This paper reports on the numerical investigation of assisting and opposing mixed convection flows and heat transfer performance of nanofluids in a cubical enclosure under turbulent conditions. The enclosure side wall consists of a partially heated heat source and the nanofluid is pumped into the enclosure through the inlet port. A three-dimensional, turbulent two-phase mixture model is developed to investigate the thermal performance of nanofluids by considering the effects of Brownian motion and thermophoresis. The base fluid is considered as water into which different nanoparticles such as aluminum oxide (Al2O3), copper (Cu) and silver (Ag) are dispersed for different particle volume fractions (ϕ) varied from 0% to 4%. The Richardson number (Ri) and Reynolds number (Re) are varied between 0.01 ≤ Ri ≤ 10 and 3.1 × 103 ≤ Re ≤ 105 and the Grashof number (Gr) is maintained at a constant value of 108. The nanoparticles are considered as monodispersed spherical particles with a uniform diameter of 100 nm. The results indicate that the temperature and turbulent kinetic energy distributions for opposing flows are higher than assisting flows. It also found that the average heat transfer rate increases with decrease in Richardson number and increase in volume fraction for both assisting and opposing flows. The effects of Brownian motion and thermophoresis are more significant in assisting flows than opposing flows. © 2020 Elsevier B.V.