Investigation On Transient Characteristics Of The Pump System Using MOC-CFD Coupled Method

In the pumping system, like chemical process, life supply water and nuclear system, etc., sudden flow variations in a pipe will be caused by the valve maloperation, by the unexpected start or stop of pumps, by the pool water level mutation, or by other unexpected events. This phenomenon is known as water hammer. In the traditional analysis, when analyzing transient characteristics of the system, the pump is often described by a simple one-dimensional(lD) model, and when analyzing transient characteristics of the pump, a three-dimensional(3D) independent analysis is done for the pump without considering the outer pipe system. Therefore, it is difficult to get the pump transient characteristics in the transient events and the pump transient effects on system running. That is to say, it failed to fully consider the interaction of pump and system. Thus, a coupled method was proposed by combining the method of characteristics(MOC) and computational fluid dynamics(CFD) together. The method can save computing resources and time when simulating the whole system with a single CFD model and avoid imposing approximated boundary conditions to the separate analysis of individual components, and avoid using inaccurate model when analyzing the whole system with MOC calculation, which cannot also provide detailed flow information. The main content of the thesis includes the following aspects:First, this thesis proposed a MOC-CFD numerical method based on 1D/3D multi-scale coupling concept, investigated coupling theory for steady and transient process, and developed the data transfer and control program. Moreover, the strategy to improve the coupling stability and efficiency was explored at the same time.Second, a pipe-valve system was designed and built to verify the accuracy of MOC and CFD modules. In water hammer calculation based on the MOC, the results from the traditional steady friction model and Brunone unsteady friction model were analysed. In addition, the 3D CFD simulation of water hammer event was successfully realised by using the sliding mesh technology and considering the water compressibility, and the main factors affecting the 3D numerical results were researched.Third, a pump-pipe-valve system was designed and built to study the dynamic interaction between water hammer and pump. Three kinds of methods including MOC-CFD coupling simulation, pure 1D MOC calculation and pure 3D CFD simulation were conducted and comparatively analysed. The resuls show that the pure 1D MOC calculation belongs to the quasi-steady state calculation and the transient pump H-Q curve overlaps with the experimental steady-state H-Q curve, which can’t reflect the transient effect.3D CFD and MOC-CFD simulation results are in good agreement with measured data. The comparative analysis validates the feasibility of MOC-CFD coupling simulation and its superiority in the transient analysis of piping system. The results also show the strong coupling effect between water hammer and pump during the transient process.Forth, MOC-CFD coupling was used to study the pump transient characteristics when the discharge valve was rapidly closed. The pump transient external characteristics and 3D internal flow evolutions were obtained in water hammer condition. The effects of valve closure time, valve closure law and pipe length on pump transient characteristic were studied. The analysis and summary provide some advices to reduce the damage of valve-induced water hammer.Fifth, MOC-CFD coupling was used to study the dynamic interaction between water hammer and pump when the discharge valve was rapidly opened, and presented the system and pump transient characteristics. The results show that downstream valve opening causes a negative pressure wave to move from the valve upstream to the pump, leading to lower pressures than a pump would usually provide. The pump head keep fluctuating at a lower value until it reaches a steady state, and the pump flow gradually increases with a slight fluctuation. In the whole process, the transient pump Q-H curve is below the steady Q-H curve, and has a slighter fluctuation when compared with the rapid valve closure condition.Sixth, MOC-CFD coupling was used to study the dynamic interaction between water hammer and pump when the pump suddenly stopped running, and presented the system and pump transient characteristics, and vortex transfer inside the pump flow channel. In the process of pump-stopping water hammer, non-synchronous pressure fluctuations were first shown at the pump inlet/outlets, and the synchronicity pressure fluctuations were presented mainly under the action of downstream transient flow. The pump flow gradually decreased with fluctuation during the whole process.In this thesis, MOC-CFD coupled simulation was proposed and established to study the transient characteristics of pumping system. The coupling code was developed and validated through experiment. Then coupling simulation was used to study the dynamic interacitons between pump and system during transient events. The research contents and conclusions have a guiding significance for the transient analysis of complex pumping system, and provide references for the analysis on the transient characteristics of the nuclear system.