Research And Modeling On Quasi-two-dimensional Transient Flow In Pipeline

The phenomenon of hydraulic transients could always happen,when valves open and close rapidly or pumps start and stop suddenly in recirculating cooling systems of thermal or nuclear power plants,civilian heating or water supplying systems,and air conditioning water systems in buildings.For these transient flow problems,one-dimensional(1D)numerical method is widely used to simulate the pressure surge.In the 1D method,a quasi-steady friction model is employed to calculate friction dissipation in transient pipe flows approximately.However,it could not accurately calculate the pressure wave attenuation process.Quasi-two-dimensional(quasi-2D)model could accurately simulate pressure fluctuations in transient flows.Because quasi-2D model calculates both two-dimensional(2D)velocity field and 1D pressure field,and it makes use of algebraic turbulence model to calculate friction dissipation in transient flows.Quasi-2D model,however,is not widely used in engineering applications due to the complicated calculation procedures of its existing solution methods.Therefore,we propose a new method,which combines 1D and quasi-2D models to solve the problems simultaneously.It is able to adopt the analysis strategy of 1D model directly and simplify the calculation procedures of the exsiting quasi-2D model solution methods.Moreover,it expands the range of applications and improves computational efficiency.Firstly,both method of characteristics(MOC)and finite volume method are presented.And an improved finite volume method is developed,in which calculation points are arranged with those in MOC correspondingly.The accuracy of the improved finite volume method is as well verified.Based on theoretical analysis,a combined solution method for both 1D and quasi-2D models is proposed.Moreover,both 1D MOC and improved finite volume method are employed to combine with quasi-2D model,respectively.The accuracy and efficiency of the two methods are studied.Results show that the method combining MOC with quasi-2D model has the same accuracy as the existing quasi-2D methods.The accuracy of method combining finite volume method with quasi-2D model is influenced by the value of weighting coefficient.The proposed method has much more efficiency than the existing quasi-2D methods,and it also could be used in 1D staggered grid.Meanwhile,a modified quasi-steady friction model is proposed.Based on experimental data in literature references,numerical results are compared and analyzed,which are calculated using quasi-steady friction model,modified quasi-steady friction model,improved Brunone friction model,Vardy-Brown friction model,and quasi-2D model,respectively.Based on numerical results of quasi-2D model,selection rules of correction factor in the proposed friction model are investigated.Secondly,both discrete vapor cavity model(DVCM)and discrete gas cavity model(DGCM)are combined with quasi-2D model separately are developed.Numerical results are compared with experimental data in literature references,which are acquired by the two models(DVCM and DGCM)combined with quasi-steady friction model,improved Brunone friction model,and quasi-2D model,respectively.Comparisons show that results of both DVCM and DGCM with quasi-2D model are more accurate than those calculated by DVCM and DGCM with 1D friction models,and DVCM and DGCM with quasi-2D model could accurately simulate pressure fluctuation in gas-liquid transient pipe flows.Then,a pump-valve-vessel model combining pump sudden stoppage model with air vessel model is proposed.According to the results of quasi-2D model,the accuracy of the traditional 1D model,which is used to simulate transient flow caused by pump sudden stoppage,is investigated.Results show that the pressure fluctuation simulated by quasi-2D model is smaller than that simulated by the 1D model,and the rate of pressure attenuation is faster.Finially,based on the results of quasi-2D model mentioned above,sequential quadratic programming is employed to optimize parameters of air vessel with three different connecting structures.Results show that optimal volume and connecting structure parameters could be directly confirmed,according to the plotted graph of both the optimal volume and the connecting structure parameters under different maximum allowable pressures.To improve the accuracy and applicability of optimization,the results of the traditional 1D model should be revised based on those of quasi-2D model.In this dissertation,the method combining 1D model with quasi-2D model to calculate transient flow is proposed.And it is used to verify the accuracy of 1D friction model,analyze transient flow in gas-liquid flow,simulate transient flow by pump sudden stoppage,and optimize air vessel parameters.The results in this dissertation expand the application range of quasi-2D model and refine the basic theory of quasi-2D model.It would be a great help to further understand mechanism of transient flow and provide guidance for practical projects.