Gas entrainment at free surface of liquid, (1)

The air entrainment mechanisms and rate at the free surface of water were investigated experimentally for a drawing flow into a vertical suction pipe in a cylindrical test vessel. It was found that the flow mechanisms of air entrainment were grouped into three typical types; (1) the vortex-induced entrainment, (2) the conically depressed surface with entrainment of relatively large bubbles, and (3) the falling flow along the side wall of the suction pipe with entrainment of fine bubbles. In the absence of a forced circulation, the air flow rate of the vortex-induced entrainment was well correlated by the Froude number. The proposed analytical model for the air entrainment rate of the vortex-induced entrainment agreed well with the experimental results in the presence of a forced circulation. The transition mechanisms between three types of entrainment were investigated, and it was found that the entrainment rate has the minimum value in the transition between the vortex-induced entrainment and the falling flow due to the stability of the air core. (author)