Three-dimensional flow effects on forced convection heat transfer in a channel with stepwise-varying width

► Laminar forced convection inside variable-width heat sink is numerically investigated. ► The topology of the contraction-induced 3-D recirculation pattern is characterized. ► Onset of two pairs of longitudinal vortices due to the contraction effect found. ► Pattern of overlapping cold and warm fluid layers formed downstream of the contraction. ► Heat transfer significantly enhanced in comparison to parallel channel flow.