Experimental Study of Entrainment and Evolution of an Entrapped Air Pocket by Flowing Water in an Inclined U-Shaped Pipe

An air pocket is very likely to be trapped in a downward-sloping pipe (DSP) when water fills an undulating pipeline consisting of a series of inclined U-shaped pipes. To investigate the entrainment and evolution behavior of the entrapped air pocket, a series of experiments was carried out using a self-built inclined U-shaped device. The results show that at the junction between the DSP and the upward-sloping pipe (USP), the water phase forms a liquid accumulation. In addition to the agreed mechanism of liquid film jet shear and turbulent pulsation, the initiation of air entrainment is promoted further by the unique entraining effect of the reverse upward movement of the liquid accumulation in the DSP. Based on this, the flow patterns in the DSP are classified into four categories, namely single long slug flow (SLSF), unsteady transitional flow (UTF), quasi-slug flow (QSF), and plug flow (PF). Among them, UTF identified in this paper is a new and unique flow pattern for U-shaped pipe. Due to the development of UTF, the additional head loss of the air pocket and the critical Froude number required to completely remove the air pocket are increased. The effects of pipe inflow Froude number, liquid film Reynolds number, and upward-inclination angle on flow pattern transition, bubble movement, and air removal are explained and discussed.