Hydrodynamics and heat transfer around a diamond-shaped cylinder in a stationary flow have been investigated using direct numerical simulation. Simulations were carried out for a steady flow with a Reynolds numbers ranging from 1 to 70 and for a Prandtl number corresponding to a gas (P r = 0.7). The study focuses on the influence of the diamond apex angle α (33 ≤ α ≤ 120 • ) on the evolution of drag, wake length and Nusselt number. A comparison with the case of a circular cylinder is performed. It is shown that the drag coefficient of a diamond-shaped cylinder remains very close to the one of a circular cylinder (±10%) for Re < 10 and that it is reduced by decreasing the apex angle for Re > 10. In the same time, compared to the circular cylinder case, the reduction of the apex angle postpones significantly the Reynolds corresponding to the wake recirculation onset. When the Reynolds reference velocity is, as often, taken as the far field velocity, the corresponding Nusselt numbers are found to decrease with the apex angle α. However, it is found that when the reference velocity is based on the maximal vorticity near the equator, the Nusselt number of diamond-shaped cylinders seems to collapse on a single master curve. This may indicate that the relevant velocity scale to describe Nusselt variation, and thus the heat transfer, is dependant on the interfacial vorticity intensity rather than on the far field velocity.

Cylinders with an elliptical, oblong, lenticular, sinus or diamond transveral shape are very interesting geometries for the design of compact heat exchangers. This work investigates the role of the porosity and of the apex angle of diamond-shaped cylinders networks on the pressure losses, at moderate Reynolds numbers, inside micro heat regenerators. The design of the geometry under test has been chosen so that the cross section of the flow remains almost constant along the path of the flow between cylinders. Experiments have been performed at 1 ? Re ? 30 and a porosity range 0.40 ? ? ? 0.90 for an apex angle of ? " 33?. Numerical simulations have been conducted using the same Reynolds and porosity ranges but varying the apex angle 33? ? ? ? 90?. Experimental measurements and dimensional analysis have shown that the friction factor can be affected by the porosity. 2D numerical simulations confirmed that the friction factor increases with the porosity but also with the apex angle. An analysis at the scale of a channel flanked by adjacent cylinders has provided a original correlation able to describe easily the evolution of the Poiseuille number and the collective effects on the drag coefficient as a function of ? and ?. Such a diamond-shaped design is found to induce much lower Poiseuille numbers than those expected from conventional stacked spheres, woven wires and circular cylinders arrays. The findings of this study can help for better understanding the optimization of low pressure drop regenerators and how to reduce associated hydraulic power.