Nihon Kikai Gakkai ronbunshu (Oct 2019)
Flow structure of a submerged vortex in a pump sump
Abstract
The flow structures of a submerged vortex that appears in a model pump sump have been fully clarified by performing large eddy simulation (LES) of a model vortex in a simplified computational model. The computational model had a sufficiently fine grid that could resolve the vortex core. The model sump is composed of a 2,500 mm-long water channel of rectangular cross section with a width of 300 mm and a water height of 150 mm and a vertical suction pipe with a 100 mm diameter installed at its downstream end. Our previous large eddy simulations (LES), which used approximately 2 billion grids and were applied to the whole pump sump, has fully clarified the origin and formation mechanism of a submerged vortex and an air-entrained vortex. In these computations, however, the static pressure in the vortex core decreased by only 5 kPa at a channel velocity of 0.37 m/s. The decrease in the static pressure was far smaller than the one for which one can expect initiation of cavitation in the vortex core. In the corresponding experiment, however, appearance of a submerged vortex was confirmed by the occurrence of cavitation in the vortex core. Therefore, the decrease in the static pressure is most likely to be underpredicted in our previous LES. Insufficient grid resolution was assumed to be one of the reasons for this underprediction. In the present study, LES with a sufficiently fine grid was applied to a simplified computational model that represents the stretch of a submerged vortex under a constant acceleration of the vertical velocity. Vertical and tangential velocities obtained by averaging those profiles of a submerged vortex computed in the previous LES were prescribed at the bottom wall of the computational domain as the inflow boundary conditions. In the present LES, the static pressure has decreased by more than 100 kPa. In addition, parametric studies with different initial swirl numbers varied from 0.08 to 10.9 have fully clarified the behavior of a submerged vortex. It is found that a strong submerged vortex appears only at a relatively small range of the swirl-number from 0.8 to 2.
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