The formation of the steady and unsteady air-entrained vortices in pump sump

Abstract

Air-entrainment is a negative phenomenon in pump stations. The process and the underlying mechanism are far from being fully understood. Based on the previous research, the present paper numerically studies the air-entrained vortex formation in a benchmark pump sump. The bifurcation model (BM) for capturing the rotating characteristics of the vortex and the simple coupled level set and volume of fluid method (S-CLSVOF) for tracking the free surface are adopted in this study. During both steady (case 1) and unsteady (case 2) air-entrained vortex formations, it is found that the wave is the predominant aspect. Exploration on the pressure signals reveals that the wave is generated by the reflection effect of the back wall and propagates from downstream to upstream. The vertical pressure gradient plays a negative role, rather than a positive one as expected. The streamwise and spanwise pressure gradients are found to drive the dimples and waves. The processes of the air-entrained vortex formations in two cases are analyzed in detail. For case 1, the formation process is wavy – inception – propagation – steady – fully developed. For case 2, there are some differences including the collision and reformation stages. Two waves will collide with each other after the inception and reform near the pipe. The process is wavy/inception – collision – reformation – propagation – fully developed. The study offers a new perspective on the air-entrainment mechanism. Accounting for the wavy characteristics, it is expected to eliminate the air-entrained vortex by suppressing the reflection wave.