Gas entrainment rate into liquid by a vortex formed on free surface was examined experimentally. Four kinds of test fluid were used; water at 25 °C, water at 60 °C, 20 cSt silicone oil and kerosene. Gas was air. The flow state of gas entrainment was visually observed by using a high speed video camera. The gas entrainment rate into liquid was measured. When liquid velocity was low, bubble-type gas entrainment occurred. As the liquid velocity increased, the gas entrainment type turned from the bubble type to a vortex type and gas entrainment rate considerably increased. The relation between gas entrainment rate and liquid velocity was mainly affected by the viscosity of liquid. As viscosity became large, higher liquid velocity was required to get the same gas entrainment rate. The effect of surface tension on the gas entertainment rate was minor or little. No systematic trend by the surface tension was noticed in the gas entrainment rate As liquid velocity increased, vortex became deep in the test vessel without gas entrainment occurrence and eventually a vortex tip reached a bottom outlet. After the vortex tip reached the bottom outlet, the tip penetrated into an outlet piping. Vortex growing speed becomes slow. Then, bubble-type gas entrainment was initiated from the vortex tip. A further increase in liquid velocity resulted in a transition from the bubble-type gas entrainment to vortex-type gas entrainment. By assuming that liquid flow was free falling film flow in an outlet pipe, liquid velocity was derived. The Kelvin-Helmholtz instability wave length was calculated for this liquid velocity. The wavelength that was observed at the vortex tip interface in the condition that bubbles were torn off from the tip was close to the Kelvin-Helmholtz instability wave length in both bubble-type and vortex-type gas entrainment.

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