Author(s): Xiangpeng Mu; Zihou Niu; Wenxue Chen; Wei Cui; Zheqi Zhang

Linked Author(s): Wenxue Chen

Keywords: CFD; VOF; CLSVOF; CFD-PBM; Air pocket

Abstract: The national condition of uneven spatial and temporal distribution of water resources in China has promoted the large-scale construction of long-distance water diversion projects. However, during the pipeline filling process, if the exhaust device is improperly installed, the air pocket cannot be smoothly discharged, and the hydraulic transient problems caused by the retained air pocket seriously threaten the project safety. This study constructs a mathematical model for the retained air pocket at the pipeline blind end. Using Computational Fluid Dynamics (CFD) methods, it systematically analyzes the applicability of three gas-liquid two-phase flow models -- VOF (Volume of fluid), CLSVOF (Coupled level set and volume of fluid), and CFD-PBM (Computational fluid dynamics - population balance model) -- and compares the differences among the models in transient pressure capture and air pocket dynamic characteristic simulation. All three models can effectively reproduce the maximum pressure wave peak (with a maximum error ≤1.45 [m]) and the minimum pressure wave trough (with a maximum error ≤0.55 [m]). In terms of air pocket simulation, the CLSVOF model demonstrates better performance in air pocket breakage, coalescence, and interface capture, with significantly improved interface smoothness; the CFD-PBM model can simulate the distribution of small-diameter bubbles. Subsequently, the CLSVOF model is applied to simulate the migration process of the retained air pocket in long-distance pipelines. Results show that there is a significant velocity difference between the retained air pocket and the water flow. Under the driving forces such as the drag force of the water flow, the air pocket migrates downstream in a slug flow pattern. Finally, the rationality of the simulation results is verified through the flow pattern diagrams of Scott and Mandhane, confirming the applicability of the CLSVOF model in simulating the dynamic evolution of air pockets in long-distance water transmission pipelines. The findings of this study provide a theoretical basis for optimizing exhaust devices and preventing/ controlling water hammer risks in engineering projects.

DOI: https://doi.org/10.64697/978-90-835589-7-4_41WC-P1664-cd

Year: 2025