Formation mechanism of suction vortices in a pump sump

Abstract

The origin and formation mechanism of a submerged vortex and an air-entraining vortex have been fully clarified by largeeddy simulation (LES) that used approximately 2 billion hexahedral elements with maximum resolution of 0.255 mm and was applied to the internal flows of a model pump sump. The model pump sump is composed of a 2,500 mm-long water channel of rectangular cross section with a width of 300 mm and a water depth of 100 mm and a vertical suction pipe with a 100 mm diameter installed at its downstream end with an offset of 10 mm from the centerline of the rectangular channel. At the upstream end of the channel, a uniform velocity of 0.37 m/s is given. LES with different wall boundary conditions have revealed that the origin of a submerged vortex is the mean shear of the approaching boundary layers that develop on the bottom and side walls of the pump sump. From detailed investigations of LES computed for a long time period of 16 seconds have revealed that deviation of the mean flow that approaches the suction pipe triggers conversion of the axis of the vorticity that was originally aligned to the lateral direction in the approaching boundary layers to that aligned to the vertical direction. The local acceleration of the vertical flow stretches the afore-mentioned vertical vortex, which results in formation of a submerged vortex. The separated flows downstream of the suction pipe generate vertical vorticity, and forms an air-entraining vortex when such a vortex is sucked into the suction pipe. Computations with a different bellmouth height and a different water-surface height have supported the above mentioned origin and formation mechanism of these vortices.

A Numerical Study on the Performance of a Pumping Station with Bell-Mouth-Based Vertical Pumps during an Accidental Shutdown

“…It is very important to predict the operation conditions when the above-mentioned phenomena take place and then to prevent their formation. That is why these phenomena are widely examined both numerically and experimentally [3][4][5][6][7][8][9][10][11]. Due to the time and financial costs, Computational Fluid Dynamics (CFD) methods prevail.…”

Evaluation of the Influence of Upstream Flow on the Energy Characteristics of a Giant Kaplan Turbine

“…Despite previous studies on the scale effect, the motion of vortices is very complex and may differ from the actual situation in prototype engineering due to factors such as Reynolds number, Weber number, and boundary conditions. In contrast, in recent years, many scholars have adopted numerical simulation methods based on computational fluid dynamics to conduct numerical simulation research on the free surface flow of the inlet pool of power plant units [13][14][15][16][17]. Constantinescu et al [18,19] simulated the critical conditions in the inlet pool, displaying the size, position, and intensity of vortices, and calculated and compared them using the k-epsion model and k-omega model.…”

Experimental Research and Numerical Analysis of Flow Phenomena in Discharge Object with Siphon

“…The analysis of the flow with free water level in the suction objects of pumping stations can be found in many references. They concentrate mainly on vortex structures and cavitation phenomena in the pump intake [5][6][7][8][9][10][11]. Especially Tokay and Constantinescu [5] give a detailed picture of unsteady vortices obtained by means of the large eddy simulations (hereinafter LES).…”

“…In the next sections, this methodology will be used as well, referring also to the terminology of three-dimensional separated flows introduced by Tobak and Peake [12]. A very interesting comparison of five different commercial CFD codes can be found in the work of Okamura et al [6] showing the ability to predict vortices in a sump of the axial-flow pump. Unfortunately, these calculations are based just on the single-phase flow and there is lack of information on the turbulence and cavitation models used.…”

“…The main singularity on the floor is the nodal point or focus of separation (the stable node), based on the pre-rotation generated by the impeller. An important part of vortices represent the free-surface ones, which could be filled with the air bubbles or with a full air core, or the submerged vortices with the vapor bubbles originating in the pressure drop inside the vortex cores [6,11]. On the contrary, in the discharge object close to the siphon outlet, the vortices are driven primarily by the water jet, which comes from the siphon tube and attaches the floor at the nodal point of attachment (the unstable node).…”