Study of Heat Transfer and Flow Mechanism in Circular Backward-Facing Step Considering Ferrofluid Flow Under Magnetic Field Effect

The backward-facing step is critical in heat transfer applications, especially in heating and cooling systems, due to its unique flow phenomena like reattachment and separation. Expansion ratio, angle, flow regime, and thermo-physical properties of the working fluid significantly impact heat transfer and flow mechanisms. In the present work a numerical study was conducted on circular cross-section backward-facing step with 2.5 expansion ratio and 90° angle. Magnetizable ferrofluid with volume concentrations of φ=1.0, 1.5, and 2.0% were used in laminar flow regimes (500≤Re≤2000), exposed to a constant magnetic field (B=0.3, 0.5, 0.7, and 1.0T) and a constant heat flux (q”=600 W/m²). Results showed that φ=1.0% ferrofluid had the highest Nusselt number and performance evaluation criterion. Besides, the increment of magnetic field strength not only enhances convective heat transfer but also causes its reduction. The magnetic field significantly influenced reattachment and recirculation, and the optimum case is acquired in B=0.5T at Re=500.