Revision as of 07:36, 11 April 2016

Swirling flow in a conical diffuser generated with rotor-stator interaction

Application Challenge AC6-14 © copyright ERCOFTAC 2024

Introduction

Confined turbulent swirling flows are encountered in many industrial applications, such as hydraulic turbines, gas turbine combustors and internal combustion engines. The flow patterns associated with turbulent swirling flows are vortex-dominated and have long been of interest to scientists and engineers. The goal of the study on which this AC is based has been to understand and control the naturally occurring phenomena. The knowledge can be applied to increase the turbulent convection in heat and mass transfer applications \cite{Javadi2015}, or to reduce unwanted pressure pulsations in e.g. hydro power systems \cite{Javadi2015c}. The present test case primarily relates to the flow in water turbines. The numerical results reported here are based on the work published in \cite{Javadi2015c}.

Relevance to Industrial Sector

The flow in water turbines running at off-design conditions often has a strong swirl which may cause vortex breakdown and pressure pulsation in the draft tube. The pulsations may damage the structure and also produce significant electrical power swings. The occurrence of the pulsation and its impact on the efficiency of the draft tube depends mainly on the flow rate of the turbine, the local pressure level, and the velocity field downstream of the runner \cite{Bosioc2012}. Under circumstances leading to the surge, the swirling flow tends to separate into two concentric flow regions. The axial flow basically occurs in the outer region, while the inner region may contain an on-axis recirculation region, also called stagnation region \cite{Javadi2015}. The breakdown of the swirling flow leads to a precessing helical vortex, also called vortex rope, which makes the recirculation region unsteady.

The swirling flow configuration of the present test case corresponds to part load operation of a Francis turbine. A swirl generator is designed to generate similar flow properties as the Francis turbine at the inlet to the draft tube, yielding a precessing vortex rope in the draft tube.