Dissolved-gas influence on the Francis part-load oscillation

• Published in: IOP conference series. Earth and environmental science Vol. 405; no. 1; pp. 12015 - 12020
• Main Author: Dörfler, P K
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 01.12.2019
• Subjects: Cavitation; Compressibility; Computational fluid dynamics; Draft tubes; Forced vibration; Mathematical models; Model testing; Pressure; Resonance; Resonant frequencies; Scale models; Turbines; Vapor pressure; Vapors; Vortices
• ISSN: 1755-1307; 1755-1315
• DOI: 10.1088/1755-1315/405/1/012015

A forced oscillation of system pressure occurs when a Francis turbine operates at partial load, typically between 50-85% of best-efficiency flow. Its cause is the precession of the draft tube vortex; due to cavitation of the vortex, the oscillation may enter in resonance. Predicting such a resonance from reduced-scale model tests is a long-standing goal of hydraulic research. Key parameter controlling the natural frequency of the underlying eigenmode is the compressibility of the cavitation zone. With the vortex cavity at vapor pressure, the natural frequency would scale by the same factor as the runner speed, thus enabling simple resonance prediction based on the model test. Based on experimental evidence, the actual pressure inside the vortex cavity is higher than vapor pressure, and the non-condensable gas content has significant influence. Due to uncontrolled variation of cavity pressure, the natural frequency in model tests may deviate by as much as 30% under nominally identical conditions. A 1D mathematical model can represent the influence of non-condensable gas on the cavity volume and compliance; even quite moderate cavity pressures explain the observed discrepancies.