Measurement of the water jet velocity at the outlet of nozzles with different profiles

Abstract

Water jets accelerated by differently configured convergent nozzles of diameters ranging from 4.5 to 5.0 mm are studied. The excess pressure at the nozzle inlet varies from 5 × 103 to 3.5 × 106 Pa. Velocity measurements are carried out with a spring dynamometer and a free-running Pelton microturbine. The jet strength at the outlet of the nozzle is found to increase by a factor of 4.0–4.5 compared with the water flow strength at the inlet, which depends on the excess pressure and volume flow at the inlet. Reasons for such an effect and a possible source of the additional energy are considered. The 2D Bernoulli equation used instead of the 1D equation routinely applied in nozzle analysis leads to a negative value of the excess pressure at the nozzle exit section and in the jet. Gas evolution and cavitation enhance this effect because of a decrease in the jet density. As a result, the jet is accelerated not only by the inlet pressure but also due to the fact that the potential energy of the flowing medium decreases because of a decrease in its absolute pressure down to the technical vacuum level. The contraction of the jet by the atmospheric air and the establishment of the equilibrium (in air) pressure in the water jet eventually raise its kinetic energy through the internal energy of the air.