This work describes the effect of steady-state laminar forced convection on multiscale rotating cylinders in cross-flow. The objective was to numerically maximise the heat-transfer-rate-density from the multiscale cylinder assembly under a prescribed pressure drop. Two main configurations were studied, the first was with two different-sized cylinders aligned along the same centreline, and the second configuration
was that in which the axis of rotation of the two cylinders was not on the same centreline but the
leading edges of the cylinders were on the same line. In both configurations, the cylinders were subjected to two types of rotations, counter-rotation and co-rotation. Numerical solutions for stationary and rotational cylinders were solved to determine the optimum cylinder diameter, spacing and the corresponding maximum heat transfer rate density. The effects of different centres of rotation and the dimensionless
pressure drop on the cylinder-to-cylinder spacing, optimal diameter of the cylinder and the maximum heat transfer rate density were reported. Results show that the optimal smaller cylinder diameter was robust with respect to the dimensionless pressure drop number, for both configurations. Results further showed that rotation was only beneficial for cylinders with the same axis of rotation and the effect was
minimal when the axis of rotation is different.