Numerical Modeling of Flow Field around the Shaft Spillways Using Piano-Key Inlet

Document Type : Research Article

Authors

Abstract

A shaft spillway is a hydraulic structure consisting of an entrance control structure and a vertical or inclined conduit discharging flow into the downstream channel. Morning glory spillway is a special case of vertical-circular crested shaft spillways.  The shape of the weir crest and the upper part of the shaft are designed to follow the trajectory of the lower flow nappe. Morning glory spillways are used where other types of spillways are useless. However, limited discharge coefficient of morning glory spillways, results in increasing the water head in the reservoir and subsequently increase the risks of operation. Consequent to the onset of submergence, the head-loss increases, swirling flow generates, vortices intensify and the performance of spillway on evacuating flow decreases. Intake vortices are the results of angular momentum conservation at the flow constriction, where angular velocity increases with a decrease in the cross sectional area. The strong vortices over the inlet of the shaft spillway reduce the flow discharge considerably. A common solution for avoiding air-entrainment and swirling flow is to provide sufficient submergence at the intake. If the required approach flow conditions cannot be met to avoid the swirl and air entrainment, other structural approaches are considered. Anti-vortex devices are used as means of reducing the strength of vortex flow. The most economic and common measure to reduce the air-entrainment and swirling flow strength, is optimized shape of inlet e.g. by installing a circular Piano-Key (PK) inlet over the shaft spillway.  
In this study, the flow field around the shaft spillways using PK inlet was investigated based on numerical modeling. Due to its special configuration, a PK inlet reduces the swirling flow strength and has significant effects on increasing the discharge coefficient of shaft spillways. The flow field around the structure and inside the vertical shaft spillway was modeled using FLUENT 6.3.26 software. For this purpose, the governing differential equations of motion in a cylindrical coordinate system were solved using finite volume method (FVM). Air-water two phase flow was applied on free water surface and standard k-e turbulence model was used. The model consists of a cylindrical tank of 2 m in diameter and 1 m high. A vertical shaft of 0.126 m in diameter was installed at the center of the tank floor equipped with the PK inlet. Accordingly, the effects of the PK inlet geometries, including the inlet length, height and angle on hydraulics of flow through the shaft and the flow discharge coefficient were investigated. The flow characteristics such as variations of pressure and 3-D flow velocities were calculated. Results show that, PK inlets increase the flow discharge considerably compared to a simple shaft or even a morning-glory spillway. Finally, the best geometries of PK inlets were suggested to achieve the maximum efficiency of the vertical shaft spillways.

Keywords


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  • Receive Date: 07 November 2015
  • Revise Date: 21 September 2016
  • Accept Date: 29 October 2016
  • First Publish Date: 29 October 2016