Study On Dynamic Characteristics Of The Last Stage Blade Of Steam Turbine Under Low Volume Flow Conditions

Due to the rapid development of renewable energy power generation in China,new energy consumption and grid connection become difficult.To further absorb renewable energy such as wind and solar energy and reduce the abandoning wind and solar energy,thermal power units will undertake the task of deep peak shaving for a long time.When the thermal power units work under low load conditions,the low volume flow conditions will appear in the last stage of the steam turbine.Low volume flow conditions mean that the volume of steam is less than the geometric volume of the flow passage,steam cannot fill the passage and forms vortex system in the passage.The vortex system causes complicated flow field and leads to higher stress on the blade,which is prone to fracture accidents.Safe operation of the last stage of steam turbine under low volume flow conditions is one of the key problems of depth peak shaving of steam turbine,and it is of great significance to study its dynamic characteristics.Firstly,considering the steam condensation flow in the last stage of steam turbine,the Eulerian-Eulerian method is used to simulate the three-dimensional condensation flow in the last two stages of steam turbine.The results show that there is a vortex system in the flow field,which is composed of back flow vortex,separation vortex and tip clearance vortex.With the decrease of volume flow,the separation vortex causing by negative attack angle forms firstly at the 40% blade height rather than at the tip.Then,the adverse pressure leads to the back flow vortex.Finally,the tip clearance vortex appears at the upper span between the rotor blade and the stator blade and the temperature of tip clearance vortex is the highest in the flow field.Below 10% THA conditions,the characteristics of steam expansion are divided into expansion-compression,compressioncompression and compression-expansion along the blade height in stator and rotor blade respectively,which leads to the degree of reaction unable to reflect the expansion characteristics.By analyzing the velocity triangles,the velocity of tip clearance vortex is higher than the tangential velocity of rotor blade at 5% THA condition.According to the calculation of the flow field,the exhaust temperature under low volume flow conditions is obtained.The numerical simulation of water spray in the exhaust passage is carried out under the conditions of over temperature of exhaust steam,and the effect of water spray on the last stage is studied.The Euler-Lagrange particle tracking method is used to study the heat and mass transfer of water spray in the exhaust passage.The temperature difference between steam and water in the exhaust passage is small,and the steam velocity is high,which results in the minimum evaporation of water.The water spray quantity and the inlet steam mass flow rate under different conditions have a negative correlation.Water spray has a certain effect on the flow field of the last stage.The pressure coefficient of the rotor blade increases because of the water spray and the maximum temperature of rotor blade reduce about 3-9?.The water spray has little effect on the tip clearance vortex and the separation vortex,but it increases the height and velocity of the back flow vortex.Meanwhile,the pressure ratio of the rotor blade increases,which results in the dissipating shaft power by the rotor blade reduces.Secondly,the strength performance of the last stage rotor blade was studied by fluid structure interaction method with cyclic symmetry analysis.The maximum deformation of the rotor blade is on the leading edge of the blade at 80% relative blade height(80%span).With the decrease of inlet steam flow,the maximum deformation and the maximum equivalent stress increase.Under20%-5%THA conditions,the maximum deformation decreases 0.47%-7.08%,the maximum equivalent stress on the suction surface decreases 1.77%-2.94%,and the maximum equivalent stress on the pressure surface decreases 1.13%-2.65%.Taking the strength performance as the prestress,the natural frequencies and modes of the last stage rotor blade are calculated.The first six modes of the blade and the 0-3 pitch diameter of the disk are analyzed.In addition,the natural frequency of the blade is increased by water spray.Ultimately,the last stage blade works with unsteady flow force and local high temperature due to the existence of vortex system.The three-dimensional transient flow field,elastic-plastic strain field and stress field of the last stage blade of steam turbine are calculated by two-way fluid structure interaction method,and the fatigue life of the last stage blade under different volume flow conditions is estimated.The self-excited unsteady frequency of tip clearance vortex is obtained at 90% span.According to the maximum equivalent stress distribution on the rotor blade surface,three dangerous points of the blade are identified,which are located at the blade tip on the pressure surface(DP1),90% span on the suction surface(DP2)and the trailing edge of the blade root on the pressure surface(DP3).With the decrease of volume flow,the equivalent stress and strain range of dangerous points increase,and the strain-life of dangerous points decreases.Under10%-5%THA conditions,the strain-life of DP2 is the shortest.The frequency of strain response of DP2 is consistent with the self-excited unsteady frequency of the tip clearance vortex,which proves that it is necessary to consider the unsteady steam flow force in strain-life calculation.This paper provides theoretical guidance for the safe operation of the last stage blade under low volume flow conditions.