| The trash-rack is an indispensable and important facility in the water inlet of hydraulic structures such as hydro-power station,pumping station and ship lock,and it is used to intercept the floating objects and suspended objects carried in the flow and protect the generator set,gate,pipeline and other related facilities from damage.The operation conditions of the trash-rack are extremely complicated.For example,trash rack need to withstand two-way water in pumped storage power station,and the problems of high flow velocity,uneven flow velocity distribution and large impact angle of trash-rack surface caused by the frequent change of flow direction under different operating conditions increase the flow induced vibration damage.If the trash-rack is damaged,it will not only be difficult to maintain,but also affect the efficiency of power station operation.More seriously,once it loosens and is sucked into the turbine,it will cause the turbine to be destroyed,and the power station will be shut down for maintenance,resulting in huge economic losses.Therefore,the scientific design,reasonable assembly,and safe use of trash-rack are of great significance to the safe and stable operation of power stations,and to improve the economic and social benefits of power stations.In the past,most of the researches on trash-rack were from the perspective of optimizing the structure and improving the bearing capacity.However,there have been few studies on the vibration damage of trash-rack caused by turbulence.The flow-induced vibration problem of the trash-rack is a typical fluid-structure interaction problem.For the simulation of this problem,due to the characteristics of water flow conditions,boundary conditions and dynamic characteristics of the trash-rack itself,its numerical modeling and accurate simulation have always been a major difficulty in engineering research.In this paper,based on previous research experience,starting from the important influence factor of the cross-sectional shape of the grid-bar,combined numerical simulation and theoretical analysis to study the stress characteristics,pressure distribution,flow velocity distribution and vortex off frequency of five kinds of section grids of circle,square,long rectangle,leading-edge semicircular rectangle and two-edge semicircular rectangles.The research content and conclusions are as follows:?1?Summarize the causes of trash rack damage,and review the research status of flow-induced vibration damage for trash-rack from three aspects:vibration generation mechanism,vibration damage characteristics and vibration research methods.?2?In order to accurately simulate the flow field structure around the grid bars and accurately capture the vortex shedding around the grid-bars,this paper selects the k-?SST turbulence model for simulation.The discrete format of the model adopts the first-order implicit format in time,the spatial convection term adopts the third-order QUICK format,and the diffusion term adopts the second-order CDS format.The PISO algorithm is used for pressure velocity decoupling,finally,the calculation domain is divided using a hybrid grid.Aiming at two typical working conditions in the operation of trash rack project,the inlet velocity is 1m/s?Re=2×104?and 3m/s?Re=2×104?,a numerical model is established.?3?The velocity field distribution results of grid bars with different sections under different velocity are analyzed,and the velocity field characteristics of each grid-bar are compared and discussed from the three aspects of axial velocity distribution,reflux zone shape and cross-sectional velocity distribution.The results show that the flow field velocity distribution is the most uniform after the grid-bar with two edges semi-circular rectangular section.The type of trash-rack with this section can reduce or avoid the generation of suction vortex at the water inlet,protect the generator set,effectively avoid local scouring and other adverse factors at the water outlet,and reduce the related hazards caused by the bad flow pattern after the trash-rack.?4?Starting from the two aspects of flow field pressure and wall pressure,the pressure field distribution results of grid bars with different cross-sections under different Reynolds numbers are analyzed.This paper calculates the total force of the grid bars'wall with the maximum value of the drag coefficient Cd,and simplify the structure and stress of the grid bars,calculate the maximum deflection?max of the grid bars,finally check the bending capacity of the grid bars.The results show that the grid bars with circular section and square section do not meet the stiffness conditions required by the project.The remaining three grid bars with long rectangular section,leading-edge semicircular rectangle,and two-edge semicircular rectangle meet the stiffness conditions and have greater affluence,that is,their stiffness conditions can withstand higher velocity water flow Cross fence.?5?This paper monitors the instantaneous velocity value of the sampling point near the grid bars,calculates the vortex shedding frequency of the flow field in different parts from the grid bars,and compares it with the natural frequency of the grid bars in the water to calculate the safety threshold for the resonance of grid bars.The results show that the grid bars with two-edge semi-circular rectangular section has the largest safety threshold for avoiding resonance.As the flow rate increases,the grid bars with this section are the least prone to resonance.The results obtained in this paper not only have important guiding value for engineering practice such as trash-rack's design,rational selection of operating conditions,and measures to avoid flow-induced vibration damage to the trash-rack,but also can provide reference for the study of similar fluid-structure interaction problems. |
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