Research On Flow-induced Vibration Characteristics Of 90-degree Elbow Pipe Based On Bidirectional Fluid-solid Coupling

Considering the safety problem of the fatigue damage of the main steam pipeline caused by the flow-induced vibration of the steam boiler,the main pipe shape of the main steam pipeline-the 90-degree elbow(vertical upward and horizontal upward)is used as the research object to capture the gas-liquid two-phase flow pattern evolution.Based on the dual-threaded analysis of dynamic characteristics and dynamic response,the dynamic change law of pipeline vibration induced by gas-liquid two-phase flow in the elbow is analyzed from the dual angles of time domain and frequency spectrum.With the increase of V_sg and V_sl,the incoming flow section of the pipe decreases in the form of"pressure ring",The internal wall pressure in the elbow falls first and then rises,and the external wall pressure is the opposite.At 15-20 D of the de-flowing section,the pressure inside and outside the pipe tends to be unanimous.The flow pattern evolves differently under different tube shapes and the same working condition,and the flow pattern formation of V_slconvection pattern is more sensitive than that of V_sg.With the increase of V_sg,the vertical elbow shows itself as a bubble loop surrounded by liquid film and circulating liquid phase.with the increase of V_sl,Taylor bubbles gradually become irregularly evolving block bubbles,which flow alternately with the liquid plug.With the increase in V_sg in horizontal bending,the liquid phase from the laminar passes to the wavy flow due to the influence of the gravity and surface tension,forming wave-slug flow.With the increase of V_sl,the slug flow is fully developed after passing through the horizontal elbow due to the interphase instability mechanism.The phase space is reconstructed by C-C algorithm,and the maximum Lyapunov exponents of each flow pattern are positive values,which showed different chaotic characteristics of different strengths.The modal analysis under the three calculation methods shows that the natural frequency of the two-way fluid structure interaction has a stronger decreasing trend,but the mode shape is not affected.The fixed support at both ends makes the natural frequency higher;The order of the influence of single fixed support-single freedom on the natural frequency of the bent pipe is from high to low:XYZ,XZ,XY,YZ,Y,X,Z directions are free,and the increase in wall thickness and inner diameter increases the natural frequency.Based on the dynamic response analysis of the time domain and frequency spectrum,the vibration trend of each monitoring point of the model is almost the same.The center of the elbow has the highest amplitude,the strongest vibration,and the largest vibration in the Y direction.With the increase of V_sg,the gas-liquid carrying capacity of the gas phase is enhanced,the gas-liquid interface fluctuation is enhanced,the amplitude and vibration frequency of displacement and acceleration are increased,the high-frequency components are reduced due to bubble aggregation,the volumetric gas holdup amplitude and dominant frequency are decreased,and the extended range of PSD is decreased by 5 Hz.With the increase of V_sl,displacement amplitude and vibration frequency decrease and increase respectively,while the increase of Vsl leads to the shortening of charging time which increases the acceleration amplitude and vibration frequency,and the PSD extends to 0-30 Hz.The dominant frequency value of vertical bend is slightly higher under the same working condition.The vertical elbow has the peak value at low frequency and high frequency,and the energy at high frequency is low,while the horizontal elbow has the opposite.The root mean square(RMS)of the excitation force of the elbow is strongly correlated with the momentum flux and the root mean square of the pressure fluctuation respectively.