Simulation study on migration characteristics of trapped air mass in long-distance pressurized water conveyance systems

Abstract

[Objective] The water-air coupling and its simulation method have long been recognized as a scientific challenge in the hydraulics of long-distance pressurized water conveyance pipelines. Comparing the performance of different water-air two-phase flow models in simulating the water-air coupling is essential for investigating the evolution and migration process of trapped air mass in long-distance pressurized water pipelines. [Methods] Three water-air two-phase flow models, VOF, CLSVOF, and CFD-PBM, were used to simulate transient flow scenarios of water flow impacting trapped air mass at pipeline dead end. A comparative analysis was conducted based on the transient pressure variations and morphological variations of the air mass. [Results] The result showed that the error range for the maximum pressure at the first peak simulated by the three models was 0.78 m to 1.45 m, and the error range for the minimum pressure at the first trough was-0.55 m to-0.03 m, both within acceptable error ranges in transient pressure simulation. The CLSVOF model outperformed both VOF and CFD-PBM models in simulating the fragmentation and coalescence of air mass, making it more suitable for analyzing the migration of trapped air mass in long-distance pipelines. [Conclusion] The result indicate that the CLSVOF model successfully simulates the migration process of trapped air mass under the impact of water flow in long-distance pipelines. A significant velocity difference between the trapped air mass and the water flow is observed. Under the drag force of the water flow, the air mass migrates downstream in the form of slug flow. The correctness of the simulation result is verified through flow pattern diagrams, demonstrating the applicability of the CLSVOF model in simulating the migration and evolution of trapped air mass in long-distance water conveyance pipelines.

Keywords

transient flow / CFD / numerical simulation / pressurized water conveyance pipeline / trapped air mass / flow pattern