| Water conservancy project is the national lifeline project,ensuring the safe operation of water pipeline is of great significance to social stability and high quality development of economy.However,the change of fluid flow state in the pipeline may lead to instantaneous pipeline high pressure and water hammer phenomenon when the long-distance transmission pipeline is operated in the process of starting pump,stopping pump and flow adjustment.The huge damage effect of water percussion has a great threat to the safe operation of the pipeline,once it appears,it will cause pipeline rupture,serious will lead to casualties.Therefore,it is important to establish a fast and accurate water strike theoretical model for water strike prediction of complex pipelines for safe and reliable operation of lifting pipelines.Water crash theory has been developed for more than 100 years,but it is still not complete because of the complexity of water crash process and the diversity of boundary conditions.The traditional single-phase model,which does not consider the water shock phase transition process,is widely used because of its simple calculation,but its assumed physical basis is inconsistent with the actual situation,and there are still great uncertainties in the calculation results.However,the two-phase flow model used for water shock prediction has a more solid physical foundation and is an important development direction of the current water shock model.More manpower and time should be invested in its research.In this paper,experiment and numerical calculation are used to verify the reliability of two-phase water impact theory and to apply it to engineering examples.The main research work and conclusions of this paper are as follows:(1)In this paper,a small water percussion test platform is built to analyze and summarize the basic characteristics of two-phase water percussion.The experimental results show that:in the process of valve closing,the water percussion curve of different phases will be superimposed,and the negative pressure water percussion will result in the phase transformation of water,forming a complex and clearly regulated water percussion wave curve.The initial flow rate of 1.4m/s and 0.35s quick closing ball valve will produce 852.4kPa water burst pressure,which will decrease significantly with the extension of closing time;Closing the valve for 1.15s generates 133.40kPa water burst pressure.When the fixed closing time is 0.35s and the initial flow rate is 1.441m/s,899.4kPa water pressure will be generated.The pressure will decrease as the initial flow rate decreases.At the initial flow rate of 0.116m/s,the water percussion pressure decreases to 69.3kPa.The experiment verified the effect of the regulator well on preventing water shock.The maximum water shock pressure of the non-regulator well was 899.4kPa,and that of the regulator well was 69.3kPa,which reduced the pressure by 92.29%.Through the in-depth analysis of the water percussion experimental data,we found a phenomenon that had not been noticed by previous studies,that is,with the shortening of the valve closing time,the water percussion wave velocity does not always decrease,but shows a trend of decreasing first and then increasing,and the reasons for this phenomenon need to be further studied.(2)The reliability of discrete steam cavity(DVCM)and discrete gas cavity(DGCM)is verified by comparison of experiments.The sensitivity analysis of grid and weight parameters was carried out,and the comparative analysis of four groups of working conditions was carried out.For the peak square root error,it was found that the maximum RMSEP value of DVCM was 221.52kPa and the maximum RMSEP value of DGCM was 51.9kPa.For the time error tcav of each pressure peak,the maximum value of DVCM and DGCM was 13.78%and 0.81%respectively.Therefore,the calculation accuracy of DGCM is higher and it is more suitable for two-phase water impact calculation.(3)In order to solve the problem of insufficient attenuation of water impact pressure calculated by DGCM model with pseudo-steady-state friction,the pseudo-steady-state friction in DGCM model was modified,and the additional dynamic friction of Vardy&Brown model based on historical acceleration and Brunone model based on instantaneous acceleration were experimentally verified.Compared with pseudo-steady-state friction,the calculation accuracy of the two additional dynamic friction has been greatly improved in terms of peak error.The minimum root-mean-square error RMSEP of Vardy&Brown is 2.7kPa and the minimum root-mean-square error RMSEP of Brunone is 3.5kPa.Overall,Vardy&Brown is slightly better than Brunone type with additional dynamic friction.At the same time,the experiment verifies that the influence of additional dynamic friction decreases with the increase of initial velocity.(4)After the reliability of the two-phase water shock model is verified by experiments,the water shock protection of the long distance pump and valve system is analyzed by commercial software based on DGCM algorithm,and the effects of different water shock protection measures are compared.The results show that the multifunctional pump control valve in front of the pump can reduce the reverse speed of the pump,but can not reduce the water pressure;Compound air valve and waterproof hit air valve can only reduce the water hit pressure,but can not ensure that the reverse speed of the pump in the safe range;Double valve combined protection performance is very good,can reduce the water pressure can also reduce the reverse speed of the pump;As the last barrier of water blow protection,the reliability of water blow relief valve is very important to the safety of water conservancy system.Through simulation calculation,this paper gives the safety scheme of stopping pumpclosing valve and starting pump-opening valve during maintenance. |
PDF Full Text Request