The flow in a tube with the narrowest width 150 μm and the depth 150 μm was studied numerically, two flow resistance coefficient curves versus cone angle θ and lengthwidth ratio l/w were obtained when the tube was subject to convergent and divergent flows at a Reynolds numbers ranged in 100-2 000. The results show that the smaller the cone angle θ, the more significantly dependent on θ and l/w the flow resistance coefficient λ, and a transition occurs in flow resistance feature. The relations of λ to θ and l/w are different at Re=100 and Re≥500 obviously. When Re=100, most of tubes present the typeⅠ tube characteristics that flow resistance coefficient in the divergent direction is less than in the convergent one, and λ reaches a maximum 1.22 at θ=20°and l/w=20. When Re≥500, most of tubes show the type Ⅱ tube characteristics which is contrary to typeⅠ, and λ reaches a minimum 0.63 at θ=20°and l/w=1. The results show that the flow resistance coefficient of flatwalled diffusers highly depends on Reynolds number. In order to improve performance of the valveless piezoelectric pump, appropriate geometrical parameters should be chosen in the pump design according to the actual working condition of the pump.
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