This paper investigates the effects of transient vaporous cavitation caused by the closure of the downstream end-ball valve against the discharge. Numerical results are compared with the results of measurements in the simple reservoir-horizontal steel pipeline-valve apparatus. Pressures measured at the end points and at two equidistant positions along the pipeline are compared with computational results as piezometric heads. Novel numerical transient cavitating model includes effects of either of steady or unsteady skin friction and entrapped gas pockets. Comparisons between the results of two distinct water column separation tests and numerical simulations using an advanced discrete gas cavity model showed good agreement when convolution-based unsteady friction model is used. This is not the case when the steady state friction model is used. Column separation phenomenon is relatively rapid transient event characterized by a number of flow situations (accelerating and decelerating flows). Finally, two distinct column separation runs include active and passive column separation cases.
COBISS.SI-ID: 14435355
This paper deals with transfer of knowledge gained during the research project into the industry. Hydraulic transient control methods in Slovenian refurbished Francis turbine hydropower schemes are presented. Transient control strategies include alteration of operational manoeuvres, transient control devices, suitable water conveyance system layout and operational limits. Computational model and modern hydraulic transient control approaches are outlined as well. The paper concludes with practical implementation of two case studies: refurbishment of Doblar 1 HPP and Moste HPP. Both hydropower plants are equipped with Francis turbine units and have undergone refurbishment in the years 2010 to 2013. Computational results agree well with the in-situ measured results. It has been shown that for relatively slow transients the steady state skin friction model used in simulations is appropriate.
COBISS.SI-ID: 14532379
A large-scale pipeline apparatus at Deltares, Delft, The Netherlands, has been used for acoustic resonance tests. The apparatus consisted of an upstream end constant-head reservoir, a horizontal 200-mm-diameter 49-m-long steel pipeline and an oscillating valve at its downstream end. In addition to standard instrumentation, two distinctive instruments have been used: hot-film wall-shear-stress sensors and a PIV set-up for measurement of velocity profiles. Pulsating flow tests have been performed with an average flow at Reynolds number 22,000. The frequency of oscillation varied between 1.5 Hz and 100 Hz. When one of the excitation frequencies met the liquid system’s natural frequency, the system went into resonance. Moreover, when one of the frequencies coincided with a structural natural frequency of the pipeline, the liquid-filled pipeline experienced fluid-structural resonance. Results of three distinctive runs (including a hydraulic resonance case with oscillating frequency fex = 5 Hz, a non-resonance case with fex = 10 Hz and a fluid-structural resonance case with fex = 12.5 (13) Hz) indicated that the axial pipe-wall vibrations for the fluid-structural resonance case are significantly higher than in the hydraulic resonance and non-resonance cases. In addition, excessive pipeline vibrations were clearly visible in the PIV images. This fact is very important for evaluation of relative fluid velocity and consequently unsteady wall shear stress (unsteady skin friction). Comprehensive coupled experimental treatment of liquid-filled pipeline in hydraulic and fluid-structural resonance conditions is presented in the literature for the first time.
COBISS.SI-ID: 14289179