Analysis Of The Flow Field And Fluctuating Pressure Characteristics Of The Sudden Expansion Section

As a type of variable cross-section pipeline,sudden expansion is widely used in fluid,gas and various forms of gas-liquid,gas-solid mixture and other practical transportation projects.The flow field analysis of the sudden expansion pipeline has a very high reference value for the design of the actual project.In this paper,the mechanism of flow field change in the pipe with sudden expansion section is revealed by studying the characteristics of flow velocity,pressure distribution and fluctuating pressure spectrum.SolidWorks is used to build a real expansion pipe with a sudden expansion ratio of1:1.5,which is imported into Fluent software for simulation analysis,set the initial flow parameters,boundary conditions and control equations.It is found that the velocity in the upstream of the sudden expansion pipe flow is significantly higher than that in the downstream.There are two velocity dead zones in the sudden expansion section,and the velocity value of the velocity dead zone is the smallest.The range of velocity dead zone is directly proportional to the Reynolds number,and the appearance of velocity dead zone is related to the back flow phenomenon at the sudden expansion section.The pressure distribution of the pipe flow with sudden expansion section is related to the flow state of the fluid.When the fluid is in laminar flow state,there will be no pressure dead zone,but in turbulent flow state,the pressure of the pipe flow with sudden expansion section starts from the upstream,decreases first and then increases,and reaches the lowest value at the sudden expansion section.A small-scale pipe flow system with sudden change of flow section is built.Two flow regulating methods,pump pumping and water tank level control,are used to control the flow velocity,and the rectifier box is used to stabilize the upstream flow.Transparent direct connection is adopted for the measurement of the sudden expansion pipe section,and the pulsating pressure characteristics of the fluid are measured under the three conditions of Re=2000,3000,5000.The characteristics of the fluctuating pressure along the flow path,time and frequency domain are analyzed.The results show that the maximum pressure build-up length downstream of the sudden expansion is directly proportional to the Reynolds number by observing the characteristics of the flow distribution along the tube.And at10 D,it has fully developed and is no longer affected by sudden expansion.Before the fully developed flow,the fluctuating pressure at the same point is directly proportional to the Reynolds number,but reaches the peak at 2D.At the same Reynolds number,the pressure fluctuation of fluid has obvious periodicity.And the pressure fluctuation energy band of the fluid first narrows,then widens and then narrows.At 4D point,the fluid pressure fluctuation energy band reaches the widest.The main frequency of the fluctuating pressure is constant at 50 Hz,Sudden expansion will not affect the main frequency of the fluctuating pressure,but only the amplitude of the fluctuating pressure.The fluctuating pressure energy band at the sudden expansion section is the largest.The fluctuating pressure energy band in the downstream of sudden expansion is lower than that in the upstream,it can be seen that the sudden expansion will narrow the frequency band of the pressure fluctuation energy of the fluid in the circular tube.