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.

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...

To simulate transient cavitating pipe flow, the discrete gas cavity model (DGCM) is combined with first-order and second-order finite-volume method (FVM) Godunov-type schemes. The earlier discrete vapor cavity model (DVCM) and DGCM based on the method of characteristics (MOC) are known to produce unrealistic pressure spikes. The new FVM-DGCM extends the previously developed FVM-DVCM through the introduction of a very small amount of free gas at the middle of each computation cell. Importantly, a pressure adjustment procedure is proposed to establish the relation between the cavity and the halves of the reach. Predictions of FVM-DGCM are compared with those of FVM-DVCM and MOC-DGCM and with experimental data. Results show that the proposed model reproduces the experimental pressure histories considerably better than the other two models. In particular, it produces fewer spikes, but—as in the old models—the first pressure peak due to cavity collapse is predicted much better than the subsequent peaks. The second-order FVM-DGCM is found to be accurate and robust, even for Courant numbers significantly less than 1.

In this study, the phenomenon of column separation that occurs in the downstream pipeline of a rapid closure valve is experimentally investigated. Special attention is paid to the dynamic behavior of the formation, growth, and collapse processes of cavities, which are observed using a high-speed camera. Synchronized images of cavity patterns and measured pressure histories are analyzed to elucidate the process of water column separation, the mechanism of column separation events, and the influence of parameters on the transient flow. Experimental results indicate that during the collapse process of vapor cavities, a superposition phenomenon involving a positive pressure wave and collapse wave occurs, resulting in a nearly three times rise of Joukowsky pressure. In all test cases, the maximum pressure of the pipeline exceeded 150 times the reservoir static pressure. A new classification for a water hammer combined with cavitation (four types of pressure oscillation patterns) is proposed based on whether the duration of column separation decreases sequentially and the maximum pipeline pressure follows the first collapse of cavities at the valve. As the initial flow velocity increases, there is generally an increase in maximum pressure; however, this trend may be scattered under certain operation conditions.

The presence of entrapped air pockets in water pipelines during rapid filling frequently causes abnormal transient pressures which are capable of inducing accidents and causing serious damage (Wyli...