Laminar mixed convective buoyancy assisting flow through a two-dimensional vertical duct with a backward-facing step using nanofluids as a medium is numerically simulated using finite volume technique. Different types of nanoparticles with 5% volume fraction are used. The wall downstream of the step was maintained at a uniform wall temperature, while the straight wall that forms the other side of the duct was maintained at constant temperature equivalent to the inlet fluid temperature. The wall upstream of the step and the backward-facing step were considered as adiabatic surfaces. The duct has a step height of 4.9 mm and an expansion ratio of 1.942, while the total length in the downstream of the step is 0.5 m. The Reynolds number was in the range of 0 ≲ Re ≤ 100. The downstream wall was fixed to be at uniform wall temperature of 20 °C higher than the inlet flow temperature. A recirculation region was developed straight behind the backward facing step which was appeared between the edge of the step and few millimeters before the corner which connect the step and the downstream wall. In the few millimeters gap a U-turn flow was developed opposite to the recirculation flow which mixed with the unrecirculated flow and travels along the channel. It is inferred that diamond nanofluid has the highest velocity in the vicinity to the heated wall.

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