Design criteria for a type of asymmetric orifice in a surge tank using CFD

Roman Gabl,Maurizio Righetti

doi:10.1080/19942060.2018.1443837

Abstract

ABSTRACTAn asymmetric orifice can be added to a surge tank of a hydro power plant to dampen the mass oscillation. This allows a reduction of the required volume and a more stable behavior of the overall hydraulic system. In this paper, the advantage of a typical asymmetric orifice is shown in comparison with a sharp-edged geometry with two different pipe diameters. The software ANSYS-CFX is used to investigate the influence of the individual geometry parameters (radius, length, angle at the forefront) on the local head loss coefficients and the ratio of the two flow directions. Furthermore, a coefficient is analyzed based on an equation from the literature for the sharp-edged structure, which can be used comparably for the asymmetric orifice. This allows us to reach preliminary assumptions of the losses at an early state of the design process. The following adaptation can be supported by the presented guidance for the optimization process, so that a suitable geometry for the specific boundary conditions can be found.

Highlights

An orifice allows the introduction of an additional head loss at a specific location of a hydraulic system, which is useful for multiple applications
This paper focuses on applications that work as a throttle in a surge tank of a hydro power plant (HPP)
This paper has focused on asymmetric throttles, which can be added in a surge tank of a HPP to dampen the mass oscillation

Summary

Introduction

An orifice allows the introduction of an additional head loss at a specific location of a hydraulic system, which is useful for multiple applications. Large mass oscillations between the surge tank and the upper reservoir can be further damped with the addition of a throttle (Adam et al, 2018a; Li & Brekke, 1989; Vereide et al, 2017) Such a structure can be added in the connection between the surge tank and head race tunnel or at a change of the cross section (Adam et al, 2016a; Richter et al, 2015). The use of an asymmetric geometry further allows us to limit the loss in the upward flow direction (into the surge tank) and to add a bigger damping effect on the downward flow direction This leads to a reduction of the required volumes in the surge tank without introducing unwanted limits in the operation of the HPP. An asymmetric orifice is often preferred, which provides a stable behavior as well as a suitable ratio of the losses depending on the flow direction

Results

Discussion

Conclusion