Investigation on the Impact of Air Admission in a Prototype Francis Turbine at Low-Load Operation

Julian Unterluggauer,Anton Maly,Eduard Doujak

doi:10.3390/en12152893

Abstract

Due to significant changes in the energy system, hydraulic turbines are required to operate over a wide power range. In particular, older turbines which are not designed for these environments will suffer under off-design conditions. In order to evaluate whether or not such a turbine could fulfill the new requirements of the energy market, a study about the behavior of a prototype plant in low-load operation is presented. Therefore, prototype site measurements are performed to determine the most damaging operating point by means of acceleration sensors and pressure transducers. Moreover, unsteady computational fluid dynamics (CFD) simulations considering two-phase flow and two hybrid turbulence models are used to analyze the flow conditions inside the turbine. The resulting pressure pulsations are mapped onto the runner blade to obtain stress and further calculate damage factors. Accordingly, the stresses are compared to those obtained by the strain gauge measurement. Moreover, the influence of active flow control by means of air injection on plant behavior and runner lifetime is discussed as well.

Highlights

The energy system is undergoing a truly disruptive transformation
After the plant start-up Machine 1 was operating with stationary load points determined by constant guide vane opening and the power output (P), while Machine 2 was operating at the rated point (RP) to ensure constant conditions and omit influence of the secondary unit on the measurement as much as possible
Both turbulence models produce similar results and predict the signal relatively well based on the pressure oscillation induced by the draft tube vortex

Summary

Introduction

The energy system is undergoing a truly disruptive transformation. The integration of renewable energy sources, driven by the need to address climate change can lead to instability and reliability issues in the electrical grid. Hydro power plants are supplying grid stabilization services of significant importance. To provide those services, hydraulic machines have to operate under off-design conditions and further deal with transient behavior and fast response times [1]. To minimize the cost of operation and prevent expensive failure events [8,9], one has to reduce those vortex-induced-vibrations (VIV), while mitigating pressure pulsations. This can be done by influencing the flow behavior by means of design modifications of the draft tube. There are various approaches of this so called passive method like draft tube fins [10] and runner cone extensions [11]

Objectives

Methods

Results

Conclusion