Aiding buoyancy driven flow and heat transfer features of converging and diverging trapezoidal cylinders

Numerical computations have been carried out to explore the influence of mixed convection heat transfer from heated trapezoidal geometries of two different configurations namely converging and diverging cylinders in a vertical domain. The recirculation length of the diverging cylinder is found to be more than that of the converging cylinder for all values of Re (5 to 40) considered in this study and this length decreases after introducing buoyancy effect. Drag coefficients decrease with increase in Re for a fixed Ri. However, drag increases for the increasing values of Ri (values considered up to 1). The drag coefficient is found to be the smaller for diverging cylinder than that of converging one. Local Nusselt number shows significant increase as Re and Ri values increase, which results in enhanced heat transfer. Keeping Ri fixed and increasing the value of Re results in the augmentation of heat transfer and is around 15% at Re = 5 and 23% at Re = 40 for Ri = 0 for a square cylinder with respect to diverging cylinder. Under the influence of aiding buoyancy, the values of average Nusselt number ( $$ \overline{Nu} $$ ) for the diverging cylinder are higher compared to that of converging cylinder. A correlation expressing functional relationship of $$ \overline{Nu} $$ with Re and Ri has also been generated.