Investigation On The Characteristics Of The Transient Flow Using CFD

The water hammer phenomenon in the pipeline system is the main cause of pipeline failure.The traditional analysis method often performs one-dimensional calculation based on the characteristic line method,ignoring the influence of the three-dimensional characteristics of the fluid.In addition,for complex fluid transport system,the influence of capacitive structure and other components on the transient flow characteristics of pipeline is not very clear.It is necessary to adopt a more reliable numerical model to study the influence of capacitive structure on the transmission characteristics of water hammer wave in pipeline system.Therefore,three-dimensional computational fluid dynamics(CFD)is used to study the phenomenon of water hammer closing valve in pipeline system.Considering the compressibility of fluid in real time,dynamic grid technology is used to simulate the movement boundary of the valve,and an accurate numerical calculation model of the pipeline system considering the compressibility of medium is established.The influence of the capacitive components of the system on the transient characteristics of the pipeline system is studied,revealing the transmission and attenuation of the pressure pulsation of the pipeline system by the capacitive structure.The research results have important guiding significance for further improving the accuracy of one-dimensional transient flow calculation model,water hammer prevention and pipeline design.The main contents of the paper are as follows:1)Straight pipeline and branch pipeline test systems were designed and built to carry out research on the applicability of CFD in analyzing pipeline transient flow.Comparing the straight pipeline test with the simulation,the results show that the numerical simulation results of the straight pipeline agree well with the experimental data.Comparing the branch pipeline test with the simulation,the results show that the test fluctuation period is slightly larger than the simulation period,the error is 0.4%,and the peak-to-valley error is small.The simulation results are in good agreement with the experimental results.The feasibility and reliability of three-dimensional CFD method for numerical calculation of transient characteristics of different pipelines are verified.2)The transient characteristic test bench of the capacitive structure system was designedand built,and the influence of the capacitive structure on the transient transfer characteristics of the pipeline was studied.The transient test of the capacitive structure with different dimensional parameters was carried out.The effects of the axial and radial dimensions of the capacitive structure on the transient characteristics of the system were studied.3)Based on the capacitive structure test model,the three-dimensional CFD method is used to simulate the transient characteristics of the capacitive structure system.Compared with the test,the results show that the numerical calculation results and the test results have less error,indicating that the reliability of the three-dimensional CFD method in calculating the transient characteristics of capacitive structural systems is verified.In addition,the three-dimensional transient flow evolution process of the capacitive structure is also given.4)The three-dimensional CFD analysis is carried out for the geometrical changes of the capacitive structure.The water hammer wave pressure variation curve,the speed variation curve,the power density spectrum and the peak-to-valley curve change under the transient state of the fast closing valve are compared and analyzed.The results show that the increase of the axial dimension will cause the cycle of the water hammer wave to become larger,delaying the attenuation of the water hammer wave,and the fluctuation is more durable.An increase in the radial dimension(the radial dimension is greater than the pipe diameter)also causes the cycle of the water hammer wave to become larger.The reduction in the radial dimension(the radial dimension is smaller than the pipe diameter)will slightly cause the cycle to become larger without affecting the attenuation of the water hammer wave.The increase in the volume of the capacitive structure leads to a larger fluctuation period and a longer fluctuation time.