Thermal–Hydraulic Performance of Nanofluids in Turbulent Separated Flow Induced by Double Forward Facing Step With Converging/Diverging Wall

Researchers have proposed multifarious passive methods for heat transfer augmentation over geometries with separation and reattachment. In this study, the turbulent forced convection flow of water-based CNT-TiO₂ hybrid nanofluid, ND-Ni hybrid nanofluid, and mono Ni nanofluid (with temperature-dependent properties) in a double forward-facing step channel with a converging/diverging bottom adiabatic wall is evaluated. The single-phase shear stress transport k-ω model is applied to solve the governing equations. Results indicate that the highest thermo-hydraulic performance (the value of figure-of-merit is equal to 1.1) can be achieved using TiO₂-CNT/water HyNf with ϕ = 0.002. Generally, as the velocity of the incoming stream is reduced, the thermal efficacy of HyNf improves. When the water-based NFs are not an effective heat transfer fluid (inducing a performance evaluation criterion lower than unity), the diverging channel (weakening of the contact between surfaces with constant heat flux and working fluid) can be employed instead of the DFFS channel while the Reynolds number of the incoming flow reduces as well. It is found that the thermal efficacy of water-based NFs in complex separated flows depends strongly not only on the deflection angle of the bottom adiabatic wall but also on the velocity of the incoming flow.