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Nanofluid flow over a backward facing step was investigated numerically at low Reynolds number and the heat transfer was analyzed and reported. Al₂O₃–H₂O nanofluids of different volume fractions (<inline-formula><math display="inline"><semantics><mi>φ</mi></semantics></math></inline-formula> = 1–5%) were used as the material with uniform heat flux (UHF) of 5000 W/m² at bottom wall for Reynolds number 200–600. The backward facing step of two geometries was investigated for two expansion ratios, 1.9432 and 3.5. The SIMPLE algorithm was used in the finite volume solver to solve the Naiver–Stokes equation. Temperature difference at inlet and boundaries, heat transfer coefficient, Nusselt number, coefficient of skin friction, and temperature contours were reported. The results show that when nanofluids are used, the coefficient of heat transfer and Nusselt number increased at all volume fractions and Reynolds number for both the expansion ratios. The coefficient of heat transfer at <inline-formula><math display="inline"><semantics><mi>φ</mi></semantics></math></inline-formula> = 5% was higher by 9.14% and 9.68% than the pure water for ER = 1.9432 and ER = 3.5 at Re. 500. At <inline-formula><math display="inline"><semantics><mi>φ</mi></semantics></math></inline-formula> = 5%, the outlet temperature for the duct decreased by 10 K and 5 K when compared to the pure water for ER = 1.9432 and ER = 3.5 at Re. 500. Coefficient of skin friction and outlet temperature decreased for both the volume fractions in both the expansion ratios.