In this paper, the convective heat transfer enhancement of aqueous suspensions of multi-walled carbon nanotubes flowing through a straight horizontal tube was investigated experimentally for a Reynolds number range of 1000-8000, which included the transitional flow regime. The tube was made out of copper with an internal diameter of 5.16 mm. Experiments were conducted at a constant heat flux of 13 kW/m(2) with 0.33%, 0.75% and 1.0% volume concentrations of multi-walled carbon nanotubes. The nanotubes had an outside diameter of 10-20 nm, an inside diameter of 3-5 nm and a length of 10-30 mu m. Temperature and pressure drop measurements were taken, from which the heat transfer coefficients and friction factors were determined as a function of Reynolds number. It was found that heat transfer was enhanced when comparing the data on a Reynolds-Nusselt graph but when comparing the data at the same velocity, it was shown that heat transfer was not enhanced. Performance evaluation of the nanofluids showed that the increase in viscosity was four times the increase in the thermal conductivity, which resulted in an inefficient nanofluid. (C) 2012 Elsevier Ltd. All rights reserved.