flow conditions. The blend of MPCMs and MWCNTs was highly viscous and displayed a non-Newtonian shear thinning behavior. Due to its high viscosity, the blend exhibited laminar behavior and lower heat transfer rate, though the maximum local heat transfer coefficient achieved by the blend was comparable to that obtained with MPCM slurry alone. The pressure drop of the blend was also lower than that of the MWCNT nanofluid. (C) 2011 Elsevier Ltd. All rights reserved.