Pressure polarization oscillation in large-scale bulb tubular pump

Abstract

Hydraulic consistency between a prototype pump and its hydraulic model is generally considered to be satisfied, that is, the energy performance and the flow structures conform to the similarity law. However, such hydraulic consistency may not always exist in actual engineering projects. Head change and vibration enhancement in large-scale bulb tubular pumps are exhibited, but the reason for this inconsistency is still unclear. In this article, a case study was performed for the South-to-North Water Diversion Project in China, and the pressure characteristics in the impeller were explored under the optimal condition. An interesting new phenomenon, pressure polarization oscillation, is found. Its most important feature is the constant difference in average pressure values among axisymmetric blades, that is, the blades with axisymmetric structures exhibit large-amplitude pressure oscillations around different average pressure values, and one revolution is equivalent to one rotating cycle due to the failure of hydraulic consistency. Further in-depth analysis reveals that pressure oscillations are realized by the adjustment of the resultant force between the Coriolis force and the potential rothalpy gradient to fluid motions under the sinusoidal disturbance of gravity, and its action path presents a hysteresis loop with the impeller rotating. This adjustment mode leads to a special conversion between the gravity potential energy, pressure energy, and the kinetic energy, and, thus, the hydrostatic pressure difference at the initial phase cannot be fully eliminated; the pressure transition between the sides along and against the gravity is inconsistent. This finding suggests that the difference induced by the hydraulic inconsistency should be considered if the conventional similarity conversion is applied to large-scale tubular pumps.