Experimental investigation on the effect of throat size on bubble transportation and breakup in small Venturi channels

Owing to high-turbulent and shear flow caused by the converging-diverging geometry, Venturi channels can serve as bubble generators to produce fine bubbles efficiently. Taking the advantage of simplicity in structure, high reliability, and low power consumption, Venturi-type bubble generators have broad application potential in various fields. Under a specified flow condition, the performance of a Venturi-type bubble generator is highly dependent on its geometric parameter. In the present work, an investigation was carried out to illustrate the effect of the throat sizes (Wth = 1 mm, 2 mm, and 4 mm) on the performance of rectangular Venturi channels for producing fine bubbles. Bubble deceleration, residence, and breakup in the diverging sections were particularly investigated and compared. Bubbles experienced dramatic deceleration and intense breakup in small Venturi channels like that in conventional size ones. The Vortex region in the diverging section moved to upstream with decreasing the throat size, resulting in more intense bubbles deformation and breakup. In Venturi channels of Wth = 2 mm and 4 mm, bubbles were always collapsed into several fragments after chaotic shape oscillations in the diverging section. For the case of Wth = 1 mm, bubbles were split by the strong liquid jet in their wake regions once they entered the diverging section. The liquid jet induced stronger vortex flow and significantly prolonged the residence time of the bubbles in the diverging section, which was beneficial to surface energy accumulation to trigger continuously shearing-off processes or bubble breakup. Overall, the Venturi channel of Wth = 1 mm presents the best performance, producing the smallest and most uniform fine bubbles with average diameter of 0.2 – 0.3 mm at the liquid velocity of Uth = 6.5 – 10 m/s.