A numerical study on flow and heat transfer of de-ionized water over in-line and staggered micro-cylinder-groups had been performed with Reynolds number varying in the range from 0 to 150. A 3-D incompressible numerical model was employed to investigate the vortex distributions and the influences of the vortexes on the flow and heat transfer characteristics at low Re numbers in micro-cylinder-groups with different geometrical parameters, including micro-cylinder diameters (100μm, 250μm and 500μm), ratios of pitch to micro-cylinder diameter (1.5 2 and 2.5) and ratios of micro-cylinder height to diameter (0.5, 1, 1.5 and 2), etc. The vortex distributions, the flow and temperature fields, and the relationships among them were investigated by solving the numerical model with the finite volume method. It was found that the vortex number became larger with the increase of pitch ratio, and the change of flow rate distribution affected the heat transfer characteristics apparently. The appearance of vortexes in micro-cylinder-group increased the differential pressure resistance; as a result the total flow resistance in micro-cylinder-groups correspondingly increased. Meanwhile, the local heat transfer coefficients nearby the locations of vortexes greatly increased due to the boundary layer separation, which further enhanced the heat transfer in micro-cylinder-groups. The new correlations which could predict Nusselt number of de-ionized water over micro-cylinders with Re number varying from 0–150 had been proposed considering the differential pressure resistance and the natural convection based on numerical calculations in this paper.

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