Study On Flow Characteristics And Waveform Construction Method Of Water Hammer With Column Separation In Pipeline System

To elucidate the internal flow mechanism of water hammer with column separation during the development of cavity,this paper uses computational fluid dynamics(CFD)to numerically calculate the water hammer with column separation of the downstream shut-off valve in three dimensions,in-depth analysis of its internal flow characteristics and reveal its pressure wave transmission law.A one-dimensional two-fluid model based on the inter-phase adaptive relaxation calculation method is used to construct an analytical scheme for the water hammer with column separation waveform.A selfprogramming method is adopted to obtain water hammer pressure sample data by onedimensional numerical calculation,and a prediction method for obtaining water hammer with column separation waveform curves and ultimate water hammer pressure surge values by fitting.The study results are important guidance for further analysis of water hammers with column separation and system safety calibration.The main work and results of this paper are as follows:(1)The flow characteristics of the water hammer with column separation transient process are analyzed using a three-dimensional numerical simulation method.The analysis of the internal flow field at several vital moments reveals a local velocity surge near the cavity.In the middle of the cavity development,the fluid velocity increases sharply,and the flow direction is toward the vacuole;in the early and late cavity development,the rate increases slightly,and the flow direction is away from the cavity.After spatially passing through the cavity bubble area,the flow direction is restored to the axial direction of the pipe,the flow state is almost symmetrical in the radial direction,and the backflow phenomenon is prevalent near the pipe wall in the downstream section.Based on the pressure distribution characteristics at each moment,combined with the pipeline’s pressure distribution curve,revealed the water hammer’s flow mechanism with column separation secondary pressure rise phenomenon is caused by the superposition of two levels of positive pressure wave transmission.(2)Numerical calculations based on a two-fluid model of gas-liquid two-phase flow and inter-phase relaxation coefficients and mass transfer model to obtain water hammer waveform data samples.Regression calculation correction of pressure values,cavity duration,a moment of secondary pressure rise,and boundary conditions for the occurrence of water hammer with column separation by single and superposition fitting,an accurate water hammer pressure waveform prediction scheme was obtained,where the prediction error of pressure values was mostly below 7%.The water hammer pressure under different hydraulic head conditions is calculated for boundary flow conditions.The maximum value of water hammer pressure under each boundary flow velocity is obtained statistically.The prediction rules of the limit water hammer pressure values and the corresponding boundary water head at a given boundary flow velocity are obtained by various fitting methods.The water hammer pressure safety threshold envelope was drawn to provide a theoretical reference for safety assessment.(3)A 180 m long,40.8 mm diameter water hammer of valve closed in downstream test system was designed and built to verify the water hammer with column separation waveform prediction scheme.By conducting a water hammer with column separation test,the test results were analyzed and compared with the waveform prediction results in this paper;for the prediction of water hammer pressure,the maximum error does not exceed 7%.This experiment verifies the accuracy and applicability of the waveform prediction model for water hammer with column separation prediction.