Nanofluids are a novel strategy to increase heat transfer characteristics of fluids by the addition of solid particles with diameters below 100 nm. Experimental measurements have shown that this approach can greatly increase heat conductivity, even above that predicted by Maxwell's theory. This paper shows a direct numerical simulation of the flow and thermal behaviour of a nanofluid loaded with alumina nanoparticles. The Boundary Element Method is used given its capabilities to deal with moving boundary problems. Results showed strong convective currents caused by the presence of the nanoparticles, which in time increase total heat flow in the cavity.