In the present study, natural convection heat transfer inside a trapezoidal enclosure filled with carbon nanotube-EG-water nanofluid using variable properties has been numerically investigated. The bottom and top walls of trapezoidal enclosure are kept at constant temperatures T-h and T-c, respectively, while the side walls of the cavity are thermally insulated. Using the finite volume method and the SIMPLER algorithm, the governing equations have been discretized. Simulations have been carried out for different aspect ratios, Rayleigh numbers of 10(3)-10(6) as well as solid volume fractions of 0.0015, 0.03, and 0.045. The results show that at low Rayleigh number (Ra <= 104), the average Nusselt number Nu(Avg) decreases with increasing the inclination angle (aspect ratio) at all solid volume fractions. While for Ra = 10(6), the NUAvg increases and then decreases with inclination angle (aspect ratio) with maximum Nu(Avg) occurring at gamma = 30. Moreover, Nu(Avg) increases with increasing Rayleigh number at fixed inclination angle (aspect ratio) and solid volume fraction. (C) 2015 Elsevier Ltd. All rights reserved.