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Experimental and Computational Multiphase Flow

Article Title

Fundamental study on chaotic transition of two-phase flow regime and free surface instability in gas deaeration process

Keywords

deaeration, instability, kink formation, critical Atwood number

Abstract

Deaeration is a process of eliminating aspirated air from liquid in hydraulic reservoirs to avoid cavitation in the downstream pump blades. The complex fluid dynamics associated with deaeration is investigated. The three-dimensional buoyancy driven chaotic behavior of gas- liquid interfacial two-phase flow is studied. Parametric study is executed to understand change in internal flow physics (bubble coalescence, disintegration, horizontal spread, bubble velocity etc.), strength of accelerating Rayleigh-Taylor instability, turbulent kinetic energy, amplitude of upward velocity near free surface, and rise in free surface level with the variation of parameters like incoming mixture flow rate, incoming volume fraction of air, liquid fill depth, and Atwood number. The computations show increment in cavitation, wavenumber and amplitude of upward velocity towards oscillating free surface with incoming flow rate (Re). Cavitation and free surface instability show incremental trend with volume fraction of incoming air forming a kink (cavitation reduces) due to bubble coalescence in a threshold range of volume fraction of incoming air. With the variation of Atwood number, initially cavitation reduces. But after a critical value (A*) of Atwood number, effect of bubble disintegration, and rise of cavitation become prominent, which is formulated with respect to incoming flow rate (Re). With liquid fill depth, cavitation shows a slight decrement with almost equal deaeration and constant wavelength of free surface oscillation at an increasing buoyancy driven upward velocity. Some glimpse of design solution to reduce the cavitation and enhance the deaeration is also studied and formulated to get better understanding.

Publisher

Tsinghua University Press

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