Water pipelines are designed to operate in pressurized flow mode, but in exceptionally air may be present within the conduits. One example is pipeline refilling operations, which follows certain maintenance procedures that require partial or complete emptying of the conduits.
Considering that ventilation system in such systems is not ideal, it is important that the predictions of such refilling process to consider the effects of air pressurization. A promising modeling approach couples an unsteady flow regime transition (mixed flow) model with a model for the air phase. To date, modeling approaches for air phase in such problems have considered for each time step of computations either uniform or spatially-varying air phase pressures. This work is part of an ongoing investigation that aims to create a model to simulate the filling of water pipelines accounting for air pressurization. The manuscript compares the alternatives of uniform and varying air phase pressures in the simulated refilling of an hypothetical water main. The model presented in this study solves the water flow applying the Saint-Venant equation modified by the Two-component Pressure Approach, and the air flow using the one-dimensional Euler equation. The numerical scheme to solve the water flow is the approximate Riemann solver by proposed by Roe, while the air phase was solved using the FORCE scheme. A simple pipeline refilling problem is used to compare these two different air formulations using various flow rates and ventilation schemes.
Preliminary results indicate that for faster moving air flows and there are significant spatial pressure gradients that should be included to improve model accuracy.