Draft Tube Flow Instabilities Of Hydro-Turbine Units Under Off-Design Conditions

In this modern era of extensive industrialization and population growth,the energy is needed more than ever.Among renewable energy resources;the hydropower has been attractive due to its outreaching advantages of storage reliability,environment friendliness,high efficiency,flexibility adjustment,predictability and sustainability.Therefore;many efforts have been made to increase the production efficiency and reduce flow instabilities.Those flow instabilities are continuous hydraulic circuit excitations developed by dynamic interactions within the fluid itself or with the structure.Beside the best efficiency point with a stable axial flow of the draft tube,off-load operations initiate a swirling flow with the severe vortex breakdown.Resultant low-frequency precession of the vortex rope introduces a high flow imbalance that influences local and remote system dynamics.The present thesis reports attempts done to identify those two influences of the draft tube flow in a Francis turbine unit.Particularly,the reference has been the draft tube outlying domain that conveys a massive flow.The effort was to approximate the transmission rate of instabilities from component to component.As part-load operations have more unstable regimes,two part-load operating points have been selected and other two recommended operating points near the best efficiency point have been chosen for comparison.A reduced-scale model of low-head Francis turbine has been used to learn flow dynamics and underlying development mechanisms.The introductory part reviews most recent insights into low-frequency excitations of hydropower and attempted palliative remedies.The second part briefly describes the applied investigation methodology and explains the reasons behind its choice for the present study.Numerical simulations have been the main approach because the zone of interest extends farther inside the flow,which may require experiments of intrusive probes with additional instability of their own.However,wall pressure measurements have been used to validate the authenticity of numerical results and a good agreement has been obtained.Chapter 3 provides a brief description of the draft tube flow causing a widespread instability.That draft tube flow is the central core activity that initiates the locally dominant instability of vortex breakdown and inlet flow conditions that display upstream-forwarded instabilities delaying the pressure recovery.Chapter 4 learns local draft tube instabilities in the outlying zone,as a part that transports the massive flow.The purpose was to learn wall-bounded instabilities and associate them with their origin.This information is useful in evaluating the depth versus performance of wall-mounted countermeasures.Two part-load operating points with different polarity in pressure pulsations have been investigated.The outlying domain of the draft tube cone has been found to be composed of three distinct zones of influence:(i)the upper cone dominated by upstream travelling influences,(ii)the middle cone dominated by the core excitation and,(iii)the downmost cone dominated by the backward influence from water collision on the elbow curvature.The inward vortex rope excitation depends on the operating point,which recommends the use wall-countermeasures with adjustable feed motion.The fifth chapter is dedicated to the flow of instabilities between components.Those mutual contaminations can be used to understand new or already known flow phenomena.The reciprocity in flow instabilities between the outlying draft tube and upstream components has been assessed.By means of averaged parameters of the runner flow,the characteristic distribution curve of traveling influences at the draft tube intake has been revealed for the first time.Moreover,the approaches to identify harmful perturbations from harmless instabilities and to determine the instability dampening rate for every unit components have been suggested.Comparing with high-frequency perturbations,it has been found that lowfrequency instabilities can override strong flow gradients to propagate farther in the hydraulic circuit.The reason behind that sustainability may be the origin of lowfrequency instabilities,which is the fluid itself,i.e.the vortex rope precession whereas dominant instabilities of high-frequency are originated from components interactions.Beside the instability component of low-frequency,its harmonic amplified by the inlet cone wall impact is less sensitive to the amplitude damping in backward direction.The blade passing frequency(BPF)dominates the inlet.Its high frequency instability collides with the wall and causes high flow perturbations that reduce the local pressure recovery.However,that traveling BPF is rapidly diffused and tapped by the dominant low-frequency core instability.Important future efforts call for performance assessment of wall-mounted countermeasures in terms of their sources so as to replace them at proper locations.Moreover,those palliative remedies of the draft tube pressure surge should be evaluated in terms of dampened traveling instability in upstream components.Further works should use traveling influences to understand some dynamic behaviors of the flow.It is thus recommended to extend the study over a wider operation range.