When pipelines are rapidly filled, dynamic and sometimes dramatic air–water interactions often occur. These air–water interactions, along with their associated pressure and temperature oscillations, are explored in this paper both experimentally and numerically. A computational fluid dynamics (CFD) approach based on a volume of fluid (VOF) formulation is used to simulate the flow field in three-dimensions (3D). Thermal conduction and convection in three different media (air, liquid water, and the pipe wall) are all considered in order to account for the key thermal processes potentially influencing an entrapped air pocket during rapid filling. The simulation accounts for the compressibility of water and the roughness of the pipe wall, considerations sometimes neglected in previous studies. Simulated pressures and air–water profiles are compared to measured data and to the dynamic images obtained through a high-speed camera. The relatively good agreement between the numerical and experimental results confirms that the proposed model can accurately simulate transient flow and also reasonably represents the associated physical processes. Significantly, the observed phenomena of white mist and of a hot pipe wall are explained through the physics represented in the 3D simulations. Indeed, the model shows that extremely intense air–water interactions appear sufficient to account for the observed efficient heat exchange and the dramatic rates of energy dissipation. Overall, the proposed model provides insight into the physical mechanisms of rapid filling and particularly those thermal effects associated with entrapped air.

The phenomenon of white mist in a rapidly filling pipeline containing an entrapped air pocket is numerically and experimentally investigated. The air-water flow patterns, pressure, and temperature histories are synchronously recorded to illustrate their interrelations. The white mist phenomenon is particularly observed during fast transients, especially during the first compression of the air pocket. Comparisons between calculations and experiments indicate that the white mist primarily reflects a condensation process. More specifically, the air temperature increases because of rapid compression of an entrapped air pocket, and the high temperature could cause water to adhere to vapor at the pipe surface. For fast transients, the first compression causes a near-adiabatic air compression, but heat exchange effects become more significant in the subsequent compression and expansion cycles. As the initial air length decreases, the maximum pressure first increases and then declines, with the most dangerous air length occurring when about 3.4% is initially occupied by air. The ratio of the maximum pressure to the driving pressure increases approximately linearly with respect to the upstream pressure. A local-interpolation elastic-water model is developed by considering air-temperature change and its validity is confirmed by comparing the model and experimental results.

Transient flow caused by air expulsion is investigated. Earlier experimental studies involved the horizontal or horizontal–vertical pipe cases, or the vertical pipe case with relatively less test range. This paper focuses on the vertical pipe case with much broader ranges of orifice size and air length to more completely characterize the transient response. Observations show air release undergoes two distinct stages: stage 1 with pressurization, expansion and release of air pocket; stage 2 with an impacting water hammer pressure when water reaches the pipe end. Two types of pressure oscillation patterns are found, depending on orifice size. When orifice sizes are small, air cushioning effect prevents high water hammer pressures from being generated. As orifice size increases, the water hammer pressure is dominant. As initial air length increases, the maximum pressure firstly increases and then decreases. An elastic-water model could well reproduce the measured air pressure oscillations and impact pressure.

Liquid column separation (LCS) in pressurized pipelines may occur if a water hammer event drops the local pressure to the liquid’s vapor point. Numerical simulations of LCS have traditionally been based on one-dimensional (1D) transient flow theory; here, a two-dimensional (2D) computational fluid dynamics (CFD) model is used to investigate the complicated nature of LCS and to help characterize the limitations of the traditional 1D models. To this end, the Schnerr–Sauer cavitation model with a shear-stress transport (SST) k−ω turbulence model is employed, whereas the Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations are solved for the mixture of liquid and vapor. 2D model results are compared to both experimental data and to those of the 1D discrete vapor-cavity model (DVCM), thus demonstrating that the 2D method effectively simulates the pressure variations while helping to visualize the associated physical processes. More specifically, the 2D simulations vividly reveal the growth and th...