Research On Cavitation Flow Characteristics And Fluidsolid Coupling Of Francis Turbine

The Francis runner with its special hydraulic design and structure enables the unit to adapt to a wide range of heads.For conventional hydropower stations,the unit has a long steady-state operation time and a large working condition span,and the health of the turbine is facing a great test.Cavitation is a common phenomenon in fluid machinery.The initial generation,flow and collapse of cavitation reduce the efficiency of the machine,cause irregular vibration and harmful noise,and even damage the structure.The occurrence and development of hydraulic cavitation rely on the characteristics of the flow pattern,and exploring the flow characteristics of the water flow in the whole flow channel is very important to clarify the cavitation mechanism,formulate a reasonable maintenance strategy and ensure the stable and economic operation of the unit.Because the turbine works in a closed water environment,its internal phenomena are difficult to understand intuitively.CFD technology is based on turbulence theory and cavitation theory,and uses computer rendering and display as the result form.Through continuous improvement of numerical calculation strategies to obtain higher calculation accuracy,the three-dimensional flow field simulation reflects a good performance in the study of cavitation flow problems.Adaptability.This paper uses CFD technology as the main tool to formulate a comprehensive analysis plan and route,and conducts research on the characteristics of the flow field of the turbine and the response of the runner structure in turn.The scope of the research object was determined to include the full flow channel model from the volute to the draft tube.Therefore,the original size geometric model was constructed based on the actual unit manufacturing drawings.The core idea of the volute and draft tube modeling method is the coordination of the positioning sketch and the crosssection profile sketch,and the implementation of global mesh size control for this type of pipe structure is meshed.The double-bladed cascade model is built by plane stretching,and the sweeping grid mode is adopted to stretch the high-quality plane grid into a threedimensional grid accordingly.In particular,the parametric modeling technology is used to configure the runner,and the airfoil structure is controlled by parameters such as the meridian section of the runner and the blade wrap angle and thickness.Furthermore,the single runner area of the runner is intercepted,an O-shaped grid suitable for the airfoil structure is introduced,the structured grid is split on the basis of the complete topology,and then the full cycle model is completed by making full use of the rotation of the runner periodically.Finally,use the Boolean operation to implement the complete machine assembly of the model of the separated components,and complete the construction of the full-flow calculation model.The reliability of the calculation model is verified by gridindependent calculation.The iterative calculation of the control equation is driven by the inlet and outlet pressure as the boundary condition,and continuous internal flow field information is obtained through post-processing steps.The cavitation phenomenon can be observed from the water flow characteristics and the internal cause can be analyzed.This article focuses on the cavitation characteristics of the runner components,and analyzes the turbulence characteristics of the upstream component,the double-bladed cascade,the surface cavitation of the runner,and the vortex characteristics of the downstream component,which is the draft tube.Based on complex operating conditions,the connection between steady-state operating conditions and operating conditions is explored by controlling a single variable of water head and power.After learning the flow characteristics,extract the transient characteristic parameter pressure pulsation data,analyze the correlation between the pressure characteristic quantity and the flow pattern from the time and frequency domain respectively,and establish the relationship between the nature of cavitation and the monitoring quantity.Based on the simulation of cavitation flow,the key parameter values of cavitation on the runner surface under multiple working conditions are taken as samples to analyze the sensitivity of the degree of cavitation to working condition parameters.After completing the analysis of the flow characteristics,the flow field is used as an external excitation to explore the response characteristics of the runner structure under cavitation flow.Implement a one-way fluid-solid coupling steady-state calculation to solve the overall stress distribution of the runner.Based on the results of fluid analysis of complex working conditions,the relationship between stress characteristics and working conditions is discussed.Then refer to the definition of material yield in the fourth theory of strength to determine whether the strength of the research object meets the material requirements under typical working conditions.After clarifying the response characteristics of the structure subjected to external force factors,starting from the nature of the structure,the various vibration frequencies and vibration patterns of the runner are discussed.The runner runs in a closed water environment and is subjected to water pressure.In the modal analysis,the effects of water and stress on the vibration frequency are studied separately.Through dynamic and static interference analysis,the pitch diameter of the runner is calculated,and the resonance area is identified according to the margin of hydraulic frequency and structural vibration frequency.In this study,a complete program was implemented to analyze the causes of cavitation of Francis turbine runners,identify cavitation sensitive factors,explore the effects of cavitation on the structure,and determine the possibility of hydraulic-mechanical resonance.It has a theory for formulating unit maintenance programs to maintain safe operation.significance.