CFD based investigations on the effects of blockage shapes on transient mixed convective nanofluid flow over a backward facing step

Abstract

The separation and reattachment flow occurs in a backward-facing step (BFS) is available in various industrial applications such as gas turbine engines, combustors, aircraft, buildings and many other devices of heat transfer. It represents the pillar key of determining the flow structure and significantly affecting the heat transfer mechanism. This work studies numerically the effects of four various types of blockage shapes on transient laminar mixed convective nanofluid flow over a horizontal BFS placed in a duct. Nanoparticles of SiO2 with ethylene glycol as a base fluid at 2% of volume fraction and 20 nm of nanoparticle diameter are considered to examine the effect of different blockage shape on the thermal and flow fields at different time scales. The downstream of the step is kept at a constant heat flux of 500 W/m2, while other walls and sides of the duct are considered thermally insulated. The Reynolds number used in this study is in the range of 50–200. The relevant governing equations (continuity, momentum and energy) along with the boundary conditions are solved with the aid of finite volume method (FVM). The results reveal that after dimensionless time of τ = 5 the trend of nanofluid flow and recirculation area near the step and behind the blockage shape does not change significantly and this time is selected as a quasi-steady state time and the point of comparison. The velocity distributions for front facing triangular blockage decreased and this blockage shape has the highest value of skin friction coefficient beyond Re = 150. The results indicate that the front facing triangular blockage has the highest value of average Nusselt number and performance evaluation index while the trapezoidal blockage has the lowest value