| Driven by the goal of "carbon emissions peak and carbon neutrality ",the new power system needs more clean energy such as wind and photovoltaic power.Due to the poor regulation stability of wind and solar power station,the pumped-storage power can match with them and generally undertake functions such as peak load regulation,valley load regulation,frequency modulation,phase modulation,store energy,spinning reserve,and black start in the power grid,and is called the "regulator" and "stabilizer" of new power systems.Because of the special tasks undertaken by pumped-storage power,various off-design operations always result in excessive pressure pulsation,while fast and frequent transitions of operating modes often lead to the imbalance hydraulic thrust.The prominent dynamic instability problems affect the safety,stability,efficiency,flexible operation of the unit.The clearance flow channel(CFC)of pump-turbine is a thin cavity with several stages composed of runner and head cover or bottom ring.Due to the small size and complex shape of CFC,most studies on the hydraulic forces and pressure fluctuations of the runner ignores the influence of CFC,and only takes the main flow channel(MFC)into account,which is difficult to fully reveal the force imbalance of runner and the vibration mechanism of the unit.Therefore,the clearance shear flow pattern,pressure distribution,pressure fluctuation characteristics and their influence mechanisms on clearance hydraulic forces are a scientific issue that need to be further studied.In this paper,numerical simulation was used as the main research method,while theoretical analysis and model test were the auxiliary methods.Using the technical route of hydraulic force in MFC combined with that in CFC,and water hammer pressure combined with pulsating pressure,to carry out the research on velocity distribution,pressure prediction,hydraulic forces calculation and pressure pulsation of CFC.Then the influence mechanism of clearance hydraulic forces on runner imbalance,as well as the hydraulic excitation mechanism caused by pressure fluctuations,were revealed.The study was divided into the following five aspects:1.Reveal the influence degree and mechanism of clearance hydraulic forces on runner during the transition processIt is found that the axial hydraulic forces in CFC and MFC show a similar significance level during the transition process for the low specific-speed pump-turbine,both are the main hydraulic factors leading to the runner unbalance.The axial hydraulic forces in CFC can be formulated as a function of the clearance inlet pressure and rotational speed,dominated by the former;while the MFC component can be expressed as a function of the momentum changing rate and the runner outlet pressure force,dominated by the former as well.The axial hydraulic force in CFC is getting the upward maximum before approaching the runaway point,resulting in the upward maximum total axial force;When the working point transits from turbine mode to turbine braking mode,the axial force in CFC reaches the downward maximum,resulting in the downward maximum total axial hydraulic thrust.This conclusion clarifies the influence extent and mechanism of clearance axial hydraulic thrust on the runner,providing a basis for the future study of axial hydraulic thrust imbalance.2.Reveal the clearance velocity and pressure distribution in CFC that dominated by strong rotational shear flowDue to the small size and complicated structure of CFC,model test is constrained by geometric scale,while prototype observation is difficult to perform.When taking the clearance flow as a part of the CFD model,the calculation cost of multi-scale grid coupling between the large grid in MFC and the small grid in CFC is extremely high,and the results are not necessarily completely accurate.In this paper,clearance flow patterns and pressure distribution of the pump-turbine were studied using a rotating–stationary disk model,which is a simplified model of the clearance flow channel of a pump-turbine.The results show that the circumferential velocity in the clearance region presents a distribution of “core region + double boundary layers”,and the water body in the core region rotates like a rigid body at a speed of0.41 ～ 0.42 times runner rotating speed.The circumferential velocity in the core region is dominated by the rotating disk speed,while the rotating coefficient k is unaffected by the disk speed,the clearance inlet pressure,and the clearance height if the location is far from the boundary effect.The clearance pressure decreases in a quadratic distribution along the radial direction,and linearly increases with the increase of the clearance inlet pressure,while decreases in a quadratic pattern with the increase of the runner rotational speed.The formulas of pressure drop caused by rotating shear flow and the axial hydraulic thrust of CFC external cavity are corrected by the k value.This conclusion provides a basis for quantitative calculation of clearance axial hydraulic thrust.3.Derive the calculation formula of the axial hydraulic thrust of CFC,and reveal the unbalance mechanism of the axial hydraulic thrust of the runnerCurrent axial hydraulic thrust calculation of pump-turbine ignores CFC,while that in MFC depends on the empirical formula based on the water head,and the calculation result has a large error.In this paper,the sources of the hydraulic loss in CFC are divided into three types: the strong rotating shear flow,the jet flow and the friction loss.By quantifying these losses,the formulas of clearance leakage and axial hydraulic thrust of CFC are derived.The leakage flowrate can be expressed as a function of the head difference across CFC,the size of clearance channel and the runner speed;The clearance axial hydraulic thrust can be expressed as a function of clearance inlet pressure,runner rotating speed and clearance leakage flowrate.After the offset of axial hydraulic thrust in the upper and lower clearance flow channel,the total clearance axial hydraulic thrust is dominated by the runner speed,and its drastic change will easily lead to the imbalance of the axial hydraulic thrust.The conclusion provides a reference for revealing the mechanism of axial force unbalance of the unit during the transition process.4.Reveal the pressure fluctuation source and its propagation laws in CFC,and propose the corresponding measures to suppress the fluctuationAt present,the source and propagation laws of pressure pulsation in CFC of pump-turbine are still very lacking.However,because the upper clearance passage is in direct contact with the head cover,the pressure fluctuation in CFC is easy to cause the head cover excitation.It is found that the pressure fluctuation in CFC mainly derives from two aspects: one is the pressure pulsation from MFC;The second is the pulsation generated by self-excitation in the CFC,which mainly comes from the imbalanced rotating vortex in the external cavity and the discontinuous vortex structure in the internal cavity.The first measure to suppress the pressure pulsation in CFC is to reduce the clearance inlet size to increase the fluctuation sudden amplitude;the second is to increase the seal size of the last sealing ring to suppress the discontinuous vortex structure in the internal cavity.The conclusions can be used as a reference for the study of head cove vibration caused by hydraulic pulsation.5.Reveal the abnormal vibration mechanism of the head cover under high-head pumping condition,and put forward the measures to eliminate the vibrationIn recent years,flow-induced vibration of head cover with a frequency of 45-65 Hz happened under high head pumping condition.The results show that the hydraulic excitation comes from three aspects:(1)unbalanced pressure wave in the spiral casing(indirect phase resonance);(2)detachment vortex in the vaneless space;(3)discontinuous vortex structure in the internal cavity of the CFC.The measures to eliminate the above three kinds of pressure pulsation excitation are as follows:(1)punching the spiring casing nose to eliminate the unbalanced pressure wave;(2)add horizontal blades on the guide vanes to eliminate the flow separation vortex in the vaneless space;(3)increase the sealing size of last seal ring to suppress the discontinuous vortex structure in the internal cavity of CFC.The measures adopted above can suppress the hydraulic excitation source of the abnormal vibration of the head cover,which provides great reference significance for other power stations with similar problem.This paper analyzed the hydraulic forces and pressure fluctuation laws of the pump-turbine considering CFC,revealed the influence mechanism and degree of clearance hydraulic thrust on the runner force.By clarifying the main influencing factors,a calculation formula for the axial hydraulic thrust of full flow channel was established.Revealing the unbalance mechanism of runner axial hydraulic thrust,which provided guidance for the mechanism of runner force unbalance during transient process.By revealing the source,generation mechanism,and propagation law of pressure pulsation in CFC,the problem of 45～65Hz hydraulic excitation of the head cover was solved.The conclusions can provide reference for similar vibration problems in other power plants. |
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