Investigating the Flow and Heat Transfer Characteristics of Two Co/Counter-Rotating Circular Cylinders at a Low Reynolds Number

In this study, the characteristics of fluid flow and heat transfer of two rotating circular cylinders are analyzed using the two-dimensional finite volume method (CFD). The unsteady, incompressible, and uniform flow around two cylinders of equal diameter has been studied at Re = 200 and Pr = 0.71 to clarify the effects of the rotation rate (1 = a = 5) , spacing ratio (1.5 = L/D = 4) , and direction of rotation (co/counter-rotation) on the heat transfer rate and the critical rotation rate of cylinders arranged in three different angular positions: staggered (?= 45(?)), side-by-side (? = 90(?)), and tandem (? = 0(?)). The results revealed that the critical rotation rate (a(cr)) of cylinders with co-rotation is nearly 20% higher than that of the cylinders with counter-rotation in all the angular positions and for all the spacing ratios. For L/D> 1.5, a(cr) increases with increasing the spacing ratio. Also, a(cr) is the maximum for cylinders in a staggered arrangement. The heat transfer rate of the cylinders in different arrangements decreases as the rotation rate increases. However, this reduction is more significant for counter rotating cylinders compared to co-rotating ones, because co-rotation mitigates the amount of rotation-induced heat reduction. The mitigating effects of co-rotation are not the same in different arrangements. The rotation-induced heat reduction is minimum in side-by-side cylinders, while the heat transfer rate of tandem cylinders is less affected by co-rotation. These results are consistent with the 30% reduction observed in the mean Nusselt number of the downstream cylinder in a tandem arrangement. When the spacing ratio between the staggered cylinders increases from 1.5 to 3, the downstream cylinder with counter-rotation shows a 40% reduction in the mean Nusselt number, while co-rotation causes a 25% increase in the mean Nusselt number of the downstream cylinder.