Transient pressure associated with a rapidly filling pipeline containing two entrapped air pockets is investigated experimentally and numerically. A multiple-air-pocket elastic-water model considering multiple moving boundaries of water columns is developed by neglecting inertia and head loss of a short water column near air–water interfaces. The proposed model is validated by experimental data. Results show that when two air pockets in length are much different, the maximum pressure always arises in the smaller air pocket regardless of the blocking column’s length. The case of the upstream air pocket with a similar length to the downstream is the most complicated and dangerous because (1) the maximum pressure may alternately arise in two air pockets as the blocking column increases, and (2) interaction of two air pockets could cause a huge pressure surge, which is likely much higher than with only one air pocket. The existing single-air-pocket model cannot effectively simulate pressure surge of the two air pockets.

The pressure variations associated with a filling undulating pipeline containing an entrapped air pocket are investigated both experimentally and numerically. The influence of entrapped air on abnormal transient pressures is often ambiguous because the compressibility of the air pocket permits the liquid flow to accelerate but also partly cushions the system, with the balance of these tendencies being associated with the initial void fraction of the air pocket. Earlier experimental research involved systems with an initial void fraction greater than 5.8%; this paper focuses on initial void fractions ranging from 0 to 10% to more completely characterize the transient response. Experimental results show that the maximum pressure increases and then decreases as the initial void fraction decreases. A simplified model is developed by neglecting the liquid inertia and energy loss of a short water column near the air-water interface. Comparisons of the calculated and observed results show that the model is able to accurately predict peak pressures as a function of void fraction and filling conditions. Rigid water column models, however, perform poorly with small void fractions.

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.

He pressure within an entrapped air pocket in a filling pipeline without air release is investigated both experimentally and numerically. Volume of Fluid (VOF) model is introduced to simulate the transient flow. Both VOF model and experimental observations show that the air-water interface moves irregularly, and the water column near bottom pipe wall firstly reaches and impacts on pipe end wall, which could induce a sudden pressure jump. Due to assumptions of “air pocket occupies entire cross section” and “air-water interface is perpendicular to pipe centerline”, the existing one-dimensional model is incapable of calculating the jump pressure. Comparisons between calculated and measured results demonstrate that VOF model is effective to simulate movement of air pocket and pressure surge. In a one-dimensional model, only the first peak pressure is close to the measured value, but the following peak pressures are not. Therefore, VOF model is a feasible and more advanced method to investigate further this problem.

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.