AbstractIn this work we present experimental results of cross-sectional speed of water flow in narrow cylindrical metal tubes at high pressure gradients up to 1.1 GPa$$\cdot$$ · m−1. The measurement draws attention to the paradoxical behaviour of flowing water in internal diameters less than 250 $$\upmu$$ μ m. At constant pressure gradient, its cross-section speed decreases with decreasing diameter in accordance with the classical hydrodynamic prediction for turbulent flow in rough cylindrical

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... ube. However for very low diameters below 250 $$\upmu$$ μ m, the cross-section speed rises again and reaches almost the maximum theoretical value of the outflow speed for the appropriate pressure without energy loss caused by contraction or hydraulic friction. Our contribution describes mainly experimental character of the new phenomenon and its motivation is to promptly provide the material for further study to the professional public.